'\" te .\" Copyright (c) 2004, Sun Microsystems, Inc. All Rights Reserved. .\" Copyright 1989 AT&T. .\" Copyright (c) 1980 Regents of the University of California. All rights reserved. The Berkeley software License Agreement specifies the terms and conditions for redistribution. .TH VFORK 2 "Dec 13, 2006" .SH NAME vfork, vforkx \- spawn new process in a virtual memory efficient way .SH SYNOPSIS .LP .nf #include \fBpid_t\fR \fBvfork\fR(\fBvoid\fR); .fi .LP .nf #include \fBpid_t\fR \fBvforkx\fR(\fBint\fR \fIflags\fR); .fi .SH DESCRIPTION .sp .LP The \fBvfork()\fR and \fBvforkx()\fR functions create a new process without fully copying the address space of the old process. These functions are useful in instances where the purpose of a \fBfork\fR(2) operation is to create a new system context for an \fBexecve()\fR operation (see \fBexec\fR(2)). .sp .LP Unlike with the \fBfork()\fR function, the child process borrows the parent's memory and thread of control until a call to \fBexecve()\fR or an exit (either abnormally or by a call to \fB_exit()\fR (see \fBexit\fR(2)). Any modification made during this time to any part of memory in the child process is reflected in the parent process on return from \fBvfork()\fR or \fBvforkx()\fR. The parent process is suspended while the child is using its resources. .sp .LP In a multithreaded application, \fBvfork()\fR and \fBvforkx()\fR borrow only the thread of control that called \fBvfork()\fR or \fBvforkx()\fR in the parent; that is, the child contains only one thread. The use of \fBvfork()\fR or \fBvforkx()\fR in multithreaded applications, however, is unsafe due to race conditions that can cause the child process to become deadlocked and consequently block both the child and parent process from execution indefinitely. .sp .LP The \fBvfork()\fR and \fBvforkx()\fR functions can normally be used the same way as \fBfork()\fR and \fBforkx()\fR, respectively. The calling procedure, however, should not return while running in the child's context, since the eventual return from \fBvfork()\fR or \fBvforkx()\fR in the parent would be to a stack frame that no longer exists. The \fB_exit()\fR function should be used in favor of \fBexit\fR(3C) if unable to perform an \fBexecve()\fR operation, since \fBexit()\fR will invoke all functions registered by \fBatexit\fR(3C) and will flush and close standard I/O channels, thereby corrupting the parent process's standard I/O data structures. Care must be taken in the child process not to modify any global or local data that affects the behavior of the parent process on return from \fBvfork()\fR or \fBvforkx()\fR, unless such an effect is intentional. .sp .LP Unlike \fBfork()\fR and \fBforkx()\fR, fork handlers are not run when \fBvfork()\fR and \fBvforkx()\fR are called. .sp .LP The \fBvfork()\fR and \fBvforkx()\fR functions are deprecated. Their sole legitimate use as a prelude to an immediate call to a function from the \fBexec\fR family can be achieved safely by \fBposix_spawn\fR(3C) or \fBposix_spawnp\fR(3C). .SS "Fork Extensions" .sp .LP The \fBvforkx()\fR function accepts a \fIflags\fR argument consisting of a bitwise inclusive-OR of zero or more of the following flags, which are defined in the header \fB\fR: .br .in +2 \fBFORK_NOSIGCHLD\fR .in -2 .br .in +2 \fBFORK_WAITPID\fR .in -2 .sp .LP See \fBfork\fR(2) for descriptions of these flags. If the \fIflags\fR argument is 0, \fBvforkx()\fR is identical to \fBvfork()\fR. .SH RETURN VALUES .sp .LP Upon successful completion, \fBvfork()\fR and \fBvforkx()\fR return \fB0\fR to the child process and returns the process ID of the child process to the parent process. Otherwise, \fB\(mi1\fR is returned to the parent process, no child process is created, and \fBerrno\fR is set to indicate the error. .SH ERRORS .sp .LP The \fBvfork()\fR and \fBvforkx()\fR functions will fail if: .sp .ne 2 .na \fB\fBEAGAIN\fR\fR .ad .RS 10n The system-imposed limit on the total number of processes under execution (either system-quality or by a single user) would be exceeded. This limit is determined when the system is generated. .RE .sp .ne 2 .na \fB\fBENOMEM\fR\fR .ad .RS 10n There is insufficient swap space for the new process. .RE .sp .LP The \fBvforkx()\fR function will fail if: .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 10n The \fIflags\fR argument is invalid. .RE .SH ATTRIBUTES .sp .LP See \fBattributes\fR(5) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ Interface Stability Obsolete _ MT-Level Unsafe .TE .SH SEE ALSO .sp .LP \fBexec\fR(2), \fBexit\fR(2), \fBfork\fR(2), \fBioctl\fR(2), \fBatexit\fR(3C), \fBexit\fR(3C), \fBposix_spawn\fR(3C), \fBposix_spawnp\fR(3C), \fBsignal.h\fR(3HEAD), \fBwait\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5) .SH NOTES .sp .LP To avoid a possible deadlock situation, processes that are children in the middle of a \fBvfork()\fR or \fBvforkx()\fR are never sent \fBSIGTTOU\fR or \fBSIGTTIN\fR signals; rather, output or ioctls are allowed and input attempts result in an \fBEOF\fR indication. .sp .LP To forestall parent memory corruption due to race conditions with signal handling, \fBvfork()\fR and \fBvforkx()\fR treat signal handlers in the child process in the same manner as the \fBexec\fR(2) functions: signals set to be caught by the parent process are set to the default action (\fBSIG_DFL\fR) in the child process (see \fBsignal.h\fR(3HEAD)). Any attempt to set a signal handler in the child before \fBexecve()\fR to anything other than \fBSIG_DFL\fR or \fBSIG_IGN\fR is disallowed and results in setting the handler to \fBSIG_DFL\fR. .sp .LP On some systems, the implementation of \fBvfork()\fR and \fBvforkx()\fR cause the parent to inherit register values from the child. This can create problems for certain optimizing compilers if \fB\fR is not included in the source calling \fBvfork()\fR or if \fB\fR is not included in the source calling \fBvforkx()\fR.