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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  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  will
       be created 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,
       including  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
       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,
       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 are held in a register.

CONFORMING TO

       4.3BSD,  POSIX.1-2001.   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

   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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