VFORK(2) BSD System Calls Manual VFORK(2)NAMEvfork — spawn new process in a virtual memory efficient way
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
vfork(void);
DESCRIPTIONVfork() 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). Vfork() 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.
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(2). It does not work, how‐
ever, 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.)
RETURN VALUES
Same as for fork(2).
SEE ALSOexecve(2), fork(2), rfork(2), sigvec(2), wait(2), _exit(2), exit(3)HISTORY
The vfork() function call appeared in 2.9BSD.
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(2) 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.
BSD June 4, 1993 BSD
