Kernel.fork  [{ block }]   → fixnum or nil
Process.fork [{ block }]   → fixnum or nil

Creates a subprocess. If a block is specified, that block is run in the subprocess, and the subprocess terminates with a status of zero. Otherwise, the fork call returns twice, once in the parent, returning the process ID of the child, and once in the child, returning nil. The child process can exit using Kernel.exit! to avoid running any at_exit functions. The parent process should use Process.wait to collect the termination statuses of its children or use Process.detach to register disinterest in their status; otherwise, the operating system may accumulate zombie processes.

The thread calling fork is the only thread in the created child process. fork doesn’t copy other threads.

Source Code

/*
*  call-seq:
*     Kernel.fork  [{ block }]   => fixnum or nil
*     Process.fork [{ block }]   => fixnum or nil
*
*  Creates a subprocess. If a block is specified, that block is run
*  in the subprocess, and the subprocess terminates with a status of
*  zero. Otherwise, the +fork+ call returns twice, once in
*  the parent, returning the process ID of the child, and once in
*  the child, returning _nil_. The child process can exit using
*  <code>Kernel.exit!</code> to avoid running any
*  <code>at_exit</code> functions. The parent process should
*  use <code>Process.wait</code> to collect the termination statuses
*  of its children or use <code>Process.detach</code> to register
*  disinterest in their status; otherwise, the operating system
*  may accumulate zombie processes.
*
*  The thread calling fork is the only thread in the created child process.
*  fork doesn't copy other threads.
*/

static VALUE
rb_f_fork(obj)
   VALUE obj;
{
#if !defined(__human68k__) && !defined(_WIN32) && !defined(__MACOS__) && !defined(__EMX__) && !defined(__VMS)
   int pid;

   rb_secure(2);

#ifndef __VMS
   fflush(stdout);
   fflush(stderr);
#endif

   switch (pid = fork()) {
     case 0:
#ifdef linux
       after_exec();
#endif
       rb_thread_atfork();
       if (rb_block_given_p()) {
           int status;

           rb_protect(rb_yield, Qundef, &status);
           ruby_stop(status);
       }
       return Qnil;

     case -1:
       rb_sys_fail("fork(2)");
       return Qnil;

     default:
       return INT2FIX(pid);
   }
#else
   rb_notimplement();
#endif
}