NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       vfork - create a child process and block parent

SYNOPSIS         top

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

       pid_t vfork(void);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       vfork():
           Since glibc 2.12:
               (_XOPEN_SOURCE >= 500) && ! (_POSIX_C_SOURCE >= 200809L)
                   || /* Since glibc 2.19: */ _DEFAULT_SOURCE
                   || /* Glibc versions <= 2.19: */ _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION         top

   Standard description
       (From POSIX.1) The vfork() function has the same effect as fork(2),
       except that the behavior 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(2) or one of the exec(3) 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 is
       created which then immediately issues an execve(2).

       vfork() differs from fork(2) in that the calling thread is suspended
       until the child terminates (either normally, by calling _exit(2), or
       abnormally, after delivery of a fatal signal), or it makes a call to
       execve(2).  Until that point, the child shares all memory with its
       parent, including the stack.  The child must not return from the
       current function or call exit(3), but may call _exit(2).

       As with fork(2), the child process created by vfork() inherits copies
       of various of the caller's process attributes (e.g., file
       descriptors, signal dispositions, and current working directory); the
       vfork() call differs only in the treatment of the virtual address
       space, as described above.

       Signals sent to the parent arrive after the child releases the
       parent's memory (i.e., after the child terminates or calls
       execve(2)).

   Historic description
       Under Linux, fork(2) is implemented using copy-on-write pages, so the
       only penalty incurred by fork(2) 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(2)
       would require making a complete copy of the caller's data space,
       often needlessly, since usually immediately afterward an exec(3) is
       done.  Thus, for greater efficiency, BSD introduced the vfork()
       system call, which 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(2) 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 were held in a register.

CONFORMING TO         top

       4.3BSD; POSIX.1-2001 (but marked OBSOLETE).  POSIX.1-2008 removes the
       specification of vfork().

       The requirements put on vfork() by the standards are weaker than
       those put on fork(2), so an implementation where the two are
       synonymous is compliant.  In particular, the programmer cannot rely
       on the parent remaining blocked until the child either terminates or
       calls execve(2), and cannot rely on any specific behavior with
       respect to shared memory.

NOTES         top

       Some consider the semantics of vfork() to be an architectural
       blemish, and the 4.2BSD man page stated: "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(2)."  However, even
       though modern memory management hardware has decreased the
       performance difference between fork(2) and vfork(), there are various
       reasons why Linux and other systems have retained vfork():

       *  Some performance-critical applications require the small
          performance advantage conferred by vfork().

       *  vfork() can be implemented on systems that lack a memory-
          management unit (MMU), but fork(2) can't be implemented on such
          systems.  (POSIX.1-2008 removed vfork() from the standard; the
          POSIX rationale for the posix_spawn(3) function notes that that
          function, which provides functionality equivalent to
          fork(2)+exec(3), is designed to be implementable on systems that
          lack an MMU.)

   Linux notes
       Fork handlers established using pthread_atfork(3) are not called when
       a multithreaded program employing the NPTL threading library calls
       vfork().  Fork handlers are called in this case in a program using
       the LinuxThreads threading library.  (See pthreads(7) for a
       description of Linux threading libraries.)

       A call to vfork() is equivalent to calling clone(2) with flags
       specified as:

            CLONE_VM | CLONE_VFORK | SIGCHLD

   History
       The vfork() system call appeared in 3.0BSD.  In 4.4BSD it was made
       synonymous to fork(2) but NetBSD introduced it again, cf.  
       ⟨http://www.netbsd.org/Documentation/kernel/vfork.html⟩.  In Linux, it
       has been equivalent to fork(2) 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.

BUGS         top

       Details of the signal handling are obscure and differ between
       systems.  The BSD man page 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."

SEE ALSO         top

       clone(2), execve(2), fork(2), unshare(2), wait(2)

COLOPHON         top

       This page is part of release 4.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-03-15                         VFORK(2)