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 creates a new process that does not have
a new virtual address space, but rather shares address space with the parent,
thus avoiding potentially expensive copy-on-write operations normally
associated with creating a new process. 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 existing 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 standard I/O data structures in
the parent process. (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).
ERRORS
Same as for
fork(2).
SEE ALSO
execve(2),
fork(2),
sigaction(2),
wait(2)
HISTORY
The
vfork() function call appeared in
3.0BSD. In
4.4BSD, the
semantics were changed to only suspend the parent and not share the address
space. The original semantics were reintroduced in
NetBSD
1.4.
BUGS
Portable applications should not depend on the memory sharing semantics of
vfork() as implementations exist that implement
vfork() as plain
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.