12-27-2008
ivhb is right, fork return twice one is the child (return 0 ), the other is the the child pid
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vfork(2) System Calls Manual vfork(2)
Name
vfork - spawn new process in a virtual memory-efficient way
Syntax
pid = vfork()
int pid;
Description
The can be used to create new processes without fully copying the address space of the old process, which is inefficient in a paged envi-
ronment. It is useful when the purpose of would have been to create a new system context for an The system call differs from in that the
child borrows the parent's memory and thread of control until a call to or an exit (either by a call to or abnormally.) The parent process
is suspended while the child is using its resources.
The 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 system call can normally be used just like It does not work, however, to return while running in the childs context from the procedure
which called because the eventual return from would then return to a nonexistent stack frame. Be careful, also, to call _exit rather than
exit if you cannot call 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 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 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
See Also
execve(2), fork(2), sigvec(2), wait(2)
vfork(2)