vfork(2)
Name
vfork − spawn new process in a virtual memory-efficient way
Syntax
pid = vfork()
int pid;
Description
The vfork 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 would have been to create a new system context for an execve. The vfork system call differs from fork in that the child borrows the parent’s memory and thread of control until a call to execve or an exit (either by a call to exit() or abnormally.) The parent process is suspended while the child is using its resources.
The vfork system call returns a value of zero (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. It does not work, however, to return while running in the childs context from the procedure which called vfork, because the eventual return from vfork would then return to a nonexistent stack frame. Be careful, also, to call _exit rather than exit if you cannot call execve, because exit will flush and close standard I/O channels and thereby cause problems in the parent process’s standard I/O data structures. Even with fork it is wrong to call exit, because buffered data would then be flushed twice.
Restrictions
To avoid a possible deadlock situation, processes which are children in the middle of a vfork are never sent SIGTTOU or SIGTTIN signals. Rather, output or ioctls are allowed, and input attempts result in an end-of-file indication.
Diagnostics
Same as for fork.