VFORK(2)

NAME

     vfork -- spawn new process in a virtual memory efficient way

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 inef-
     ficient 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 can't 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.)

SEE ALSO

     execve(2), _exit(2), fork(2), rfork(2), sigvec(2), wait(2), exit(3)

RETURN VALUES

     Same as for fork(2).

BUGS

     This system call will be eliminated when proper system sharing mechanisms
     are implemented.  Users should not depend on the memory sharing semantics
     of vfork() as it will, in that case, be made synonymous to fork(2).

     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.

HISTORY

     The vfork() system call appeared in 2.9BSD.