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Name

Synopsis

#include <unistd.h>

pid_t vfork(void);

Description

vfork() can be used to create new processes without fully copying the address space of the old process. It is useful when the purpose of fork() would have been to create a new system context for an execve(). vfork() 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() (see exit(2) ) or abnormally). The parent process is suspended while the child is using its resources. In a multi-threaded application, vfork() borrows only the thread of control which called vfork() in the parent; that is, the child contains only one thread. In that sense, in a multi-threaded application vfork() behaves like fork().

vfork() can normally be used just like fork(). It does not work, however, to return while running in the child’s context from the procedure which called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful, also, to call _exit() rather than exit(3C) if you cannot execve(), since exit(3C) will flush and close standard I/O channels, and thereby corrupt the parent processes standard I/O data structures. Even with fork() it is wrong to call exit(3C) since buffered data would then be flushed twice.

Return Values

Upon successful completion, vfork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error.

Errors

vfork() will fail and no child process will be created if one or more of the following are true:

EAGAIN

The system-imposed limit on the total number of processes under execution would be exceeded. This limit is determined when the system is generated.

EAGAIN

The system-imposed limit on the total number of processes under execution by a single user would be exceeded. This limit is determined when the system is generated.

ENOMEM

There is insufficient swap space for the new process.

See Also

Notes

This function will be eliminated in a future release. The memory sharing semantics of vfork() can be obtained through other mechanisms.

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 ioctls are allowed and input attempts result in an EOF indication.

On some systems, the implementation of vfork() causes the parent to inherit register values from the child. This can create problems for certain optimizing compilers if <unistd.h> is not included in the source calling vfork().

Table of Contents

• Name
• Synopsis
• Description
• Return Values
• Errors
• See Also
• Notes