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       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:
               _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 ||


   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 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 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.   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.


   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.)

       The  vfork()  system  call  appeared  in 3.0BSD.  In 4.4BSD it was made
       synonymous  to  fork(2)   but   NetBSD   introduced   it   again,   cf.  .   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.


       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."


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


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