if Statement
switch Statement
while Statement
do Statement
for Statement
goto Statement
break Statement
continue Statement
return Statement
typedef Statement
This is the GNU C reference manual.
This is a reference manual for the GNU C programming language: the C programming language as implemented by the GNU C compiler.
GCC supports several variants of C; this manual ultimately aims to explicitly document three of them:
By default, GCC will compile code as C89 plus GNU-specific extensions. Much of C99 is supported; once full support is available, the default compilation dialect will be C99 plus GNU-specific extensions. (Note that some of the GNU extensions to C89 ended up, sometimes slightly modified, as standard language features in C99.)
Presently, this manual describes C89 only. Descriptions of C99 and GNU extensions will be included in future releases, labeled as such alongside the original C89 material. While most users of GCC are free to use the latest and greatest additions to the compiler, some users must continue to use older versions of GCC. (For example, avionics programmers cannot switch to newer compilers on a whim, and indeed often use the same compiler for years until a new version is approved for use.) For this reason, we feel that a clear distinction between C dialects will be useful to our readers.
The C language includes a set of preprocessor directives, which are used for things such as macro text replacement, conditional compilation, and file inclusion. Although normally described in a C language manual, the GNU C preprocessor has been thoroughly documented in The C Preprocessor, a separate manual which covers preprocessing for C, C++, and Objective C programs, so it is not included here.
A number of people have contributed to this manual. Most of the text was written by Trevis Rothwell. Other contributors, who have offered editing, proofreading, ideas, and/or help with typesetting or administrative details, include: Karl Berry, Robert Chassell, Lisa Goldstein, Robert Hansen, Jean-Christophe Helary, Joe Humphries, J. Wren Hunt, Adam Johansen, Steve Morningthunder, Richard Stallman, J. Otto Tennant, Ole Tetlie, and T.F. Torrey.
Please send bug reports and suggestions to gnu-c-manual@gnu.org.
This chapter describes the lexical elements that make up C source code after preprocessing. These basic elements are called tokens, and there are several distinct types of tokens: keywords, identifiers, constants, operators, and separators. White space, sometimes required to separate tokens, is also described in this chapter.
Identifiers are strings of characters used for naming variables, functions, new data types, and preprocessor macros. The characters can be letters, decimal digits, the underscore character `_', or the dollar sign character `$'.
The first character of an identifier cannot be a digit.
Lowercase letters and uppercase letters are distinct, so squid
and SQUID are two different identifiers.
Keywords are special identifiers, reserved for use by the programming language itself. You cannot use them for any other purpose.
Here is a list of keywords recognized by ANSI C89:
auto break case char const continue default do double else enum extern
float for goto if int long register return short signed sizeof static
struct switch typedef union unsigned void volatile while
A constant is a literal numeric or character value, such as 5 or `m'. All constants are of a particular data type; you can use type casting to explicitly specify the type of a constant, or let the compiler use the default type based on the value of the constant.
An integer constant is a sequence of digits.
If the sequence of digits is preceded by 0x or 0X (zero x or
zero X), then the constant is considered to be hexadecimal (base 16). Hexadecimal
values may use the digits from 0 to 9, as well as the letters a to
f and A to F.
0x2f
0x88
0xAB43
0xAbCd
0x1
If the first digit is 0 (zero), and the next character is not x or
X, then the constant is considered to be octal (base 8).
Octal values may only use the digits from 0 to 7; 8 and 9 are not allowed.
057
012
03
0841
In all other cases, the sequence of digits is assumed to be decimal (base 10). Decimal values may use the digits from 0 to 9.
459
23901
8
12
There are various integer data types, for short integers, long integers, signed integers, and unsigned integers. You can force an integer constant to be of a long and/or unsigned integer type by appending a sequence of one or more letters to the end of the constant:
uUlLFor example, 45U is an unsigned int constant. You can
also combine letters: 45UL is an unsigned long int constant.
(The letters may be used in any order.) You can use two L's to
get a long long int constant; add a U and you have an
unsigned long long int constant.
There is no way to force an integer constant to be interpreted as a short integer. In addition, an integer constant will never be interpreted as a short integer by default, even if its value is small enough to be represented as one.
A character constant is usually a single character enclosed
within single quotation marks, such as 'Q'. A character constant
is of type int by default.
Some characters, such as the single quotation mark character ' itself,
cannot be represented using only one character. To represent such characters,
there are several “escape sequences” that you can use:
\\\?\'\"\a\b\e\f\n\r\t\v\ooo\xhhTo use any of these escape sequences, enclose the sequence in single
quotes, and treat it as if it were any other character. For example,
the letter m is 'm' and the newline character is '\n'.
The octal number escape sequence is the backslash character followed by
one, two, or three octal digits (0 to 7). For example, 101 is the
octal equivalent of 65, which is the ASCII character 'A'. Thus,
the character constant '\101' is the same as the character
constant 'A'.
The hexadecimal escape sequence is the backslash character, followed
by x and an unlimited number of hexadecimal digits (0 to 9, and
a to f or A to F).
While the number of possible hexadecimal digits is unlimited, the number of character constants is not. (The much-used extended ASCII character set has only 256 characters in it.) If you try to use a hexadecimal value that is outside the range of characters, you will get a compile-time error.
A real number constant is a value that represents a fractional (floating point) number. It consists of a sequence of digits which represents the integer (or “whole”) part of the number, a decimal point, and a sequence of digits which represents the fractional part.
Either the integer part or the fractional part may be omitted, but not both. Here are some examples:
double a, b, c, d, e, f;
a = 4.7; /* This is okay. */
b = 4.; /* This is okay. */
c = 4; /* This is okay. */
d = .7; /* This is okay. */
e = 0.7; /* This is okay. */
f = .; /* This is NOT okay! */
Real number constants can also be followed by e or
E, and an integer exponent. The exponent can be either positive
or negative.
double x, y;
x = 5e2; /* x is 5 x (10 ^ 2), or 500.0. */
y = 5e-2; /* y is 5 x (10 ^ -2), or 0.05. */
You can append a letter to the end of a real number constant
to cause it to be of a particular type. If you append the letter
F (or f) to a real number constant, then its type is float.
If you append the letter L (or l), then its type is long double.
If you do not append any letters, then its type is double.
A string constant is a sequence of characters, digits, and/or escape
sequences enclosed within double quotation marks. A string constant
is of type “array of characters”. All string constants contain a
null termination character (\0) as their last character. Strings
are stored as arrays of characters, with no inherent size attribute.
The null termination character lets string-processing functions know
where the string ends.
Adjacent string constants are concatenated (combined) into one string, with the null termination character added to the end of the final concatenated string.
A string cannot contain double quotation marks, as double
quotation marks are used to enclose the string. To include
the double quotation mark character in a string, use the \"
escape sequence. You can use any of the escape sequences that can
be used as character constants in strings. Here are some example
of string constants:
/* This is a single string constant. */ "tutti frutti ice cream" /* These string constants will be concatenated, same as above. */ "tutti " "frutti" " ice " "cream" /* This one uses two escape sequences. */ "\"hello, world!\""
If a string is too long to fit on one line, you can use a backslash \
to break it up onto separate lines.
"Today's special is a pastrami sandwich on rye bread with \
a potato knish and a cherry soda."
Adjacent strings are automatically concatenated, so you can also have string constants span multiple lines by writing them as separate, adjacent, strings. For example:
"Tomorrow's special is a corned beef sandwich on "
"pumpernickel bread with a kasha knish and seltzer water."
is the same as
"Tomorrow's special is a corned beef sandwich on \
pumpernickel bread with a kasha knish and seltzer water."
To insert a newline character into the string, so that when the string is printed it will be printed on two different lines, you can use the newline escape sequence `\n'.
printf ("potato\nknish");
prints
potato
knish
An operator is a special token that performs an operation, such as addition or subtraction, on either one, two, or three operands. Full coverage of operators can be found in a later chapter. See Expressions and Operators.
A separator separates tokens. White space (see next section) is a separator, but it is not a token. The other separators are all single-character tokens themselves:
( ) [ ] { } ; , . :
White space is the collective term used for a number of characters, including the space character, the tab character, and the newline character. In C programs, white space is ignored (outside of string and character constants), and is therefore optional, except when it is used to separate tokens. This means that
#include <stdio.h>
int
main()
{
printf( "hi there\n" );
return 0;
}
and
#include <stdio.h> int main(){printf("hi there\n");
return 0;}
are functionally the same program.
Although you must use white space to separate many tokens, no white space is required between operators and operands, nor is it required between other separators and that which they separate.
/* All of these are valid. */
x++;
x ++ ;
x=y+z;
x = y + z ;
x=array[2];
x = array [ 2 ] ;
fraction=numerator/*denominator_ptr;
fraction = numerator / * denominator_ptr ;
Furthermore, wherever one space is allowed, any amount of white space is allowed.
/* These two statements are functionally identical. */
x++;
x
++ ;
In string constants, spaces and tabs are not ignored; rather, they are part of the string. Therefore,
"potato knish"
is not the same as
"potato knish"
The integer data types range in size from at least 8 bits to at least 64 bits.
