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download PDF document: vfork.2.pdf

VFORK(2) FreeBSD System Calls Manual VFORK(2)
NAME vfork - create a new process without copying the address space
LIBRARY Standard C Library (libc, -lc)
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.)
RETURN VALUES Same as for fork(2).
SEE ALSO _exit(2), execve(2), fork(2), rfork(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.