NAME
vfork —
create a new process without copying
the address space
LIBRARY
library “libc”
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 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). The vfork() system call differs
from fork(2) in that the child borrows the parent process's address space
and the calling thread's stack until a call to
execve(2) or an exit (either by a call to
_exit(2) or abnormally). The calling thread is suspended while the
child is using its resources. Other threads continue to run.
The
vfork()
system call returns 0 in the child's context and (later) the pid of the
child in the parent's context.
Many problems can occur when replacing
fork(2) with
vfork().
For example, it does not work 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. Also, changing process state which is
partially implemented in user space such as signal handlers with
libthr(3) will corrupt the parent's state.
Be careful, also, to call _exit(2) rather than exit(3) if you cannot 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
_exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3), posix_spawn(3)
HISTORY
The vfork() system call appeared in
3BSD.
BUGS
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.