You should use them for storing whole number values (and the char
data type for storing characters). (Note that the sizes and ranges listed
for these types are minimums; depending on your computer platform, these sizes
and ranges may be larger.)
signed char
signed char data type can hold integer values in
the range of −128 to 127.
unsigned char
unsigned char data type can hold integer values in
the range of 0 to 255.
char
char data type is defined as
equivalent to either the signed char or the unsigned char
data type. By convention, you should use the char data type
specifically for storing ASCII characters (such as `m'), including escape
sequences (such as `\n').
short int
short int data type can hold integer values in the
range of −32,768 to 32,767. You may also refer to this data type as
short, signed short int, or signed short.
unsigned short int
unsigned short int data type can hold integer values
in the range of 0 to 65,535. You may also refer to this data type
as unsigned short.
int
int data type can hold integer values in the range
of −2,147,483,648 to 2,147,483,647. You may also refer to this data type
as signed int or signed.
unsigned int
unsigned int data type can hold integer values in
the range of 0 to 4,294,967,295. You may also refer to this data type
simply as unsigned.
long int
long int data type can hold integer values in
the range of at least −2,147,483,648 to 2,147,483,647. (Depending on
your system, this data type might be 64-bit, in which case its range is
identical to that of the long long int data type.) You may also
refer to this data type as long, signed long int,
or signed long.
unsigned long int
unsigned long int data type can hold integer values
in the range of at least 0 to 4,294,967,295. (Depending on your
system, this data type might be 64-bit, in which case its range is
identical to that of the unsigned long long int data type.) You may
also refer to this data type as unsigned long.
long long int
long int data type can hold integer values in
the range of -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. You
may also refer to this data type as long long,
signed long long int or signed long long.
unsigned long long int
unsigned long long int data type can hold integer
values in the range of at least 0 to 18,446,744,073,709,551,615. You may
also refer to this data type as unsigned long long.
There are several data types that represent fractional numbers, such as
3.14159 and 2.83. The exact sizes and ranges for the floating point data
types can vary from system to system; these values are stored in macro
definitions in the library header file float.h. In this section,
we include the names of the macro definitions in place of their possible
values:
float
float data type is the smallest of the three floating point
types, if they differ in size at all. Its minimum value is stored in
the FLT_MIN, and is should be no greater than 1e(−37).
Its maximum value is stored in FLT_MAX, and is should be no
less than 1e37.
double
double data type is at least as large as the float
type, and it may be larger. Its minimum value is stored in
DBL_MIN, and its maximum value is stored in DBL_MAX.
long double
long double data type is at least as large as the
float type, and it may be larger. Its minimum value is stored in
LDBL_MIN, and its maximum value is stored in LDBL_MAX.
All floating point data types are signed; trying to use unsigned float,
for example, will cause a compile-time error.
An enumeration is a custom data type used for storing constant integer
values and referring to them by names. By default, these values are
of type signed int; however, you can use the -fshort-enums
GCC compiler option to cause the smallest possible integer type to be
used instead.
You define an enumeration using the enum keyword, followed by
the name of the enumeration (this is optional), followed by a list of
constant names (separated by commas and enclosed in braces), and ending
with a semicolon.
enum fruit {grape, cherry, lemon, kiwi};
That example defines an enumeration, fruit, which contains four
constant integer values, grape, cherry, lemon, and
kiwi, whose values are, by default, 0, 1, 2, and 3, respectively.
You can also specify one or more of the values explicitly:
enum more_fruit {banana = -17, apple, blueberry, mango};
That example defines banana to be −17, and the remaining
values are incremented by 1—apple is −16,
blueberry is −15, and mango is -14. Unless
specified otherwise, an enumeration value is equal to one more than
the previous value (and the first value defaults to 0).
You can also refer to an enumeration value defined earlier in the same enumeration:
enum yet_more_fruit {kumquat, raspberry, peach,
plum = peach + 2};
In that example, kumquat is 0, raspberry is 1,
peach is 2, and plum is 4.
You can declare variables of an enumeration type both when the
enumeration is defined and afterward. This example declares one
variable, named my_fruit of type enum fruit:
enum fruit {banana, apple, blueberry, mango} my_fruit;
While this example declares the type and variable separately:
enum fruit {banana, apple, blueberry, mango};
enum fruit my_fruit;
(Of course, you couldn't declare it that way if you hadn't named the enumeration.)
Although such variables are considered to be of an enumeration type, you
can assign them any value that you could assign to an int variable,
including values from other enumerations. Furthermore, any variable that
can be assigned an int value can be assigned a value from an
enumeration.
However, you cannot change the values in an enumeration once it has been defined; they are constant values. For example, this won't work:
enum fruit {banana, apple, blueberry, mango};
banana = 15; /* You can't do this! */
A union is a custom data type used for storing several variables in the same memory space. Although you can access any of those variables at any time, you should only read from one of them at a time—assigning a value to one of them overwrites the values in the others.
You define a union using the union keyword followed by
the declarations of the union's members, enclosed in
braces. You declare each member of a union just as you would
normally declare a variable—using the data type followed by one
or more variable names separated by commas, and ending with a
semicolon. Then end the union definition with a semicolon after
the closing brace.
You should also include a name for the union between the union
keyword and the opening brace. This is syntactically optional, but if
you leave it out, you can't refer to that union data type later on
(without a typedef, see The typedef Statement).
Here is an example of defining a simple union for holding an integer value and a floating point value:
union numbers
{
int i;
float f;
};
That defines a union named numbers, which contains two
members, i and f, which are of type int and
float, respectively.
You can declare variables of a union type when both you initially define the union and after the definition, provided you gave the union type a name.
You can declare variables of a union type when you define the union type by putting the variable names after the closing brace of the union definition, but before the final semicolon. You can declare more than one such variable by separating the names with commas.
union numbers
{
int i;
float f;
} first_number, second_number;
That example declares two variables of type union numbers,
first_number and second_number.
You can declare variables of a union type after you define the
union by using the union keyword and the name you
gave the union type, followed by one or more variable names
separated by commas.
union numbers
{
int i;
float f;
};
union numbers first_number, second_number;
That example declares two variables of type union numbers,
first_number and second_number.
You can initialize the first member of a union variable when you declare it:
union numbers
{
int i;
float f;
};
union numbers first_number = { 5 };
In that example, the i member of first_number gets the
value 5. The f member is left alone.
Another way to initialize a union member is to specify the name of the member to initialize. This way, you can initialize whichever member you want to, not just the first one. There are two methods that you can use—either follow the member name with a colon, and then its value, like this:
union numbers first_number = { f: 3.14159 };
or precede the member name with a period and assign a value with the assignment operator, like this:
union numbers first_number = { .f = 3.14159 };
You can also initialize a union member when you declare the union variable during the definition:
union numbers
{
int i;
float f;
} first_number = { 5 };
You can access the members of a union variable using the member access operator. You put the name of the union variable on the left side of the operator, and the name of the member on the right side.
union numbers
{
int i;
float f;
};
union numbers first_number;
first_number.i = 5;
first_number.f = 3.9;
Notice in that example that giving a value to the f member overrides
the value stored in the i member.
This size of a union is equal to the size of its largest member. Consider the first union example from this section:
union numbers
{
int i;
float f;
};
The size of the union data type is the same as sizeof (float),
because the float type is larger than the int type. Since all
of the members of a union occupy the same memory space, the union data type
size doesn't need to be large enough to hold the sum of all their sizes; it
just needs to be large enough to hold the largest member.
A structure is a programmer-defined data type made up of variables of other data types (possibly including other structure types).
You define a structure using the struct keyword followed by
the declarations of the structure's members, enclosed in
braces. You declare each member of a structure just as you would
normally declare a variable—using the data type followed by one
or more variable names separated by commas, and ending with a
semicolon. Then end the structure definition with a semicolon after
the closing brace.
You should also include a name for the structure in between the
struct keyword and the opening brace. This is optional, but if
you leave it out, you can't refer to that structure data type later
on (without a typedef, see The typedef Statement).
Here is an example of defining a simple structure for holding the X and Y coordinates of a point:
struct point
{
int x, y;
};
That defines a structure type named struct point, which contains two
members, x and y, both of which are of type int.
You can declare variables of a structure type when both you initially define the structure and after the definition, provided you gave the structure type a name.
You can declare variables of a structure type when you define the structure type by putting the variable names after the closing brace of the structure definition, but before the final semicolon. You can declare more than one such variable by separating the names with commas.
struct point
{
int x, y;
} first_point, second_point;
That example declares two variables of type struct point,
first_point and second_point.
You can declare variables of a structure type after defining the
structure by using the struct keyword and the name you
gave the structure type, followed by one or more variable names
separated by commas.
struct point
{
int x, y;
};
struct point first_point, second_point;
That example declares two variables of type struct point,
first_point and second_point.
You can initialize the members of a structure type to have certain values when you declare structure variables. One way is to specify the values in a set of braces and separated by commas. Those values are assigned to the structure members in the same order that the members are declared in the structure in definition.
struct point
{
int x, y;
};
struct point first_point = { 5, 10 };
In that example, the x member of first_point gets the
value 5, and the y member gets the value 10.
Another way to initialize the members is to specify the name of the member to initialize. This way, you can initialize the members in any order you like, and even leave some of them uninitialized. There are two methods that you can use—either follow the member name with a colon, and then its value, like this:
struct point first_point = { y: 10, x: 5 };
or precede the member name with a period and assign a value with the assignment operator, like this:
struct point first_point = { .y = 10, .x = 5 };
You can also initialize the structure variable's members when you declare the variable during the structure definition:
struct point
{
int x, y;
} first_point = { 5, 10 };
You can also initialize fewer than all of a structure variable's members:
struct point
{
int x, y;
};
struct point first_point = { 5 };
In that example, only one member, x, is initialized. y is
left uninitialized. (This is because the members are initialized in
order of declaration, and x was declared before y.)
Here is another example that initializes a structure's members which are structure variables themselves:
struct point
{
int x, y;
};
struct rectangle
{
struct point top_left, bottom_right;
};
struct rectangle my_rectangle = { {0, 5}, {10, 0} };
That example defines the rectangle structure to consist of
two point structure variables. Then it declares one variable
of type struct rectangle and initializes its members. Since
its members are structure variables, we used an extra set of braces
surrounding the members that belong to the point structure
variables. However, those extra braces are not necessary; they just
make the code easier to read.
