NAME
vfork —
spawn new process in a virtual memory
efficient way
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
pid_t
vfork(void);
DESCRIPTION
Thevfork 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
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
SIGTSTP, SIGTTOU or
SIGTTIN signals; rather, output or
ioctl(2) calls are allowed and input attempts result in an
end-of-file indication.