VFORK(2)

NAME
vfork - create a new process without copying the address space

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
pid_t
vfork(void);

DESCRIPTION
Since this function is hard to use correctly from application software, it is recommended to use posix_spawn(3) or fork(2) instead.

The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is 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 vfork() system call differs from fork(2) in that the child borrows the parent process's address space and the calling thread's stack until a call to execve(2) or an exit (either by a call to _exit(2) or abnormally). The calling thread is suspended while the child is using its resources. Other threads continue to run.

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

Many problems can occur when replacing fork(2) with vfork(). For example, it does not work 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 existent stack frame. Also, changing process state which is partially implemented in user space such as signal handlers with libthr(3) will corrupt the parent's state.

Be careful, also, to call _exit(2) 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.)

Unlike fork(2), vfork() does not run pthread_atfork(3) handlers.

RETURN VALUES
Same as for fork(2).

ERRORS
The vfork() system call will fail for any of the reasons described in the fork man page. In addition, it will fail if:

[EINVAL]
    A system call other than _exit() or execve() (or libc functions that make no system calls other than those) is called following calling a vfork() call.

SEE ALSO
_exit(2), execve(2), fork(2), sigaction(2), wait(2), exit(3), posix_spawn(3)

HISTORY
The vfork() system call appeared in 3BSD.

BUGS
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.

CAVEATS
There are limits to what you can do in the child process. To be totally safe you should restrict yourself to only executing async-signal safe operations until such time as one of the exec functions is called. All APIs, including global data symbols, in any framework or library should be assumed to be unsafe after a vfork() unless explicitly documented to be safe or async-signal safe. If you need to use these frameworks in the child process, you must exec. In this situation it is reasonable to exec yourself.