The main() function will usually call other functions to help perform its job. Functions may be written by the programmer, or provided by existing libraries; the latter are accessed by including "standard headers" via the #include preprocessing directive. Certain library functions, such as printf() in the above example, are defined by the C standards; these are referred to as the standard library. (An implementation of C providing all of the standard library functions is called a "hosted implementation"; some implementations are not hosted, usually because they are not intended to be used with an operating system.) Other libraries can provide extra functionality, such as a graphical interface, advanced mathematical operations, or access to platform-specific features.

A function may return a value to the environment which called it. This is usually another C function. The main() function's calling environment is the operating system. Hence, in the "Hello, world!" example above, the operating system receives a value of 0 when the program terminates. (The printf function above returns how many characters were printed -- in the case above, 14 -- but its value is effectively ignored.)

A C function consists of a return type (void if no value is returned), a unique name, a list of parameters in parentheses (void if there are none) and a function body delimited by braces. The syntax of the function body is equivalent to that of a compound statement.

Table of contents

1 Control structures 1.1 Compound statements 1.2 Expression statements 1.3 Selection statements 1.4 Iteration statements 1.5 Jump statements 2 Operator precedence in C89 3 Data declaration 3.6 Elementary data types 3.7 Arrays 3.8 Pointers 4 Strings 5 File Input / Output 5.9 Standard I/O 6 Passing command line arguments 7 Undefined behaviors

Control structures

free-form

Note: bracing style varies from programmer to programmer and can be the subject of great debate ("religious wars"). See Indent style for more details.

Compound statements

Compound statements in C have the form

  {   }

Expression statements

A statement of the form

   ;

Selection statements

C has three types of selection statements: two kinds of if and the switch statement.

The two kinds of if statement are

  if () 
     

  if () 
     
  else 
     

The switch statement causes control to be transferred to one of several statements depending on the value of an expression, which must have integral type. The substatement controlled by a switch is typically compound. Any statement within the substatement may be labeled with one or more case labels, which consist of the keyword case followed by a constant expression and then a colon (:). No two of the case constants associated with the same switch may have the same value. There may be at most one default label associated with a switch; control passes to the default label if none of the case labels are equal to the expression in the parentheses following switch. Switches may be nested; a case or default label is associated with the smallest switch that contains it. Switch statements can "fall-through", that is, when one case section has completed its execution, statements will continue to be executed downward until a break statement is encountered. This may prove useful in certain circumstances, newer programming languages forbid case statements to "fall-through". In the below example, if is reached, the statements are executed and nothing more inside the braces. However if is reached, both and are executed since there is no break to separate the two case statements.

  switch () {
     case  :
        
     case  :
        
        break;
     default :
        
  }

  do 
     
  while ();

  while ()
     

  for ( ;  ; )
     

If all three expressions are present in a for, the statement

  for (e1; e2; e3)
     s;

  e1;
  while (e2) {
     s;
     e3;
  }

Jump statements

Jump statements transfer control unconditionally. There are four types of jump statements in C: goto, continue, break, and return.

The goto statement looks like this:

  goto <identifier>;

A continue statement may appear only within an iteration statement and causes control to pass to the loop-continuation portion of the smallest enclosing such statement. That is, within each of the statements

  while (expression) {
     /* ... */
     cont: ;
  }

  do {
     /* ... */
     cont: ;
  } while (expression);

  for (optional-expr; optexp2; optexp3) {
     /* ... */
     cont: ;
  }   

The break statement is used to get out of a for loop, while loop, do loop, or switch statement. Control passes to the statement following the terminated statement.

A function returns to its caller by the return statement. When return is followed by an expression, the value is returned to the caller of the function. Flowing off the end of the function is equivalent to a return with no expression. In either case, the returned value is undefined.

Operator precedence in C89

     () [] -> . ++ -- (cast)     postfix operators
     ++ -- * & ~ ! + - sizeof    unary operators
     * / %                       multiplicative operators
     + -                         additive operators
     << >>                       shift operators
     <  <=  >  >=                relational operators
     == !=                       equality operators
     &                           bitwise and
     ^                           bitwise exclusive or
     |                           bitwise inclusive or
     &&                          logical and
     ||                          logical or
     ?:                          conditional operator
     = += -= *= /= %= <<= >>=
         &= |= ^=                assignment operators 
     ,                           comma operator

Elementary data types

The values in the <limits.h> and <float.h> headers determine the ranges of the fundamental data types. The ranges of the float, double, and long double types are typically those mentioned in the IEEE 754 Standard.

name	minimum range
char	-127..127 or 0..255
unsigned char	0..255
signed char	-127..127
int	-32767..32767
short int	-32767..32767
long int	-2147483647..2147483647
float	1e-37..1e+37 (positive range)
double	1e-37..1e+37 (positive range)
long double	1e-37..1e+37 (positive range)

Arrays

If a declaration is suffixed by a number in square brackets ([]), the declaration is said to be an array declaration. Strings are just character arrays. They are terminated by a character zero (represented in C by '\\0', the null character). Array bounds are not checked, and if a memory location beyond the array is written to, it may result in a segmentation fault.

