For a namespace clean function, implement _fork, otherwise implement fork. The implementation of this functionality will be tightly bound to any operating infrastructure for handling multiple processes.

A minimal implementation, such as that for bare metal coding, only offers a single user thread of control. It is thus impossible to start a new process, so this function always fails.

#include <errno.h>

#undef errno
extern int  errno;

int
_fork ()
{
  errno = EAGAIN;
  return -1;                    /* Always fails */

}       /* _fork () */
	  

The choice of errno is again somewhat arbitrary. However no value for "no processes available" is provided, and EAGAIN is the closest in meaning to this.