vfork man page

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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  SUSv2  / POSIX draft.)  The vfork() function has the same effect
       as fork(), except that the behaviour is undefined if the  process  cre-
       ated  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.

LINUX DESCRIPTION
       vfork(), just like fork(2), creates a child process of the calling pro-
       cess.  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  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 mem-
       ory  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's memory.

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  struc-
       ture  for  the  child.   However,  in  the  bad old days a fork() would
       require making a complete copy of the caller's data space, often  need-
       lessly,  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 bor-
       rowed 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, 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."

       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 4.4BSD it was made syn-
       onymous   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
       4.3BSD, POSIX.1-2001.

       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.

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.)

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



Linux 2.2.0                       1999-11-01                          VFORK(2)

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