Standard C Library - 思维火花 - 博客频道 - CSDN.NET

Standard C Library - 思维火花 - 博客频道 - CSDN.NET

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

error.
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(), 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(),
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 describes 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, ... );
    int main( void ) {
   
      int answer = sum( 4, 4, 3, 2, 1 );
      printf( "The answer is %d/n", answer );
   
      return( 0 );
    }
   
    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 );
    }

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