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NAME

       vfork - create a child process and block parent

SYNOPSIS

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

       pid_t vfork(void);

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

       vfork(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION

   Standard Description
       (From  SUSv2  / POSIX draft.)  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  will
       be created which then immediately issues an execve(2).

       vfork()  differs from fork(2) 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(2) is issued
       by the child.  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.

   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 afterwards an exec(3) 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(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 are held in a register.

CONFORMING TO

       4.3BSD, POSIX.1-2001.  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 a call of execve(2) or
       _exit(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
       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

       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,
       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

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

COLOPHON

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