VFORK(2) |
System Calls Manual |
VFORK(2) |
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.)
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.