vfork (2)

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(The comments found at the beginning of the groff file "man2/vfork.2freebsd".)

NAME

vfork - create a new process without copying the address space

LIBRARY

Lb libc

SYNOPSIS

In unistd.h Ft pid_t Fn vfork void

DESCRIPTION

The Fn vfork system call can be used to create new processes without fully copying the address space of the old process, which is horrendously 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 Fn 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 _exit2 or abnormally). The parent process is suspended while the child is using its resources.

The Fn vfork system call returns 0 in the child's context and (later) the pid of the child in the parent's context.

The Fn 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 Fn vfork since the eventual return from Fn vfork would then return to a no longer existent stack frame. Be careful, also, to call _exit2 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

execve(2), _exit2, fork(2), rfork(2), sigaction(2), wait(2), exit(3)

HISTORY

The Fn vfork system call appeared in BSD 2.9

BUGS

To avoid a possible deadlock situation, processes that are children in the middle of a Fn 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.