The vfork() function creates new processes without fully copying the address space of the old process. This function is useful in instances where the purpose of a fork(2) operation would be to create a new system context for an execve() operation (see exec(2)).

Unlike with the fork() function, the child process borrows the parent's memory and thread of control until a call to execve() or an exit (either abnormally or by a call to _exit() (see exit(2)). The parent process is suspended while the child is using its resources.

In a multithreaded application, vfork() borrows only the thread of control that called vfork() in the parent; that is, the child contains only one thread. The use of vfork() in multithreaded applications, however, is not advised.

The vfork() function can normally be used the same way as fork(). The procedure that called vfork(), however, should not return while running in the child's context, since the eventual return from vfork() would be to a stack frame that no longer exists. The _exit() function should be used in favor of exit(3C) if unable to perform an execve() operation, since exit() will flush and close standard I/O channels, and thereby corrupt the parent process's standard I/O data structures. The _exit() function should be used even with fork() to avoid flushing the buffered data twice.

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

The vfork() function will fail if:

EAGAIN

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.

ENOMEM

There is insufficient swap space for the new process.

See attributes(5) for descriptions of the following attributes:

exec(2), exit(2), fork(2), ioctl(2), wait(2), exit(3C), attributes(5)

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

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().