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

NAME

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

SYNOPSIS

     #include <unistd.h>

     pid_t
     vfork(void);

DESCRIPTION

     vfork() 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.  vfork() differs
     from fork 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.

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

     vfork() can normally be used just like fork.  It does not work, however, to return while running in the childs 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 rather than exit if you can't execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes
     standard I/O data structures.  (Even with fork it is wrong to call exit since buffered data would then be flushed twice.)

SEE ALSO

     execve(2), fork(2), sigaction(2), wait(2)

ERRORS

     The vfork() system call will fail for any of the reasons described in the fork man page.  In addition, it will fail if:

     [EINVAL]		A system call other than _exit() or execve() (or libc functions that make no system calls other than those) is called fol-
			lowing calling a vfork() call.

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.

     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() function call appeared in 3.0BSD.

4th Berkeley Distribution					   June 4, 1993 					 4th Berkeley Distribution