Man

Command Section
VFORK(2)                  FreeBSD 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
     Since this function is hard to use correctly from application software,
     it is recommended to use posix_spawn(3) or fork(2) instead.

     The vfork() system call can be used to create new processes without fully
     copying the address space of the old process, which is 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 process's
     address space and the calling thread's stack until a call to execve(2) or
     an exit (either by a call to _exit(2) or abnormally).  The calling thread
     is suspended while the child is using its resources.  Other threads
     continue to run.

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

     Many problems can occur when replacing fork(2) with vfork().  For
     example, it does not work 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.  Also,
     changing process state which is partially implemented in user space such
     as signal handlers with libthr(3) will corrupt the parent's state.

     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),
     posix_spawn(3)

HISTORY
     The vfork() system call appeared in 3BSD.

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.

FreeBSD 11.1-RELEASE-p4          May 22, 2016          FreeBSD 11.1-RELEASE-p4
Command Section