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 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)). 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 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>:

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 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.

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.

Attributes

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

See Also

exec(2), exit(2), fork(2), ioctl(2), atexit(3C), exit(3C), posix_spawn(3C), posix_spawnp(3C), signal.h(3HEAD), wait(3C), attributes(5), standards(5)

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().