You can access the members of a structure variable using the member access operator. You put the name of the structure variable on the left side of the operator, and the name of the member on the right side.
struct point
{
int x, y;
};
struct point first_point;
first_point.x = 0;
first_point.y = 5;
You can also access the members of a structure variable which is itself a member of a structure variable.
struct rectangle
{
struct point top_left, bottom_right;
};
struct rectangle my_rectangle;
my_rectangle.top_left.x = 0;
my_rectangle.top_left.y = 5;
my_rectangle.bottom_right.x = 10;
my_rectangle.bottom_right.y = 0;
You can create structures with integer members of nonstandard sizes, called
bit fields. You do this by specifying an integer (int,
char, long int, etc.) member as usual, and inserting a colon
and the number of bits that the member should occupy in between the
member's name and the semicolon.
struct card
{
unsigned int suit : 2;
unsigned int face_value : 4;
};
That example defines a structure type with two bit fields, suit and
face_value, which take up 2 bits and 4 bits, respectively. suit
can hold values from 0 to 3, and face_value can hold values from 0 to
15. Notice that these bit fields were declared as unsigned int; had
they been signed integers, then their ranges would have been from
−2 to 1, and from −8 to 7, respectively.
More generally, the range of an unsigned bit field of N bits is from 0 to 2^N - 1, and the range of a signed bit field of N bits is from -(2^N) / 2 to ((2^N) / 2) - 1.
The size of a structure type is equal to the sum of the size of all of its members, possibly including padding to cause the structure type to align to a particular byte boundary. The details vary depending on your computer platform, but it would not be atypical to see structures padded to align on four- or eight-byte boundaries. This is done in order to speed up memory accesses of instances of the structure type.
If you wish to explicitly omit padding from your structure types (which may,
in turn, decrease the speed of structure memory accesses), then GCC provides
multiple methods of turning packing off. The quick and easy method is to
use the -fpack-struct compiler option. For more details on omitting
packing, please see the GCC manual which corresponds to your version of the
compiler.
An array is a data structure that lets you store one or more elements consecutively in memory. In C, array elements are indexed beginning at position zero, not one.
You declare an array by specifying the data type for its elements, its name, and the number of elements it can store. Here is an example that declares an array that can store ten integers:
int my_array[10];
The number of elements in an array must be positive.
You can initialize the elements in an array when you declare it by listing the initializing values, separated by commas, in a set of braces. Here is an example:
int my_array[5] = { 0, 1, 2, 3, 4 };
You don't have to initialize all of the array elements. For example, this code initializes only the first three elements:
int my_array[5] = { 0, 1, 2 };
That leaves the last two elements uninitialized.
You can access the elements of an array by specifying the array name, followed by the element index, enclosed in brackets. Remember that the array elements are numbered starting with zero. Here is an example:
my_array[0] = 5;
That assigns the value 5 to the first element in the array, at position zero. You can treat individual array elements like variables of whatever data type the array is made up of. For example, if you have an array made of a structure data type, you can access the structure elements like this:
struct point
{
int x, y;
};
struct point point_array[2] = { {4, 5}, {8, 9} };
point_array[0].x = 3;
You can make multidimensional arrays, or “arrays of arrays”. You do this by adding an extra set of brackets and array lengths for every additional dimension you want your array to have. For example, here is a declaration for a two-dimensional array that holds five elements in each dimension (a two-element array consisting of five-element arrays):
int two_dimensions[2][5] { {1, 2, 3, 4, 5}, {6, 7, 8, 9, 10} };
Multidimensional array elements are accessed by specifying the desired index of both dimensions:
two_dimensions[1][3] = 12;
You can use an array of characters to hold a string (see String Constants). The array may be built of either signed or unsigned characters.
When you declare the array, you can specify the number of elements it will have. That number will be the maximum number of characters that should be in the string, including the null character used to end the string. If you choose this option, then you do not have to initialize the array when you declare it. Alternately, you can simply initialize the array to a value, and its size will then be exactly large enough to hold whatever string you used to initialize it.
There are two different ways to initialize the array. You can specify of comma-delimited list of characters enclosed in braces, or you can specify a string literal enclosed in double quotation marks.
Here are some examples:
char blue[26];
char yellow[26] = {'y', 'e', 'l', 'l', 'o', 'w', '\0'};
char orange[26] = "orange";
char gray[] = {'g', 'r', 'a', 'y', '\0'};
char salmon[] = "salmon";
In each of these cases, the null character \0 is included at the
end of the string, even when not explicitly stated. (Note that if you
initialize a string using an array of individual characters, then the
null character is not guaranteed to be present. It might be,
but such an occurrence would be one of chance, and should not be relied
upon.)
After initialization, you cannot assign a new string literal to an array using the assignment operator. For example, this will not work:
char lemon[26] = "custard";
lemon = "steak sauce"; /* Fails! */
However, there are functions in the GNU C library that perform operations (including copy) on string arrays. You can also change one character at a time, by accessing individual string elements as you would any other array:
char name[] = "bob";
name[0] = 'r';
It is possible for you to explicitly state the number of elements in the array, and then initialize it using a string that has more characters than there are elements in the array. This is not a good thing. The larger string will not override the previously specified size of the array, and you will get a compile-time warning. Since the original array size remains, any part of the string that exceeds that original size is being written to a memory location that was not allocated for it.
You can create an array of a union type just as you can an array of a primitive data type.
union numbers
{
int i;
float f;
};
union numbers number_array [3];
That example creates a 3-element array of union numbers
variables called number_array. You can also initialize the
first members of the elements of a number array:
struct point point_array [3] = { {3}, {4}, {5} };
The additional inner grouping braces are optional.
After initialization, you can still access the union members in the array using the member access operator. You put the array name and element number (enclosed in brackets) to the left of the operator, and the member name to the right.
union numbers number_array [3];
number_array[0].i = 2;
You can create an array of a structure type just as you can an array of a primitive data type.
struct point
{
int x, y;
};
struct point point_array [3];
That example creates a 3-element array of struct point
variables called point_array. You can also initialize the
elements of a structure array:
struct point point_array [3] = { {2, 3}, {4, 5}, {6, 7} };
As with initializing structures which contain structure members, the additional inner grouping braces are optional. But, if you use the additional braces, then you can partially initialize some of the structures in the array, and fully initialize others:
struct point point_array [3] = { {2}, {4, 5}, {6, 7} };
In that example, the first element of the array has only its x
member initialized. Because of the grouping braces, the value 4 is
assigned to the x member of the second array element,
not to the y member of the first element, as would be
the case without the grouping braces.
After initialization, you can still access the structure members in the array using the member access operator. You put the array name and element number (enclosed in brackets) to the left of the operator, and the member name to the right.
struct point point_array [3];
point_array[0].x = 2;
point_array[0].y = 3;
Pointers hold memory addresses of stored constants or variables. For any data type, including both primitive types and custom types, you can create a pointer that holds the memory address of an instance of that type.
You declare a pointer by specifying a name for it and a data type. The data type indicates of what type of variable the pointer will hold memory addresses.
To declare a pointer, include the indirection operator (see The Indirection Operator) before the identifier. Here is the general form of a pointer declaration:
data-type * name;
You can also put the operator either directly next to name, or directly next to data-type, like these:
data-type *name;
data-type* name;
Any of these three is fine, and they all work the same way (white
space is not significant, as usual). Here is an example of declaring
a pointer to hold the address of an int variable:
int *ip;
Be careful, though: when declaring multiple pointers in the same statement, you must explicitly declare each as a pointer, using the indirection operator:
int *bob, *emily; /* Two pointers. */ int *rob, laura; /* A pointer and an integer variable. */
You can initialize a pointer when you first declare it by specifying
a variable address to store in it. For example, the following code
declares an int variable `i', and a pointer which is
initialized with the address of `i':
int i;
int *ip = &i;
Note the use of the address operator (see The Address Operator), used to get the memory address of a variable. Be careful, though: after you declare a pointer, you do not use the indirection operator with the pointer's name when assigning it a new address to point to. On the contrary, that would change the value of the variable that the points to, not the value of the pointer itself. For example:
int i, j;
int *ip = &i; /* `ip' now holds the address of `i'. */
ip = &j; /* `ip' now holds the address of `j'. */
*ip = &i; /* `j' now holds the address of `i'. */
The value stored in a pointer is an integral number: a location within the computer's memory space. If you are so inclined, you can assign pointer values explicitly using literal integers, casting them to the appropriate pointer type. However, we do not recommend this practice unless you need to have extremely fine-tuned control over what is stored in memory, and you know exactly what you are doing. It would be all too easy to accidentally overwrite something that you did not intend to.
You can create a pointer to a union type just as you can a pointer to a primitive data type.
union numbers
{
int i;
float f;
};
union numbers foo = {4};
union numbers *number_ptr = &foo;
That example creates a new union type, union numbers, and
declares (and initializes the first member of) a variable of that type
named foo. Finally, it declares a pointer to the type
union numbers, and gives it the address of foo.
You can access the members of a union variable through a pointer, but
you can't use the regular member access operator anymore. Instead,
you have to use the indirect member access operator (see Member Access Operators). Continuing with the previous example, the
following example will change the value of the first member of
foo:
number_ptr -> i = 450;
Now the i member in foo is 450.
You can create a pointer to a structure type just as you can a pointer to a primitive data type.
struct fish
{
float length, weight;
};
struct fish salmon = {4.3, 5.8};
struct fish *fish_ptr = &salmon;
That example creates a new structure type, struct fish, and
declares (and initializes) a variable of that type named salmon.
Finally, it declares a pointer to the type struct fish, and
gives it the address of salmon.
