cppreference.com > Other Standard C Functions abort Syntax:
      #include <stdlib.h>
      void abort( void );
The function abort() terminates the current program. Depending on the implementation, the return value can indicate failure. Related topics: assert atexit exit

assert
Syntax:

      #include <assert.h>
      void assert( int exp );

The assert() macro is used to test for errors. If exp evaluates to zero, assert() writes information to STDERR and exits the program. If the macro NDEBUG is defined, the assert() macros will be ignored.

Related topics:
abort

atexit
Syntax:

      #include <stdlib.h>
      int atexit( void (*func)(void) );

The function atexit() causes the function pointed to by func to be called when the program terminates. You can make multiple calls to atexit() (at least 32, depending on your compiler) and those functions will be called in reverse order of their establishment. The return value of atexit() is zero upon success, and non-zero on failure.

Related topics:
abort
exit

bsearch
Syntax:

      #include <stdlib.h>
      void *bsearch( const void *key, const void *buf, size_t num, size_t size, int (*compare)(const void *, const void *) );

The bsearch() function searches buf[0] to buf[num-1] for an item that matches key, using a binary search. The function compare should return negative if its first argument is less than its second, zero if equal, and positive if greater. The items in the array buf should be in ascending order. The return value of bsearch() is a pointer to the matching item, or NULL if none is found.

Related topics:
qsort

exit
Syntax:

      #include <stdlib.h>
      void exit( int exit_code );

The exit() function stops the program. exit_code is passed on to be the return value of the program, where usually zero indicates success and non-zero indicates an error.

Related topics:
abort
atexit
system

getenv
Syntax:

      #include <stdlib.h>
      char *getenv( const char *name );

The function getenv() returns environmental information associated with name, and is very implementation dependent. NULL is returned if no information about name is available.

Related topics:
system

longjmp
Syntax:

      #include <setjmp.h>
      void longjmp( jmp_buf envbuf, int status );

The function longjmp() causes the program to start executing code at the point of the last call to setjmp(). envbuf is usually set through a call to setjmp(). status becomes the return value of setjmp() and can be used to figure out where longjmp() came from. status should not be set to zero.

Related topics:
setjmp

qsort
Syntax:

      #include <stdlib.h>
      void qsort( void *buf, size_t num, size_t size, int (*compare)(const void *, const void *) );

The qsort() function sorts buf (which contains num items, each of size size) using Quicksort. The compare function is used to compare the items in buf. compare should return negative if the first argument is less than the second, zero if they are equal, and positive if the first argument is greater than the second. qsort() sorts buf in ascending order.

Example code:

For example, the following bit of code uses qsort() to sort an array of integers:

     int compare_ints( const void* a, const void* b ) {
       int* arg1 = (int*) a;
       int* arg2 = (int*) b;
       if( *arg1 < *arg2 ) return -1;
       else if( *arg1 == *arg2 ) return 0;
       else return 1;
     }

     int array[] = { -2, 99, 0, -743, 2, 3, 4 };
     int array_size = 7;

     ...

     printf( "Before sorting: " );
     for( int i = 0; i < array_size; i++ ) {
       printf( "%d ", array[i] );
     }
     printf( "\n" );

     qsort( array, array_size, sizeof(int), compare_ints );

     printf( "After sorting: " );
     for( int i = 0; i < array_size; i++ ) {
       printf( "%d ", array[i] );
     }
     printf( "\n" );

When run, this code displays the following output:

     Before sorting: -2 99 0 -743 2 3 4
     After sorting: -743 -2 0 2 3 4 99

Related topics:
bsearch
(C++ Algorithms) sort

raise
Syntax:

      #include <signal.h>
      int raise( int signal );

The raise() function sends the specified signal to the program. Some signals:

Signal | Meaning
--- | ---
SIGABRT | Termination error
SIGFPE | Floating pointer error
SIGILL | Bad instruction
SIGINT | User presed CTRL-C
SIGSEGV | Illegal memory access
SIGTERM | Terminate program

The return value is zero upon success, nonzero on failure.

Related topics:
signal

rand
Syntax:

      #include <stdlib.h>
      int rand( void );

The function rand() returns a pseudorandom integer between zero and RAND_MAX. An example:

       srand( time(NULL) );
       for( i = 0; i < 10; i++ )
         printf( "Random number #%d: %d\n", i, rand() );

Related topics:
srand

setjmp
Syntax:

      #include <setjmp.h>
      int setjmp( jmp_buf envbuf );

The setjmp() function saves the system stack in envbuf for use by a later call to longjmp(). When you first call setjmp(), its return value is zero. Later, when you call longjmp(), the second argument of longjmp() is what the return value of setjmp() will be. Confused? Read about longjmp().

