#include <stdlib.h>
  void abort( void );

  #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.

  #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.

  #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.

  #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.

  #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.

  #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.

  #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.

 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" );              

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

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

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

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

  #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() );          

  #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().

  #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):

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

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

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;
}               

  #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() );          

  #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.

  #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