The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


vfork - create a new process; share virtual memory


[OB XSI] [Option Start] #include <unistd.h>

pid_t vfork(void); [Option End]


The vfork() function shall be equivalent to fork(), except that the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions.


Upon successful completion, vfork() shall return 0 to the child process and return the process ID of the child process to the parent process. Otherwise, -1 shall be returned to the parent, no child process shall be created, and errno shall be set to indicate the error.


The vfork() function shall fail if:

The system-wide limit on the total number of processes under execution would be exceeded, or the system-imposed limit on the total number of processes under execution by a single user would be exceeded.
There is insufficient swap space for the new process.

The following sections are informative.




Conforming applications are recommended not to depend on vfork(), but to use fork() instead. The vfork() function may be withdrawn in a future version.

On some implementations, vfork() is equivalent to fork().

The vfork() function differs from fork() only in that the child process can share code and data with the calling process (parent process). This speeds cloning activity significantly at a risk to the integrity of the parent process if vfork() is misused.

The use of vfork() for any purpose except as a prelude to an immediate call to a function from the exec family, or to _exit(), is not advised.

The vfork() function can be used to create new processes without fully copying the address space of the old process. If a forked process is simply going to call exec, the data space copied from the parent to the child by fork() is not used. This is particularly inefficient in a paged environment, making vfork() particularly useful. Depending upon the size of the parent's data space, vfork() can give a significant performance improvement over fork().

The vfork() function can normally be used just like fork(). It does not work, however, to return while running in the child's context from the caller of vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Care should be taken, also, to call _exit() rather than exit() if exec cannot be used, since exit() flushes and closes standard I/O channels, thereby damaging the parent process' standard I/O data structures. (Even with fork(), it is wrong to call exit(), since buffered data would then be flushed twice.)

If signal handlers are invoked in the child process after vfork(), they must follow the same rules as other code in the child process.




This function may be withdrawn in a future version.


exec(), exit(), fork(), wait(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 6

This function is marked obsolescent.

End of informative text.

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