You can access the members of a structure variable through a pointer,
but you can't use the regular member access operator anymore.
Instead, you have to use the indirect member access operator
(see Member Access Operators). Continuing with the previous
example, the following example will change the values of the members
of salmon:
fish_ptr -> length = 5.1;
fish_ptr -> weight = 6.2;
Now the length and width members in salmon are
5.1 and 6.2, respectively.
You can define structures, unions, and enumerations without listing their members (or values, in the case of enumerations). Doing so results in an incomplete type. You can't declare variables of incomplete types, but you can work with pointers to those types.
struct point;
At some time later in your program you will want to complete the type. You do this by defining it as you usually would:
struct point
{
int x, y;
};
There are two type specifiers that you can prepend to your variable declarations
which change how the variables may be accessed: const and volatile.
const causes the variable to be read-only; after initialization, its
value may not be changed.
const float pi = 3.14159;
In addition to helping to prevent accidental value changes, declaring variables
with const can aid the compiler in code optimization.
volatile tells the compiler that the variable is explicitly changeable,
and seemingly useless accesses of the variable (for instance, via pointers) should
not be optimized away. You might use volatile variables to store data
that is updated via callback functions.
volatile float currentTemperature = 40.0;
There are four storage class specifiers that you can prepend to your variable
declarations which change how the variables are stored in memory:
auto, extern, register, and static.
You use auto for variables which are local to a function, and whose
values should be discarded upon return from the function in which they are
declared. This is the default behavior for variables declared within functions.
void
foo (int value)
{
auto int x = value;
...
return;
}
register is nearly identical in purpose to auto, except that
it also suggests to the compiler that the variable will be heavily used, and,
if possible, should be stored in a register in memory. You cannot use the
address-of operator to obtain the address of a variable declared with
register.
static is essentially the opposite of auto: when applied to
variables within a function or block, these variables will retain their
value even when the function or block is finished.
int
sum (int x)
{
static int sumSoFar = 0;
sumSoFar = sumSoFar + x;
return x;
}
You can also declare variables outside of functions to be static; such
variables are visible (global) to the current source file (but not
other source files).
extern is useful for declaring variables that you want to be visible to
all source files that are linked into your project. You cannot initialize a
variable in an extern declaration, as no space is actually allocated
during the declaration. You must make both an extern declaration
(typically in a header file that is included by the other source files which
need to access the variable) and a non-extern declaration which is where
space is actually allocated to store the variable. The extern declaration
may be repeated multiple times.
extern int numberOfClients;
...
int numberOfClients = 0;
See Program Structure and Scope, for related information.
Sometimes it is conveninent to give a new name to a type. You can do this using
the typedef statement. See The typedef Statement, for more information.
An expression consists of at least one operand and zero or more operators. The operands may be any value, including constants, variables, and function calls that return values. Here are some examples:
47
2 + 2
function()
The last of those, function(), is only an expression if
function() has a return type other than void.
You can use parentheses to group subexpressions:
( 2 * ( ( 3 + 10 ) - ( 2 * 6 ) ) )
Innermost expressions are evaluated first. In the above example,
3 + 10 and 2 * 6 evaluate to 13 and 12,
respectively. Then 12 is subtracted
from 13, resulting in 1. Finally, 1 is multiplied by
2, resulting in 2. The outermost parentheses are completely
optional.
An operator specifies an operation to be performed on its operand(s). Operators may have one, two, or three operands, depending on the operator.
Unary operators perform an operation on a single operand.
The increment operator ++ adds 1 to its operand. The operand must
be a either a variable of one of the primitive data types, a pointer, or an
enumeration variable. Here are some examples:
int x = 5;
char y = 'B';
float z = 5.2;
int *p = &x;
x++; /* x is now 6. */
y++; /* y is now `C'. */
z++; /* z is now 6.2. */
p++; /* p is now &x + sizeof(int). */
You can use the increment operator either before or after the operand. A prefix increment adds 1 before the operand is evaluated. A postfix increment adds 1 after the operand is evaluated. In the previous examples, that wouldn't have made any difference. However, there are cases where it does make a difference:
int x = 5;
printf ("%d \n", x++); /* Print x and then increment it. */
/* x is now equal to 6. */
printf ("%d \n", ++x); /* Increment x and then print it. */
The output of the above example is:
5
7
The decrement operator -- subtracts 1 from its operand. The operand
must be a either a variable of one of the primitive data types, a pointer, or
an enumeration variable. Here are some examples:
int x = 5;
char y = 'B';
float z = 5.2;
int *p = &x;
x--; /* x is now 4. */
y--; /* y is now `A'. */
z--; /* z is now 4.2. */
p--; /* p is now &x - sizeof(int). */
You can use the decrement operator either before or after the operand. A prefix decrement subtracts 1 before the operand is evaluated. A postfix increment subtracts 1 after the operand is evaluated. In the previous examples, that wouldn't have made any difference. However, there are cases where it does make a difference:
int x = 5;
printf ("%d \n", x--); /* Print x and then decrement it. */
/* x is now 4 */
printf ("%d \n", --x); /* Decrement x and then print it. */
The output of the above example is:
5
3
You can use the positive operator + on numeric values
to indicate that their value is positive. By default, values are positive
unless explicitly stated to be negative, so there is no need to use this
operator as far as the compiler is concerned. However, you can use it to
visually reinforce the fact that a value is positive. Here are some examples:
int x = +5;
float y = +3.14159;
You can use the negative operator - on numeric variables and constants
to indicate that their value is negative. Here are some examples:
int x = -5;
float y = -3.14159;
If the operand you use with the negative operator is of an unsigned data type, then the result is not negative, but rather the maximum value of the unsigned data type, minus the value of the operand.
You can use the logical negation operator ! to get the logical
opposite of its operand. If its operand is 0 (or null, if the operand is
a pointer), then the result of the logical negation operator is 1. If its
operand is anything other than 0 (or null), then the result of the logical
negation operator is 0. In any case, the result is an integer value. Here
are some examples:
int x = !5; /* x is 0. */
if (!x)
printf ("x is 0");
You can use the bitwise complement operator ~ to get the one's
complement of its operand. The operand must be an integer or character type.
The bitwise complement operator examines its operand's bits, and
changes all 0 bits to 1 and all 1 bits to 0. Here is an example:
unsigned int x = 500;
unsigned int y;
y = ~x;
Using signed data types with the bitwise complement operator may cause portability problems, so use unsigned data types for maximum portability.
You can use the address operator & to obtain the memory address
of its operand. You can use this operator both with variables of any data type
(including arrays and structures) and with functions, but you can't
use it with literal values. You should only store the result of the
address operator in pointer variables.
int x = 5;
int *ptr = &x;
You can use the indirection operator * to obtain the value stored
at the address specified by its operand. This is known as
dereferencing its operand. Its operand must be a pointer.
int x = 5;
int y;
int *ptr;
ptr = &x; /* ptr now holds the address of x. */
y = *ptr; /* y gets the value stored at the address
stored in ptr. */
The result of using the indirection operator with pointers that have not been initialized is unspecified; usually the program will crash.
You can use the sizeof operator to obtain the size (in bytes)
of the data type of its operand. The operand may be an actual type
specifier (such as int or float), as well as any valid
expression. You must enclose the operand in parentheses after the
operator. Here are some examples:
size_t a = sizeof(int);
size_t b = sizeof(float);
size_t c = sizeof(5);
size_t d = sizeof(5.143);
The result of the sizeof operator is of a type called size_t,
which is defined in the header file <stddef.h>. size_t is
an unsigned integer type, perhaps identical to unsigned int or
unsigned long int; it varies from system to system.
You can use a type cast to explicitly cause an expression to be of a specified data type. A type cast consists of a type specifier enclosed in parentheses, followed by an expression. To ensure proper casting, you should also enclose the expression that follows the type specifier in parentheses. Here is an example:
float x;
int y = 7;
int z = 3;
x = (float) (y / z);
In that example, since y and z are both integers, integer
division is performed, and even though x is a floating-point
variable, it receives the value 2. By explicitly casting the result
of the division to float, the floating-point value 2.333...
is retained and assigned to x.
Type casting also works with custom data types. Here is an example of converting an array of 8 bytes to a 8-byte structure type:
struct fooType
{
float f;
unsigned short int a;
unsigned short int b;
};
struct fooType foo;
unsigned char byteArray[8];
foo = (struct fooType) byteArray;
In practice, you should also pack your structure type to ensure that the compiler doesn't add any padding, thereby increasing the size of the type. In general, different compilers may use different representations for data types, so such constructs are not portable.
You can access array elements by specifying the name of the array, and the
array subscript (or index, or element number) enclosed in brackets. Here is
an example, supposing an integer array called my_array:
my_array[0] = 5;
A call to any function which returns a value is an expression.
int function(void);
...
a = 10 + function();
You use the addition operator + to add two operands. You put
the operands on either side of the operator, and it does not matter
which operand goes on which side (in the absence of side effects):
3 + 5 and 5 + 3 both result in 8. The operands must be
either expressions of a primitive data type or pointers.
x = 5 + 3;
y = 10 + 37;
z = 1 + 2 + 3 + 4 + 5;
When you use more than one addition operator (and more than two operands), such as in the last example, the expression is evaluated from left to right.
You use the subtraction operator - to subtract its second operand
from its first operand. You put the operands on either side of the operator,
and it does matter which operand goes on which side: 3 - 5 and
5 - 3 do not have the same result. The operands must be either
expressions of a primitive data type or pointers.
x = 5 - 3;
y = 57 - 10;
z = 5 - 4 - 3 - 2 - 1;
When you use more than one subtraction operator (and more than two operands), such as in the last example, the expression is evaluated from left to right.
