freebsd man page for vfork

VFORK(2)						      BSD System Calls Manual							  VFORK(2)

NAME
     vfork -- create a new process without copying the address space

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     pid_t
     vfork(void);

DESCRIPTION
     The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously
     inefficient in a paged environment.  It is useful when the purpose of fork(2) would have been to create a new system context for an
     execve(2).  The vfork() system call differs from fork(2) in that the child borrows the parent's memory and thread of control until a call to
     execve(2) or an exit (either by a call to _exit(2) or abnormally).  The parent process is suspended while the child is using its resources.

     The vfork() system call returns 0 in the child's context and (later) the pid of the child in the parent's context.

     The vfork() system call can normally be used just like fork(2).  It does not work, however, to return while running in the child's context
     from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame.  Be
     careful, also, to call _exit(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and
     thereby mess up the parent processes standard I/O data structures.  (Even with fork(2) it is wrong to call exit(3) since buffered data would
     then be flushed twice.)

RETURN VALUES
     Same as for fork(2).

SEE ALSO
     _exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3)

HISTORY
     The vfork() system call appeared in 2.9BSD.

BUGS
     To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals;
     rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication.

BSD
								 November 13, 2009							       BSD