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.