NAME
vfork —
spawn new process in a virtual memory
efficient way
LIBRARY
library “libc”
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(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, 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.)
RETURN VALUES
Same as for fork(2).
SEE ALSO
execve(2), fork(2), rfork(2), sigaction(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() 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.