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# vfork() - Unix, Linux System Call

# NAME

vfork - create a child process and block parent

# SYNOPSIS

    #include <sys/types.h>
    #include <unistd.h>
    pid_t vfork(void);

# STANDARD DESCRIPTION

(From SUSv2 / POSIX draft.) The vfork() function has the same effect as fork(), except that the behaviour is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec() family of functions.

# LINUX DESCRIPTION

vfork(), just like fork(2), creates a child process of the calling process. For details and return value and errors, see fork(2).

vfork() is a special case of clone(2). It is used to create new processes without copying the page tables of the parent process. It may be useful in performance sensitive applications where a child will be created which then immediately issues an execve().

vfork() differs from fork() in that the parent is suspended until the child makes a call to execve(2) or _exit(2). The child shares all memory with its parent, including the stack, until execve() is issued by the child. The child must not return from the current function or call exit(), but may call _exit().

Signal handlers are inherited, but not shared. Signals to the parent arrive after the child releases the parent’s memory.

# HISTORIC DESCRIPTION

Under Linux, fork() is implemented using copy-on-write pages, so the only penalty incurred by fork() is the time and memory required to duplicate the parent’s page tables, and to create a unique task structure for the child. However, in the bad old days a fork() would require making a complete copy of the caller’s data space, often needlessly, since usually immediately afterwards an exec() is done. Thus, for greater efficiency, BSD introduced the vfork() system call, that did not fully copy the address space of the parent process, but borrowed the parent’s memory and thread of control until a call to execve() or an exit occurred. The parent process was suspended while the child was using its resources. The use of vfork() was tricky: for example, not modifying data in the parent process depended on knowing which variables are held in a register.

# BUGS

It is rather unfortunate that Linux revived this spectre from the past. The BSD manpage states: "This system call will be eliminated when proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of vfork() as it will, in that case, be made synonymous to fork(). "

Formally speaking, the standard description given above does not allow one to use vfork() since a following exec() might fail, and then what happens is undefined.

Details of the signal handling are obscure and differ between systems. The BSD manpage states: "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 end-of-file indication."

Currently (Linux 2.3.25), strace(1) cannot follow vfork() and requires a kernel patch.

# HISTORY

The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made synonymous to fork() but NetBSD introduced it again, cf. http://www.netbsd.org/Documentation/kernel/vfork.html . In Linux, it has been equivalent to fork() until 2.2.0-pre6 or so. Since 2.2.0-pre9 (on i386, somewhat later on other architectures) it is an independent system call. Support was added in glibc 2.0.112.

# CONFORMING TO

4.3BSD, POSIX.1-2001. The requirements put on vfork() by the standards are weaker than those put on fork(), so an implementation where the two are synonymous is compliant. In particular, the programmer cannot rely on the parent remaining blocked until a call of execve() or _exit() and cannot rely on any specific behaviour w.r.t. shared memory.

# SEE ALSO

- clone (2)

- execve (2)

- fork (2)

- unshare (2)

- wait (2)

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