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VFORK(2) System Calls Manual VFORK(2)


vforkspawn new process and block parent


#include <unistd.h>


vfork() was originally used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. It was useful when the purpose of fork(2) would have been to create a new system context for an execve(2). Since fork(2) is now efficient, even in the above case, the need for vfork() has diminished. vfork() differs from fork(2) in that the parent is suspended until the child makes a call to execve(2) or an exit (either by a call to _exit(2) or abnormally). In addition, fork handlers established using pthread_atfork(3) are not called when a multithreaded program calls vfork().
vfork() returns 0 in the child's context and (later) the PID of the child in the parent's context.


Same as for fork(2).


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


The vfork() function call appeared in 3.0BSD with the additional semantics that the child process ran in the memory of the parent until it called execve(2) or exited. That sharing of memory was removed in 4.4BSD, leaving just the semantics of blocking the parent until the child calls execve(2) or exits. On many other systems the original behavior has been restored, making this interface particularly unportable.


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.
September 10, 2015 OpenBSD-current