You use the multiplication operator * to multiply two operands
together. You put the operands on either side of the operator, and it does
not matter which operand goes on which side: 3 * 5 and 5 * 3
both result in 15. The operands must be expressions of a primitive
data type.
x = 5 * 3;
y = 47 * 1;
z = 1 * 2 * 3 * 4 * 5;
When you use more than one multiplication operator (and more than two operands), such as in the last example, the expression is evaluated from left to right.
You use the division operator / to divide its first operand
by its second operand. You put the operands on either side of the
operator, and it does matter which operand goes on which side: 3 / 5
and 5 / 3 do not have the same result. The operands must be
expressions of a primitive data type.
x = 5 / 3;
y = 940 / 20;
z = 100 / 2 / 2;
When you use more than one division operator (and more than two operands), such as in the last example, the expression is evaluated from left to right.
You use the modulus operator % to obtain the remainder produced
by dividing its two operands. You put the operands on either side of
the operator, and it does matter which operand goes on which
side: 3 % 5 and 5 % 3 do not have the same result.
The operands must be expressions of a primitive data type.
x = 5 % 3;
y = 74 % 47;
z = 47 % 32 % 21;
When you use more than one modulus operator (and more than two operands), like in the last example, the expression is evaluated from left to right.
A common application of the modulus operator is to determine if one number is divisible by another number. If it is divisible, then the remainder is zero. Here is an example of that:
int counter;
for (counter = 0; counter <= 100; counter++)
{
if (counter % 5 == 0)
printf ("%d\n", counter);
}
That prints all of the integers from 0 to 100 that are divisible by 5.
You use the left-shift operator << to shift its first operand's bits
to the left. You specify the number of bit-places shifted with the second
operand. If there is a 1 bit in the leftmost bit position, it will be
discarded. New bits that are added to the rightmost bit position will all
be 0.
x = 47; /* 47 is 00101111 in binary. */ x << 1; /* 00101111 << 1 is 01011110. */
You use the right-shift operator >> to shift its first operand's bits
to the right. You specify the number of bit-places shifted with the second
operand. If there is a 1 bit in the rightmost bit position, it will be
discarded. New bits that are added to the leftmost bit position
may be either 0 or 1. If the first operand is unsigned, then the added
bits will be 0. If it is signed, the added bits will be either 0 or
whatever value was previously in the leftmost bit position.
x = 47; /* 47 is 00101111 in binary. */ x >> 1; /* 00101111 >> 1 is 00010111. */
The bitwise AND operator & examines each bit in its two
operands, and when two corresponding bits are both 1, the resulting bit
is 1. In every other case the result is 0. Here is an example of
how this operator works, using binary numbers:
11001001 & 10011011 = 10001001
If you look closely at that, you'll see that when a bit is 1 in both operands, the corresponding bit in the result is set to 1. Otherwise it is set to 0. Here is another example, this time in C:
char x = 149, y = 34, z;
z = x & y;
The bitwise inclusive OR operator | examines each bit
in its two operands, and when two corresponding bits are both
0, the resulting bit is 0. In every other case the resulting
bit is 1. Here is an example of how this operator works, using
binary numbers:
11001001 | 10011011 = 11011011
Here is another example, this time in C:
char x = 149, y = 34, z;
z = x | y;
The bitwise exclusive OR operator ^ (also known as XOR) examines
each bit in its two operands, and when two corresponding bits are different,
the resulting bit is 1. When they are the same, the resulting bit is 0.
Here is an example of how this operator works, using binary numbers:
11001001 | 10011011 = 01011001
Here is another example, this time in C.
char x = 149, y = 34, z;
z = x ^ y;
You use the comparison operators to determine how two operands relate to each other: are they equal to each other, is one larger than the other, is one smaller than the other, and so on. When you use any of the comparison operators, the result is either 1 or 0, meaning true or false, respectively.
In the following code examples, the variables x and y stand
for any two expressions of primitive types, or pointers.
Use the equal-to operator == to test two operands for
equality. It evaluates to 1 if the two operands are equal, and 0 if the
two operands are not equal.
if (x == y)
puts ("x is equal to y");
else
puts ("x is not equal to y");
Use the not-equal-to operator != to test two operands for
inequality. If the two operands are not equal, the result is 1. Otherwise,
if the two operands are equal, the result is 0.
if (x != y)
puts ("x is not equal to y");
else
puts ("x is equal to y");
Use the less-than operator < to determine if the first operand
is less than the second operand. If it is, the result is 1. Otherwise, the
result is 0.
if (x < y)
puts ("x is less than y");
else
puts ("x is not less than y");
Use the less-than-or-equal-to operator <= to determine if the first
operand is less than or equal to the second operand. If it is, the result
is 1. Otherwise, the result is 0.
if (x <= y)
puts ("x is less than or equal to y");
else
puts ("x is not less than or equal to y");
Use the greater-than operator > to determine if the first operand
is greater than the second operand. If it is, the result is 1. Otherwise,
the result is 0.
if (x > y)
puts ("x is greater than y");
else
puts ("x is not greater than y");
Use the greater-than-or-equal-to operator >= to determine if the
first operand is greater than or equal to the second operand. If it is, the
result is 1. Otherwise, the result is 0.
if (x >= y)
puts ("x is greater than or equal to y");
else
puts ("x is not greater than or equal to y");
You can use the logical operators to test the truth value of two operands. The operands can be expressions of a primitive type, or pointers.
Note that while the comparison operators return the value 1 for a true expression, any nonzero expression is considered true in C.
Use the logical AND operator && to test if two expressions are
both true. If the first one is false, then the second one is not
evaluated.
if ((x == 5) && (y == 10))
printf ("x is 5 and y is 10");
You can also build an expression using more than one AND operator, and more than two operands, like this:
if ((x == 5) && (y == 10) && (z == 15))
printf ("x is 5 and y is 10 and z is 15");
Use the logical OR operator || to test if at least one of two
expressions is true. If the first expression is true, then the
second expression is not evaluated.
if ((x == 5) || (y == 10))
printf ("x is 5 or y is 10");
You can also build an expression using more than one OR operator, and more than two operands, like this:
if ((x == 5) || (y == 10) || (z == 15))
printf ("x is 5 or y is 10 or z is 15");
You use the assignment operators to give values to variables. The first operand—the operand to which a value is being assigned, also known as the “lvalue”—cannot be a literal value or any other constant value. Except as noted, the operands must be of a primitive data type, or a pointer.
Use the standard assignment operator = to assign the value of its
right operand to its left operand. Unlike the other assignment operators,
you can use this operator with variables of a structure type, in addition
to primitive types and pointers.
x = 10;
y = 45 + 2;
z = (2 * (3 + function () ));
You use the compound assignment operators to perform an operation on both the left and right operands, and then assign the resulting expression to the left operand. Here is a list of the compound assignment operators, and a brief description of what they do:
+=
This operator adds its two operands together, and then assigns the result of the addition to the left operand.
-=
This operator subtracts its right operand from its left operand, and assigns the result of the subtraction to the left operand.
*=
This operator multiplies its two operands together, and then assigns the result of the multiplication to the left operand.
/=
This operator divides its left operand by its right operand, and assigns the result of the division to the left operand.
%=
This operator performs modular division on its operands, and assigns the result of the division to the left operand.
<<=
This operator performs a left shift operation on its left operand, shifting by the number of bits specified by the right operand, and assigns the result of the shift to the left operand.
>>=
This operator performs a right shift operation on its left operand, shifting by the number of bits specified by the right operand, and assigns the result of the shift to the left operand.
&=
This operator performs a bitwise AND operation on its two operands, and assigns the result of the operation to the left operand.
^=
This operator performs a bitwise exclusive OR operation on its two operands, and assigns the result of the operation to the left operand.
|=
This operator performs a bitwise inclusive OR operation on its two operands, and assigns the result of the operation to the left operand.
Here is an example of using one of the compound assignment operators:
x += y;
That produces the same result (since there no side effects in the
simple variable x as an lvalue) as:
x = x + y;
You use the comma operator , to separate two expressions. For instance,
the first expression might produce a value that is used by the second expression:
x++, y = x * x;
More commonly, the comma operator is used in for statements, like
this:
/* Using the comma operator in a for statement. */
for (x = 1, y = 10; x <=10 && y >=1; x++, y--)
{
...
}
This lets you conveniently set, monitor, and modify multiple control
expressions for the for statement.
A comma is also used to separate function parameters; however, this is not the comma operator in action. In fact, if the comma operator is used as we have discussed here in a function call (without enclosing it in an additional set of parentheses), then the compiler will interpret that as calling the function with an extra parameter.
function (x, y=47, y, z);
Even though what may be intended by such a function call is to
call the function with the parameters x, y, and
z, with y set to 47, what will happen is that the
function will be called with the parameters x, y=47,
y, and z. So if you want to include expressions that
use the comma operator in a function call, surround the comma operator
expression with parentheses.
function (x, (y=47, y), z);
That will call function with the parameters x,
y, and z, with y set to 47.
You can use the member access operator . to access the members
of a structure or union variable. You put the name of the structure
variable on the left side of the operator, and the name of the
member on the right side.
struct point
{
int x, y;
};
struct point first_point;
first_point.x = 0;
first_point.y = 5;
You can also access the members of a structure or union variable via
a pointer by using the indirect member access operator ->.
x->y is equivalent to (*x).y.
struct fish
{
int length, weight;
};
struct fish salmon;
struct fish *fish_pointer = &salmon;
fish_pointer->length = 3;
fish_pointer->weight = 9;
See Pointers.
C has only one ternary operator — the conditional operator ? :.
You use the conditional operator to cause the entire conditional expression to evaluate to either its second or its third operand, based on the truth value of its first operand.
