vfork(S)

vfork -- spawn new process in a virtual memory efficient way

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 or abnormally.) The parent process is suspended while the child is using its resources.

vfork returns 0 in the child's context and (later) the process ID (PID) of the child in the parent's context.

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 if you cannot execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. Even with fork it is wrong to call exit since buffered data would then be flushed twice.

Return values

On success, vfork returns 0 to the child process and returns the process ID of the child process to the parent process. On failure, vfork returns -1, sets errno to identify the error, and no child process is created.

Errors

In the following conditions, vfork fails and sets errno to:

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.

References

exec(S), exit(S), fork(S), ioctl(S), wait(S)

Notices

vfork will be eliminated in a future release. System implementation changes are making the efficiency gain of vfork over fork smaller. 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.

Considerations for lightweight processes

vfork will fail if the process has more than one LWP.