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VFORK(2)                   Linux Programmer's Manual                  VFORK(2)

NAME
       vfork - create a child process and block parent

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       pid_t vfork(void);

STANDARD DESCRIPTION
       (From XPG4 / 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.

ERRORS
       EAGAIN Too many processes - try again.

       ENOMEM There is insufficient swap space for the new process.

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.

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 BSD 4.4 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
       The vfork call may be a bit similar to calls with the same name in
       other operating systems. 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), wait(2)

Linux 2.2.0                       1999-11-01                          VFORK(2)

NAME | SYNOPSIS | STANDARD DESCRIPTION | ERRORS | LINUX DESCRIPTION | HISTORIC DESCRIPTION | BUGS | HISTORY | CONFORMING TO | SEE ALSO

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