Put the first operand is before the question mark; this operand may be any expression. If it evaluates to true (nonzero), then the second operand (which is also an expression), which is placed in between the question mark and the colon, is evaluated, and becomes the value of the conditional expression. Otherwise, the third operand (also an expression), which is placed after the colon, is evaluated, and becomes the value of the conditional expression. Here is an example
a = (x == 5) ? y : z;
In that example, if x equals 5, then a will receive
the value y. Otherwise, a will receive the value
z. This can be considered a shorthand method for writing a simple
if...else statement. The following example will
accomplish the same task as the previous one:
if (x != 0)
a = y;
else
a = z;
If the first operand of the conditional operator is true, then the third operand is never evaluated. Similarly, if the first operand is false, then the second operand is never evaluated. The first operand is always evaluated.
When an expression consists of subexpressions, such as
a + b * f(), the subexpressions are not all evaluated at once,
nor are they necessarily evaluated left to right as they appear in the
expression. In the preceding example, for instance, the function call
f() will be evaluated first, the result of which will be multiplied
by b, the result of which will be added to a. That is not
intuitive simply by looking at the expression, but there is a definite
order in which subexpressions are evaluated.
The following is a list of types of expressions, presented in the same order that they are evaluated. The expressions that are evaluated first are said to have the highest precedence, and those that are evaluated last the low precedence. Some types of expressions have the same precedence. If two or more subexpressions have the same precedence, then they are usually evaluated left to right. If they are not evaluated left to right, then that is explicitly stated in the list.
sizeof expressions. When used as subexpressions, these are evaluated
right to left.
?:). When used as
subexpressions, these are evaluated right to left.
You write statements to cause actions and to control flow within your programs. You can also write statements that do not do anything at all, or do things that are uselessly trivial.
You can use labels to identify a section of source code. A label consists of an identifier (such as those used for variable names) followed by a colon. Here is an example:
treet:
You should be aware that label names do not interfere with other identifier names:
int treet = 5; /*treetthe variable. */ treet: /*treetthe label. */
The ISO C standard mandates that a label must be followed by at least one statement (possibly a null statement). GCC will compile code that does not meet this requirement, but be aware that if you violate it, your code may have portability issues.
You can turn any expression into a statement by adding a semicolon to the end of the expression. Here are some examples:
5;
2 + 2;
10 >= 9;
In each of those, all that happens is that each expression is evaluated. However, they are useless because they do not store a value anywhere, nor do they actually do anything, other than the evaluation itself. The compiler is free to ignore such statements.
Expression statements are only useful when they have some kind of side effect, such as storing a value, calling a function, or (this is esoteric) causing a fault in the program. Here are some more useful examples:
x++;
y = x + 25;
puts ("Hello, user!");
*cucumber;
The last of those statements, *cucumber;, could potentially
cause a fault in the program if the value of cucumber is both
not a valid pointer and has been declared as volatile.
if Statement
You can use the if statement to conditionally execute part of your
program, based on the truth value of a given expression. Here is the
general form of an if statement:
if (test)
then-statement
else
else-statement
If test evaluates to true, then then-statement is executed and
else-statement is not. If test evaluates to false, then
else-statement is executed and then-statement is not. The
else clause is optional.
Here is an actual example:
if (x == 10)
puts ("x is 10");
If x == 10 evaluates to true, then the statement
puts ("x is 10"); is executed. If x == 10 evaluates to
false, then the statement puts ("x is 10"); is not executed.
Here is an example using else:
if (x == 10)
puts ("x is 10");
else
puts ("x is not 10");
You can use a series of if statements to test for multiple
conditions:
if (x == 1)
puts ("x is 1");
else if (x == 2)
puts ("x is 2");
else if (x == 3)
puts ("x is 3");
else
puts ("x is something else");
switch Statement
You can use the switch statement to compare one expression with others,
and then execute a series of sub-statements based on the result of the
comparisons. Here is the general form of a switch statement:
switch (test)
{
case compare-1:
if-equal-statement-1
case compare-2:
if-equal-statement-2
...
default:
default-statement
}
The switch statement compares test to each of the
compare expressions, until it finds one that is
equal to test. Then, the statements following the successful
case are executed. All of the expressions compared must be of an integer
type, and the compare-N expressions must be of a constant integer
type (e.g., a literal integer or an expression built of literal integers).
Optionally, you can specify a default case. If test doesn't match any of the specific cases listed prior to the default case, then the statements for the default case are executed. Traditionally, the default case is put after the specific cases, but that isn't required.
switch (x)
{
case 0:
puts ("x is 0");
break;
case 1:
puts ("x is 1");
break;
default:
puts ("x is something else");
break;
}
Notice the usage of the break statement in each of the cases. This
is because, once a matching case is found, not only are its statements
executed, but so are the statements for all following cases:
int x = 0;
switch (x)
{
case 0:
puts ("x is 0");
case 1:
puts ("x is 1");
default:
puts ("x is something else");
}
The output of that example is:
x is 0
x is 1
x is something else
This is often not desired. Including a break statement at the
end of each case redirects program flow to after the switch statement.
while Statement
The while statement is a loop statement with an exit test at
the beginning of the loop. Here is the general form of the while
statement:
while (test)
statement
The while statement first evaluates test. If test
evaluates to true, statement is executed, and then test is
evaluated again. statement continues to execute repeatedly as long as
test is true after each execution of statement.
This example prints the integers from zero through nine:
int counter = 0;
while (counter < 10)
printf ("%d ", counter++);
do Statement
The do statement is a loop statement with an exit test at the end
of the loop. Here is the general form of the do statement:
do
statement
while (test);
The do statement first executes statement. After that,
it evaluates test. If test is true, then statement is
executed again. statement continues to execute repeatedly as long as
test is true after each execution of statement.
This example also prints the integers from zero through nine:
int x = 0;
do
printf ("%d ", x++);
while (x < 10);
for Statement
The for statement is a loop statement whose structure allows
easy variable initialization, expression testing, and variable
modification. It is very convenient for making counter-controlled
loops. Here is the general form of the for statement:
for (initialize; test; step)
statement
The for statement first evaluates the expression initialize.
Then it evaluates the expression test. If test is false, then
the loop ends and program control resumes after statement. Otherwise,
if test is true, then statement is executed. Finally,
step is evaluated, and the next iteration of the loop begins with
evaluating test again.
Most often, initialize assigns values to one or more variables, which are generally used as counters, test compares those variables to a predefined expression, and step modifies those variables' values. Here is another example that prints the integers from zero through nine:
int x;
for (x = 0; x < 10; x++)
printf ("%d ", x);
First, it evaluates initialize, which assigns x the value
0. Then, as long as x is less than 10, the value of x
is printed (in the body of the loop). Then x is incremented in
the step clause and the test re-evaluated.
All three of the expressions in a for statement are optional, and any
combination of the three is valid. Since the first expression is evaluated
only once, it is perhaps the most commonly omitted expression. You could
also write the above example as:
int x = 1;
for (; x <= 10; x++)
printf ("%d ", x);
In this example, x receives its value prior to the beginning of the
for statement.
If you leave out the test expression, then the for statement
is an infinite loop (unless you put a break or goto statement
somewhere in statement). This is like using 1 as
test; it is never false.
This for statement starts printing numbers at 1 and then
continues indefinitely, always printing x incremented by 1:
for (x = 1; ; x++)
printf ("%d ", x);
If you leave out the step expression, then no progress is made
toward completing the loop—at least not as is normally expected with
a for statement.
This example prints the number 1 over and over, indefinitely:
for (x = 1; x <= 10;)
printf ("%d ", x);
You can use the comma operator (see The Comma Operator) for
monitoring and modifying multiple variables in a for statement:
for (x = 1, y = 10; x <= 10, y >= 1; x++, y--)
printf ("%d %d ", x, y);
This example assigns values to both x and y, checks that
x is less than or equal to 10 and that y is greater
than or equal to 1, and increments x and decrements y.
A block is a set of zero or more statements enclosed in
braces. Often, a block is used as the body of an if
statement or a loop statement, to group statements together.
for (x = 1; x <= 10; x++)
{
printf ("x is %d\n", x);
if ((x % 2) == 0)
printf ("%d is even\n", x);
else
printf ("%d is odd\n", x);
}
You can also put blocks inside other blocks:
for (x = 1; x <= 10; x++)
{
if ((x % 2) == 0)
{
printf ("x is %d\n", x);
printf ("%d is even\n", x);
}
else
{
printf ("x is %d\n", x);
printf ("%d is odd\n", x);
}
}
You can declare variables inside a block; such variables are local to that block.
{
int x = 5;
printf ("%d\n", x);
}
printf ("%d\n", x); /* Compilation error! x exists only
in the preceding block. */
The null statement is merely a semicolon alone.
;
A null statement does not do anything. It does not store a value anywhere. It does not cause time to pass during the execution of your program.
Most often, a null statement is used as the body of
a loop statement, or as one or more of the expressions in a for
statement. Here is an example of a for statement that uses the
null statement as the body of the loop (and also calculates the integer
square root of n, just for fun):
for (i = 1; i*i < n; i++)
;
Here is another example that uses the null statement as the body
of a for loop and also produces output:
for (x = 1; x <= 5; printf ("x is now %d\n", x), x++)
;
goto Statement
You can use the goto statement to unconditionally jump to a different
place in the program. Here is the general form of a goto statement:
goto label;
You have to specify a label to jump to; when the goto statement
is executed, program control jumps to that label. See Labels. Here
is an example:
goto end_of_program;
...
end_of_program:
The label can be anywhere in the same function as the goto
statement that jumps to it, but a goto statement cannot jump to a
label in a different function.
