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.)

RETURN VALUES

Same as for fork(2).

ERRORS

Same as for fork(2).

SEE ALSO

execve(2), fork(2), sigaction(2), wait(2), posix_spawn(3)

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.

New applications should prefer posix_spawn(3) over vfork followed by execve(2).