Related topics:
longjmp

signal
Syntax:

      #include <signal.h>
      void ( *signal( int signal, void (* func) (int)) ) (int);

The signal() function sets func to be called when signal is recieved by your program. func can be a custom signal handler, or one of these macros (defined in signal.h):

Macro | Explanation
--- | ---
SIG_DFL | default signal handling
SIG_IGN | ignore the signal

Some basic signals that you can attach a signal handler to are:

Signal | Description
--- | ---
SIGTERM | Generic stop signal that can be caught.
SIGINT | Interrupt program, normally ctrl-c.
SIGQUIT | Interrupt program, similar to SIGINT.
SIGKILL | Stops the program. Cannot be caught.
SIGHUP | Reports a disconnected terminal.

The return value of signal() is the address of the previously defined function for this signal, or SIG_ERR is there is an error.

Example code:

The following example uses the signal() function to call an arbitrary number of functions when the user aborts the program. The functions are stored in a vector, and a single "clean-up" function calls each function in that vector of functions when the program is aborted:

    void f1() {
      cout << "calling f1()..." << endl;
    }

    void f2() {
      cout << "calling f2()..." << endl;
    }

    typedef void(*endFunc)(void);
    vector<endFunc> endFuncs;

    void cleanUp( int dummy ) {
      for( unsigned int i = 0; i < endFuncs.size(); i++ ) {
        endFunc f = endFuncs.at(i);
        (*f)();
      }
      exit(-1);
    }

    int main() {

      // connect various signals to our clean-up function
      signal( SIGTERM, cleanUp );
      signal( SIGINT, cleanUp );
      signal( SIGQUIT, cleanUp );
      signal( SIGHUP, cleanUp );

      // add two specific clean-up functions to a list of functions
      endFuncs.push_back( f1 );
      endFuncs.push_back( f2 );

      // loop until the user breaks
      while( 1 );

      return 0;
    }

Related topics:
raise

srand
Syntax:

      #include <stdlib.h>
      void srand( unsigned seed );

The function srand() is used to seed the random sequence generated by rand(). For any given seed, rand() will generate a specific "random" sequence over and over again.

       srand( time(NULL) );
       for( i = 0; i < 10; i++ )
         printf( "Random number #%d: %d\n", i, rand() );

Related topics:
rand
(Standard C Date & Time) time

system
Syntax:

      #include <stdlib.h>
      int system( const char *command );

The system() function runs the given command as a system call. The return value is usually zero if the command executed without errors. If command is NULL, system() will test to see if there is a command interpreter available. Non-zero will be returned if there is a command interpreter available, zero if not.

Related topics:
exit
getenv

va_arg
Syntax:

      #include <stdarg.h>
      type va_arg( va_list argptr, type );
      void va_end( va_list argptr );
      void va_start( va_list argptr, last_parm );

The va_arg() macros are used to pass a variable number of arguments to a function.

1. First, you must have a call to va_start() passing a valid va_list and the mandatory first argument of the function. This first argument can be anything; one way to use it is to have it be an integer describing the number of parameters being passed.
2. Next, you call va_arg() passing the va_list and the type of the argument to be returned. The return value of va_arg() is the current parameter.
3. Repeat calls to va_arg() for however many arguments you have.
4. Finally, a call to va_end() passing the va_list is necessary for proper cleanup.

For example:

      int sum( int num, ... ) {
        int answer = 0;
        va_list argptr;

        va_start( argptr, num );

        for( ; num > 0; num-- ) {
          answer += va_arg( argptr, int );
        }

        va_end( argptr );

        return( answer );
      }

      int main( void ) {

        int answer = sum( 4, 4, 3, 2, 1 );
        printf( "The answer is %d\n", answer );

        return( 0 );
      }

This code displays 10, which is 4+3+2+1.

Here is another example of variable argument function, which is a simple printing function:

     void my_printf( char *format, ... ) {
       va_list argptr;

       va_start( argptr, format );

       while( *format != '\0' ) {
         // string
         if( *format == 's' ) {
           char* s = va_arg( argptr, char * );
           printf( "Printing a string: %s\n", s );
         }
         // character
         else if( *format == 'c' ) {
           char c = (char) va_arg( argptr, int );
           printf( "Printing a character: %c\n", c );
           break;
         }
         // integer
         else if( *format == 'd' ) {
           int d = va_arg( argptr, int );
           printf( "Printing an integer: %d\n", d );
         }

         *format++;
       }

       va_end( argptr );
     }

     int main( void ) {

       my_printf( "sdc", "This is a string", 29, 'X' );

       return( 0 );
     }

This code displays the following output when run:

     Printing a string: This is a string
     Printing an integer: 29
     Printing a character: X