You can use goto statements to simulate loop statements,
but we do not recommend it—it makes the program harder to read, and GCC
cannot optimize it as well. You should use for,
while, and do statements instead of goto statements,
when possible.
break Statement
You can use the break statement to terminate a while, do,
for, or switch statement. Here is an example:
int x;
for (x = 1; x <= 10; x++)
{
if (x == 8)
break;
else
printf ("%d ", x);
}
That example prints numbers from 1 to 7. When x is incremented
to 8, x == 8 is true, so the break statement is executed,
terminating the for loop prematurely.
If you put a break statement inside of a loop or switch
statement which itself is inside of a loop or switch statement, the
break only terminates the innermost loop or switch statement.
continue Statement
You can use the continue statement in loops to terminate an
iteration of the loop and begin the next iteration. Here is an
example:
for (x = 0; x < 100; x++)
{
if (x % 2 == 0)
continue;
else
sum_of_odd_numbers + = x;
}
If you put a continue statement inside a loop which itself is
inside a loop, then it affects only the innermost loop.
return Statement
You can use the return statement to end the execution of a function
and return program control to the function that called it. Here is the
general form of the return statement:
return return-value;
return-value is an optional expression to return. If the
function's return type is void, then it is invalid to return
an expression. You can, however, use the return statement
without a return value.
If the function's return type is not the same as the type of return-value, and automatic type conversion cannot be performed, then returning return-value is invalid.
If the function's return type is not void and no return value
is specified, then the return statement is valid unless the
function is called in a context that requires a return value. For
example:
x = cosine (y);
In that case, the function cosine was called in a context that
required a return value, so the value could be assigned to x.
Even in contexts where a return value is not required, it is a bad idea
for a non-void function to omit the return value. With GCC, you
can use the command line option -Wreturn-type to issue a warning
if you omit the return value in such functions.
Here are some examples of using the return statement, in both
a void and non-void function:
void
print_plus_five (int x)
{
printf ("%d ", x + 5);
return;
}
int
square_value (int x)
{
return x * x;
}
typedef Statement
You can use the typedef statement to create new names for data
types. Here is the general form of the typedef statement:
typedef old-type-name new-type-name
old-type-name is the existing name for the type, and may consist
of more than one token (e.g., unsigned long int).
new-type-name is the resulting new name for the type, and must
be a single identifier. Creating this new name for the type does
not cause the old name to cease to exist. Here are some examples:
typedef unsigned char byte_type;
typedef double real_number_type;
In the case of custom data types, you can use typedef to make a
new name for the type while defining the type:
typedef struct fish
{
float weight;
float length;
float probability_of_being_caught;
} fish_type;
To make a type definition of an array, you first provide the type of the element, and then establish the number of elements at the end of the type definition:
typedef char array_of_bytes [5];
array_of_bytes five_bytes = {0, 1, 2, 3, 4};
When selecting names for types, you should avoid ending your type names with
a _t suffix. The compiler will allow you to do this, but the
POSIX standard reserves use of the _t suffix for standard library
type names.
You can write functions to separate parts of your program into distinct subprocedures. To write a function, you must at least create a function definition. It is a good idea also to have an explicit function declaration; you don't have to, but if you leave it out, then the default implicit declaration might not match the function itself, and you will get some compile-time warnings.
Every program requires at least one function, called main.
That is where the program's execution begins.
You write a function declaration to specify the name of a function, a list of parameters, and the function's return type. A function declaration ends with a semicolon. Here is the general form:
return-type function-name (parameter-list);
return-type indicates the data type of the value returned by the function.
You can declare a function that doesn't return anything by using the return
type void.
function-name can be any valid identifier (see Identifiers).
parameter-list consists of zero or more parameters, separated by commas.
A typical parameter consists of a data type and an optional name for the
parameter. You can also declare a function that has a variable number of
parameters (see Variable Length Parameter Lists), or no parameters using
void. Leaving out parameter-list entirely also indicates
no parameters, but it is better to specify it explicitly with void.
Here is an example of a function declaration with two parameters:
int foo (int, double);
If you include a name for a parameter, the name immediately follows the data type, like this:
int foo (int x, double y);
The parameter names can be any identifier (see Identifiers), and if you have more than one parameter, you can't use the same name more than once within a single declaration. The parameter names in the declaration need not match the names in the definition.
You should write the function declaration above the first use of
the function. You can put it in a header file and use the #include
directive to include that function declaration in any source code files that
use the function.
You write a function definition to specify what a function actually does. A function definition consists of information regarding the function's name, return type, and types and names of parameters, along with the body of the function. The function body is a series of statements enclosed in braces. Here is the general form of a function definition:
return-type
function-name (parameter-list)
{
function-body
}
return-type and function-name are the same as what you use in the function declaration (see Function Declarations).
parameter-list is the same as the parameter list used in the function declaration (see Function Declarations), except you must include names for the parameters in a function definition.
Here is an simple example of a function definition—it takes two integers as its parameters and returns the sum of them as its return value:
int
add_values (int x, int y)
{
return x + y;
}
For compatibility with the original design of C, you can also specify the type of the function parameters after the closing parenthesis of the parameter list, like this:
int
add_values (x, y)
int x, int y;
{
return x + y;
}
However, we strongly discourage this style of coding; it can cause subtle problems with type casting, among other problems.
You can call a function by using its name and supplying any needed parameters. Here is the general form of a function call:
function-name (parameters)
A function call can make up an entire statement, or it can be used as a subexpression. Here is an example of a standalone function call:
foo (5);
In that example, the function `foo' is called with the
parameter 5.
Here is an example of a function call used as a subexpression:
a = square (5);
Supposing that the function `square' squares its parameter, the
above example assigns the value 25 to a.
If a parameter takes more than one argument, you separate parameters with commas:
a = quux (5, 10);
Function parameters can be any expression—a literal value, a value stored in variable, an address in memory, or a more complex expression built by combining these.
Within the function body, the parameter is a local copy of the value passed into the function; you cannot change the value passed in by changing the local copy.
int x = 23;
foo (x);
...
/* Definition for function foo. */
int foo (int a)
{
a = 2 * a;
return a;
}
In that example, even though the parameter a is modified in the
function `foo', the variable x that is passed to
the function does not change. If you wish to use the function to change
the original value of x, then you would have to incorporate the
function call into an assignment statement:
x = foo (x);
If the value that you pass to a function is a memory address, then you can access (and change) the data stored at the memory address. This achieves a similar effect as pass-by-reference function parameters in other languages, but is not the same: the memory address is simply a value, just like any other value, and cannot itself be changed. The difference between passing a pointer and passing an integer lies in what you can do using the value within the function.
Here is an example of calling a function with a memory address parameter:
void
foo (int *x)
{
*x = *x + 42;
}
...
int a = 15;
foo (&a);
The formal parameter for the function is of type pointer-to-int, and we
call the function by passing it the address of a variable of type int. By
dereferencing the pointer within the function body, we can both see and change
the value stored in the address. The above changes the value of
a to `57'.
Even if you don't want to change the value stored in the address, passing the address of a variable rather than the variable itself can be useful if the variable type is large and you need to conserve memory space. For example:
struct foo
{
int x;
float y;
double z;
};
void bar (struct foo *a);
In this case, unless you are working on a computer with very large memory addresses, it will take less memory to pass a pointer to the structure than to pass an instance of the structure.
One type of parameter that is always passed as a memory address is any sort of array:
void foo (int a[]);
...
int x[100];
foo (x);
In this example, calling the function foo with the parameter a
does not copy the entire array into a new local parameter within foo;
rather, it passes x as a pointer to the first element in x.
Be careful, though: within the function, you cannot use sizeof to determine
the size of the array x—sizeof instead tells you the size of the
pointer x. Indeed, the above code is equivalent to:
void foo (int *a);
...
int x[100];
foo (x);
You can write a function that takes a variable number of arguments. To do this, the function needs to have at least one parameter of a known data type, but the remaining parameters are optional, and can vary in both quantity and data type.
You list the first parameter as normal, but then as the second parameter, use an ellipsis: `...'. Here is an example function prototype:
int add_multiple_values (int number, ...);
To work with the optional parameters in the function definition, you need
to use macro functions that are defined in the library header file
`stdarg.h', so you must #include that file. Those functions
are described in a separate manual, but here is an example of using them:
int
add_multiple_values (int number, ...)
{
int counter, total = 0;
/* Declare a variable of type `va_list'. */
va_list parameters;
/* Call the `va_start' function. */
va_start (parameters, number);
for (counter = 0; counter < number; counter++)
{
/* Get the values of the optional parameters. */
total += va_arg (parameters, int);
}
/* End use of the `parameters' variable. */
va_end (parameters);
return total;
}
To use optional parameters, you need to have a way to know how many there are. This can vary, so it can't be hard-coded, but if you don't know how many optional parameters you have, then you could have difficulty knowing when to stop using the `va_arg' function. In the above example, the first parameter to the `add_multiple_values' function, `number', is the number of optional parameters actually passed. So, we might call the function like this:
sum = add_multiple_values (3, 12, 34, 190);
The first parameter indicates how many optional parameters follow it.
Also, note that you don't actually need to use `va_end' function. In fact, with GCC it doesn't do anything at all. However, you might want to include it to maximize compatibility with other compilers.
See Variadic Functions.
main Function
Every program requires at least one function, called `main'.
This is where the program begins executing. You do not need to write a
declaration or prototype for main, but you do need to define it.
The return type for main is always int. You do not have
to specify the return type for main, but you can. However, you
cannot specify that it has a return type other than int.
In general, the return value from main indicates the program's
exit status. Usually a value of zero indicates success and any
other value indicates an error, but this is not guaranteed. You can
#include <stdlib.h> and use the symbolic names
EXIT_SUCCESS and EXIT_FAILURE if portability is a concern.
