vfork - create a child process and block parent


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

   pid_t vfork(void);

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

       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


   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

   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.


   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.


   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:


   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.


   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."


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


   This  page  is  part of release 4.09 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


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