vfork


SYNOPSIS
       #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:
               _BSD_SOURCE ||
                   (_XOPEN_SOURCE >= 500 ||
                       _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
                   !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
               _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

DESCRIPTION
   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  success-
       fully 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  pro-
       cesses  without  copying the page tables of the parent process.  It may
       be useful in performance-sensitive applications where a child  is  cre-
       ated which then immediately issues an execve(2).

       vfork()  differs from fork(2) in that the parent 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,  includ-
       ing  the stack.  The child must not return from the current function or
       call exit(3), but may call _exit(2).

       Signal handlers are inherited, but not shared.  Signals 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  struc-
       ture  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  need-
       than those put on fork(2), so an implementation where the two are  syn-
       onymous 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
   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.)

   History
       The vfork() system call appeared in 3.0BSD.  In 4.4BSD it was made syn-
       onymous  to fork(2) but NetBSD introduced it again, cf. http://www.net-
       bsd.org/Documentation/kernel/vfork.html .  In Linux, it has been equiv-
       alent  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
       It is rather unfortunate that Linux revived this specter from the past.
       The BSD man page 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(2)."

       Details  of the signal handling are obscure and differ between systems.
       The BSD man page states: "To avoid a possible deadlock situation,  pro-
       cesses  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
       clone(2), execve(2), fork(2), unshare(2), wait(2)

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-09-20                          VFORK(2)
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