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VFORK(2) NetBSD System Calls Manual VFORK(2)
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vfork -- spawn new process in a virtual memory efficient way
Standard C Library (libc, -lc)
The vfork system call creates a new process that does not have a new vir-
tual address space, but rather shares address space with the parent, thus
avoiding potentially expensive copy-on-write operations normally associ-
ated 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
Same as for fork(2).
Same as for fork(2).
execve(2), fork(2), sigaction(2), wait(2)
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.
Portable applications should not depend on the memory sharing semantics
of vfork() as implementations exist that implement vfork() as plain
To avoid a possible deadlock situation, processes that are children in
the middle of a vfork() are never sent SIGTSTP, SIGTTOU or SIGTTIN sig-
nals; rather, output or ioctl(2) calls are allowed and input attempts
result in an end-of-file indication.
NetBSD 9.99 May 19, 2018 NetBSD 9.99
Modified for NetBSD
by Kimmo Suominen