You can write your main function to have no parameters, or to
accept parameters from the command line. To have no parameters, it is best
to specify void as the parameter list.
Here is a very simple main function with no parameters:
int
main (void)
{
puts ("Hi there!");
return 0;
}
To accept command line parameters, you need to have two parameters in the
main function, int argc followed by char *argv[]. You
can change the names of those parameters, but they must have those data
types—int and array of pointers to char. argc is the
number of command line parameters, including the name of the program itself.
argv is an array of the parameters, as character strings.
argv[0], the first element in the array, is the name of the program
as typed at the command line; any following array elements are the
parameters that followed the name of the program.
Here is an example main function that accepts command line
parameters, and prints out what those parameters are:
int
main (int argc, char *argv[])
{
int counter;
for (counter = 0; counter < argc; counter++)
printf ("%s\n", argv[counter]);
return 0;
}
You can write a function that is recursive—a function that calls itself. Here is an example that computes the factorial of an integer:
int
factorial (int x)
{
if (x < 1)
return x;
else
return (x * factorial (x - 1));
}
Be careful that you do not write a function that is infinitely recursive. In
the above example, once x is 1, the recursion stops. However, in the
following example, the recursion does not stop until the program is interrupted
or runs out of memory:
int
watermelon (int x)
{
return (watermelon (x));
}
You can define a function to be static if you want it to be callable only within the source file where it is defined:
static int
foo (int x)
{
return x + 42;
}
This is useful if you are building a reusable library of functions and need to include some subroutines that should not be callable by the end user.
Now that we have seen all of the fundamental elements of C programs, it's time to look at the big picture.
A C program may exist entirely within a single source file, but more commonly, any non-trivial program will consist of several custom header files and source files, and will also include and link with files from existing libraries.
By convention, header files (with a “.h” extension) contain variable and function declarations, and source files (with a “.c” extension) contain the corresponding definitions. Source files may also store declarations, if these declarations are not for objects which need to be seen by other files. However, header files almost certainly should not contain any definitions.
For example, if you write a function that computes square roots, and you wanted this function to be accessible to files other than where you define the function, then you would put the function declaration into a header file (with a “.h” file extension):
/* sqrt.h */
double
computeSqrt (double x);
This header file could be included by other source files which need to use your function, but do not need to know how it was implemented.
The implementation of the function would then go into a corresponding source file (with a “.c” file extension):
/* sqrt.c */
#include "sqrt.h"
double
computeSqrt (double x)
{
double result;
...
return result;
}
Scope refers to what parts of the program can “see” a declared object. A declared
object can be visible only within a particular function, or within a particular file,
or may be visible to an entire set of files by way of including header files and
using extern declarations.
Unless explicitly stated otherwise, declarations made at the top-level of a file (i.e., not within a function) are visible to the entire file, including from within functions, but are not visible outside of the file.
Declarations made within functions are visible only within those functions.
A declaration is not visible to declarations that came before it; for example:
int x = 5;
int y = x + 10;
will work, but:
int x = y + 10;
int y = 5;
will not.
See Storage Class Specifiers, for more information on changing the scope of declared objects.
To conclude our description of C, here is a complete program written in C,
consisting of both a C source file and a header file. This
program is an expanded version of the quintessential “hello world” program, and
serves as an example of how to format and structure C code for use in programs
for FSF Project GNU. (You can always download the most recent version of this
program, including sample makefiles and other examples of how to produce GNU
software, from http://www.gnu.org/software/hello.)
This program uses features of the preprocessor; for a description of preprocessor macros, see The C Preprocessor, available as part of the GCC documentation.
hello.c /* hello.c -- print a greeting message and exit.
Copyright (C) 1992, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002,
2005, 2006, 2007 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
#include <config.h>
#include "system.h"
/* String containing name the program is called with. */
const char *program_name;
static const struct option longopts[] =
{
{ "greeting", required_argument, NULL, 'g' },
{ "help", no_argument, NULL, 'h' },
{ "next-generation", no_argument, NULL, 'n' },
{ "traditional", no_argument, NULL, 't' },
{ "version", no_argument, NULL, 'v' },
{ NULL, 0, NULL, 0 }
};
static void print_help (void);
static void print_version (void);
int
main (int argc, char *argv[])
{
int optc;
int t = 0, n = 0, lose = 0;
const char *greeting = NULL;
program_name = argv[0];
/* Set locale via LC_ALL. */
setlocale (LC_ALL, "");
#if ENABLE_NLS
/* Set the text message domain. */
bindtextdomain (PACKAGE, LOCALEDIR);
textdomain (PACKAGE);
#endif
/* Even exiting has subtleties. The /dev/full device on GNU/Linux
can be used for testing whether writes are checked properly. For
instance, hello >/dev/full should exit unsuccessfully. On exit,
if any writes failed, change the exit status. This is
implemented in the Gnulib module "closeout". */
atexit (close_stdout);
while ((optc = getopt_long (argc, argv, "g:hntv", longopts, NULL)) != -1)
switch (optc)
{
/* One goal here is having --help and --version exit immediately,
per GNU coding standards. */
case 'v':
print_version ();
exit (EXIT_SUCCESS);
break;
case 'g':
greeting = optarg;
break;
case 'h':
print_help ();
exit (EXIT_SUCCESS);
break;
case 'n':
n = 1;
break;
case 't':
t = 1;
break;
default:
lose = 1;
break;
}
if (lose || optind < argc)
{
/* Print error message and exit. */
if (optind < argc)
fprintf (stderr, _("%s: extra operand: %s\n"),
program_name, argv[optind]);
fprintf (stderr, _("Try `%s --help' for more information.\n"),
program_name);
exit (EXIT_FAILURE);
}
/* Print greeting message and exit. */
if (t)
printf (_("hello, world\n"));
else if (n)
/* TRANSLATORS: Use box drawing characters or other fancy stuff
if your encoding (e.g., UTF-8) allows it. If done so add the
following note, please:
[Note: For best viewing results use a UTF-8 locale, please.]
*/
printf (_("\
+---------------+\n\
| Hello, world! |\n\
+---------------+\n\
"));
else
{
if (!greeting)
greeting = _("Hello, world!");
puts (greeting);
}
exit (EXIT_SUCCESS);
}
/* Print help info. This long message is split into
several pieces to help translators be able to align different
blocks and identify the various pieces. */
static void
print_help (void)
{
/* TRANSLATORS: --help output 1 (synopsis)
no-wrap */
printf (_("\
Usage: %s [OPTION]...\n"), program_name);
/* TRANSLATORS: --help output 2 (brief description)
no-wrap */
fputs (_("\
Print a friendly, customizable greeting.\n"), stdout);
puts ("");
/* TRANSLATORS: --help output 3: options 1/2
no-wrap */
fputs (_("\
-h, --help display this help and exit\n\
-v, --version display version information and exit\n"), stdout);
puts ("");
/* TRANSLATORS: --help output 4: options 2/2
no-wrap */
fputs (_("\
-t, --traditional use traditional greeting format\n\
-n, --next-generation use next-generation greeting format\n\
-g, --greeting=TEXT use TEXT as the greeting message\n"), stdout);
printf ("\n");
/* TRANSLATORS: --help output 5 (end)
TRANSLATORS: the placeholder indicates the bug-reporting address
for this application. Please add _another line_ with the
address for translation bugs.
no-wrap */
printf (_("\
Report bugs to <%s>.\n"), PACKAGE_BUGREPORT);
}
/* Print version and copyright information. */
static void
print_version (void)
{
printf ("hello (GNU %s) %s\n", PACKAGE, VERSION);
/* xgettext: no-wrap */
puts ("");
/* It is important to separate the year from the rest of the message,
as done here, to avoid having to retranslate the message when a new
year comes around. */
printf (_("\
Copyright (C) %s Free Software Foundation, Inc.\n\
License GPLv3+: GNU GPL version 3 or later\
<http://gnu.org/licenses/gpl.html>\n\
This is free software: you are free to change and redistribute it.\n\
There is NO WARRANTY, to the extent permitted by law.\n"),
"2007");
}
system.h /* system.h: system-dependent declarations; include this first.
Copyright (C) 1996, 2005, 2006, 2007 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
#ifndef HELLO_SYSTEM_H
#define HELLO_SYSTEM_H
/* Assume ANSI C89 headers are available. */
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Use POSIX headers. If they are not available, we use the substitute
provided by gnulib. */
#include <getopt.h>
#include <unistd.h>
/* Internationalization. */
#include "gettext.h"
#define _(str) gettext (str)
#define N_(str) gettext_noop (str)
/* Check for errors on write. */
#include "closeout.h"
#endif /* HELLO_SYSTEM_H */
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auto storage class specifier: Storage Class Specifiersbreak statement: The break Statementchar data type: Integer Typesconst type specifier: Type Specifierscontinue statement: The continue Statementdo statement: The do Statementdouble data type: Real Number Typeselse statements: The if StatementEXIT_FAILURE: The main FunctionEXIT_SUCCESS: The main Functionextern storage class specifier: Storage Class Specifiersfloat data type: Real Number Typesfor statement: The for Statementgoto statement: The goto Statementif statements: The if Statementint data type: Integer Typeslong double data type: Real Number Typeslong int data type: Integer Typeslong long int data type: Integer Typesregister storage class specifier: Storage Class Specifiersreturn statement: The return Statementshort int data type: Integer Typessigned char data type: Integer Typesstatic storage class specifier: Storage Class Specifiersswitch statement: The switch Statementtypedef statement: The typedef Statementunsigned char data type: Integer Typesunsigned int data type: Integer Typesunsigned long int data type: Integer Typesunsigned long long int data type: Integer Typesunsigned short int data type: Integer Typesvolatile type specifier: Type Specifierswhile statement: The while Statement