VFORK(2) System Calls VFORK(2)

NAME

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

SYNOPSIS

#include <unistd.h>

pid_t
vfork(void);

#include <sys/fork.h>

pid_t
vforkx(int flags);

DESCRIPTION

The vfork() and vforkx() functions create a new process without fully
copying the address space of the old process. These functions are
useful in instances where the purpose of a fork(2) operation is to
create a new system context for an exec(2) operation.

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(2)). Any modification made
during this time to any part of memory in the child process is
reflected in the parent process on return from vfork() or vforkx().
The parent process is suspended while the child is using its resources.

In a multithreaded application, vfork() and vforkx() borrow only the
thread of control that called vfork() or vforkx() in the parent; that
is, the child contains only one thread. The use of vfork() or vforkx()
in multithreaded applications, however, is unsafe due to race
conditions that can cause the child process to become deadlocked and
consequently block both the child and parent process from execution
indefinitely.

The vfork() and vforkx() functions can normally be used the same way as
fork() and forkx(), respectively. The calling procedure, however,
should not return while running in the child's context, since the
eventual return from vfork() or vforkx() in the parent 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 invoke all functions registered by atexit(3C) and will
flush and close standard I/O channels, thereby corrupting the parent
process's standard I/O data structures. Care must be taken in the
child process not to modify any global or local data that affects the
behavior of the parent process on return from vfork() or vforkx(),
unless such an effect is intentional.

Unlike fork() and forkx(), fork handlers are not run when vfork() and
vforkx() are called.

The vfork() and vforkx() functions are deprecated. Their sole
legitimate use as a prelude to an immediate call to a function from the
exec(2) family can be achieved safely by posix_spawn(3C) or
posix_spawnp(3C).

Fork Extensions

The vforkx() function accepts a flags argument consisting of a bitwise
inclusive-OR of zero or more of the following flags, which are defined
in the header <sys/fork.h>:

FORK_NOSIGCHLD
FORK_WAITPID

See fork(2) for descriptions of these flags. If the flags argument is
0, vforkx() is identical to vfork().

RETURN VALUES

Upon successful completion, vfork() and vforkx() return 0 to the child
process and return 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.

ERRORS

The vfork() and vforkx() functions 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.

The vforkx() function will fail if:

EINVAL The flags argument is invalid.

INTERFACE STABILITY

The vfork() function is Obsolete Standard.

The vforkx() function is Obsolete Uncommitted.

MT-LEVEL
Unsafe.

SEE ALSO

exec(2), exit(2), fork(2), ioctl(2), atexit(3C), exit(3C),
posix_spawn(3C), posix_spawnp(3C), wait(3C), signal.h(3HEAD),
standards(7)

NOTES

To avoid a possible deadlock situation, processes that are children in
the middle of a vfork() or vforkx() are never sent SIGTTOU or SIGTTIN
signals; rather, output or ioctls are allowed and input attempts result
in an EOF indication.

To forestall parent memory corruption due to race conditions with
signal handling, vfork() and vforkx() treat signal handlers in the
child process in the same manner as the exec(2) functions: signals set
to be caught by the parent process are set to the default action
(SIG_DFL) in the child process (see signal.h(3HEAD)). Any attempt to
set a signal handler in the child before execve() to anything other
than SIG_DFL or SIG_IGN is disallowed and results in setting the
handler to SIG_DFL.

On some systems, the implementation of vfork() and vforkx() cause 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() or if <sys/fork.h> is not included in the
source calling vforkx().

STANDARDS

The vfork() function is available in the following compilation
environments. See standards(7).

+o X/Open Portability Guide Issue 4, Version 2 ("XPG4.2")
+o Version 2 of the Single UNIX Specification ("SUSv2")
+o Version 3 of the Single UNIX Specification ("SUSv3")

It was marked obsolete in Version 3 of the Single UNIX Specification
("SUSv3") and removed from IEEE Std 1003.1-2008 ("POSIX.1").

The vforkx() function is a local extension and not available in any
strictly standards-compliant compilation environment.

illumos August 20, 2014 illumos