Examples:

   int myvector [100];
   char mystring [80]; 
   float mymatrix [3] [2] = {2.0 , 10.0, 20.0, 123.0, 1.0, 1.0}
   char lexicon  [10000] [300] ;  /* 10000 entries with max 300 chars each. */
   int a[3][4];

a[0][0]	a[0][1]	a[0][2]	a[0][3]
a[1][0]	a[1][1]	a[1][2]	a[1][3]
a[2][0]	a[2][1]	a[2][2]	a[2][3]

Pointers

If a variable has an asterisk (*) in its declaration it is said to be a pointer.

Examples:

   int *pi; /* pointer to int */
   int *api[3]; /* array of 3 pointers to int */
   char **argv; /* pointer to pointer to char */

Another operator, the & (ampersand), called the address-of operator, returns the address of variable, array, or function. Thus, given the following

  int i, *pi; /* int and pointer to int */
  pi = &i;

Strings

Strings may be manipulated without using the standard library. However, the library contains many useful functions for working with both zero-terminated strings and unterminated arrays of char.

The most commonly used string functions are:

• strcat(dest, source) - appends the string source to the end of string dest
• strchr(s, c) - finds the first instance of character c in string s and returns a pointer to it or a null pointer if c is not found
• strcmp(a, b) - compares strings a and b (lexical ordering); returns negative if a is less than b, 0 if equal, positive if greater.
• strcpy(dest, source) - copies the string source to the string dest
• strlen(st) - return the length of string st
• strncat(dest, source, n) - appends a maximum of n characters from the string source to the end of string dest; characters after the null terminator are not copied.
• strncmp(a, b, n) - compares a maximum of n characters from strings a and b (lexical ordering); returns negative if a is less than b, 0 if equal, positive if greater.
• strncpy(dest, source, n) - copies a maximum of n characters from the string source to the string dest
• strrchr(s, c) - finds the last instance of character c in string s and returns a pointer to it or a null pointer if c is not found

• strcoll(s1, s2) - compare two strings according to a locale-specific collating sequence
• strcspn(s1, s2) - returns the index of the first character in s1 that matches any character in s2
• strerror(err) - returns a string with an error message corresponding to the code in err
• strpbrk(s1, s2) - returns a pointer to the first character in s1 that matches any character in s2 or a null pointer if not found
• strspn(s1, s2) - returns the index of the first character in s1 that matches no character in s2
• strstr(st, subst) - returns a pointer to the first occurrence of the string subst in st or a null pointer if no such substring exists.
• strtok(s1, s2) - returns a pointer to a token within s1 delimited by the characters in s2.
• strxfrm(s1, s2, n) - transforms s2 into s1 using locale-specific rules

• fopen
• fclose

• stdin standard input
• stdout standard output
• stderr standard error

The following example demonstrates how a filter program is typically structured:

\r\n#include \r\n\r\nint main()\r\n{\r\n   int c;\r\n\r\n   while (( c = getchar()) != EOF ) {\r\n         /* do various things \r\n            to the characters */\r\n\r\n          if (anErrorOccurs) {\r\n              fputs("an error eee occurred\\n", stderr);\r\n              break;\r\n          }\r\n\r\n         /* ... */\r\n         putchar(c);\r\n         /* ... */\r\n\r\n    }\r\n    return 0;\r\n}\r\n

Passing command line arguments

The parameters given on a command line are passed to a C program with two predefined variables - the count of the command line arguments in argc and the individual arguments as character arrays in the pointer array argv. So the command

 myFilt p1 p2 p3 

(Note: there is no guarantee that the individual strings are contiguous.)

The individual values of the parameters may be accessed with argv[1], argv[2], and argv[3].

Undefined behaviors

An interesting (though certainly not unique) aspect of the C standards is that the behavior of certain code is said to be "undefined". In practice, this means that the program produced from this code can do anything, from (accidentally) working as intended to crashing every time it is run

For example, the following code produces undefined behavior, because the variable b is operated on more than once in the expression a = b + b++;:

\r\n#include \r\n\r\nint main (void)\r\n{\r\n  int a, b = 1;\r\n  a = b + b++;\r\n  printf ("%d\\n", a);\r\n  return 0;\r\n}\r\n