VFORK(2)

NAME

vfork − spawn new process in a virtual memory efficient way

SYNOPSIS

int vfork()

HP-UX COMPATIBILITY

Level: HP-UX/STANDARD

Origin: UCB

Remarks: Vfork is provided as a higher performance version of fork on those systems which choose to provide it and for which there is a performance advantage.

Vfork differs from fork only in that the child process may share code and data with the calling process (parent process). This speeds the 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 exec or exit is not supported. Any program which relies upon the differences between fork and vfork is not portable across HP-UX systems.

All implementations of HP-UX must provide the entry vfork, but it is permissible for them to treat it identically to fork. Some implementations may not choose to distinguish the two because their implementation of fork is as efficient as possible, and others may not wish to carry the added overhead of two similar calls.

DESCRIPTION

Vfork 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 do an exec(2), the data space copied from the parent to the child by fork(2) is not used. This is particularly inefficient in a paged environment. Vfork is useful in this case. Depending upon the size of the parent’s data space, it can give a significant performance improvement over fork.

Vfork differs from fork in that the child borrows the parent’s memory and thread of control until a call to exec or an exit (either by a call to exit(2) 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 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 can’t exec, 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.)

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

[EAGAIN] The system-wide limit on the total number of processes under execution would be exceeded.

[EAGAIN] The system-imposed limit on the total number of processes under execution by a single user would be exceeded.

HARDWARE DEPENDENCIES

Series 200 and 500: The [vfork,exec] window begins at the vfork call and ends when the child completes its exec call.

Series 500:

Process times for the parent and child processes within the [vfork,exec] window may be inaccurate.

Shared memory segments generated with the EMS intrinsics will be inherited over vfork. Private memory segments will not be copied over vfork.

Vfork will also fail in the following cases:

[ENOMEM] There is not enough physical memory to create the new process.

[EAGAIN] The child process attempts to do a second vfork or a fork while in the [vfork,exec] window.

The parent and child processes share the same stack space within the [vfork,exec] window. If the size of the stack has been changed within this window by the child process (return from or call to a function, for example), it is likely that the parent and child processes will be killed with signal SIGSEGV.

A child process which does not call exec will not generate a process accounting record.

Series 200:
A call to signal(2) in the [vfork,exec] window which is used to catch a signal can affect handling of the signal by the parent. This is not true if the signal is set to SIG_DFL or SIG_IGN, or if sigvector(2) is used.

Integral PC:
On the Integral PC, vforked children have a unique 2K-byte stack allocated to them. Any stack space used beyond this 2K limit is shared between the child and the parent.

RETURN VALUE

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, no child process is created, and errno is set to indicate the error.

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