vfork(2)



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);

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

       vfork():
           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

DESCRIPTION
   Standard description
       (From POSIX.1) The vfork() function has the same effect as fork(2), ex-
       cept 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 success-
       fully 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 pro-
       cesses without copying the page tables of the parent process.   It  may
       be  useful  in performance-sensitive applications where a child is cre-
       ated 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 par-
       ent, including the stack.  The child must not return from  the  current
       function  or  call exit(3) (which would have the effect of calling exit
       handlers established by the parent process and  flushing  the  parent's
       stdio(3) buffers), 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  de-
       scribed 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 du-
       plicate 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 vari-
       ables were held in a register.

CONFORMING TO
       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 re-
       maining blocked until the child either terminates or  calls  execve(2),
       and cannot rely on any specific behavior with respect to shared memory.

NOTES
       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 be-
       tween  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 pro-
          vides functionality equivalent to fork(2)+exec(3), is designed to be
          implementable on systems that lack an MMU.)

       *  On systems where memory is constrained, vfork() avoids the  need  to
          temporarily commit memory (see the description of /proc/sys/vm/over-
          commit_memory in proc(5)) in order to execute a new program.   (This
          can  be especially beneficial where a large parent process wishes to
          execute a small helper program in a child  process.)   By  contrast,
          using  fork(2) in this scenario requires either committing an amount
          of memory equal to the size of the parent process (if  strict  over-
          committing  is in force) or overcommitting memory with the risk that
          a process is terminated by the out-of-memory (OOM) killer.

   Caveats
       The child process should take care not to modify the  memory  in  unin-
       tended ways, since such changes will be seen by the parent process once
       the child terminates or executes another program.  In this regard, sig-
       nal handlers can be especially problematic: if a signal handler that is
       invoked in the child of vfork() changes memory, those changes  may  re-
       sult  in an inconsistent process state from the perspective of the par-
       ent process (e.g., memory changes would be visible in the  parent,  but
       changes to the state of open file descriptors would not be visible).

       When  vfork()  is  called  in a multithreaded process, only the calling
       thread is suspended until the child terminates or executes a  new  pro-
       gram.  This means that the child is sharing an address space with other
       running code.  This can be dangerous if another thread  in  the  parent
       process  changes  credentials (using setuid(2) or similar), since there
       are now two processes with different privilege levels  running  in  the
       same  address space.  As an example of the dangers, suppose that a mul-
       tithreaded program running as root creates a child using vfork().   Af-
       ter the vfork(), a thread in the parent process drops the process to an
       unprivileged user in order to run some untrusted  code  (e.g.,  perhaps
       via plug-in opened with dlopen(3)).  In this case, attacks are possible
       where the parent process uses mmap(2) to map in code that will be  exe-
       cuted by the privileged child process.

   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 speci-
       fied as:

            CLONE_VM | CLONE_VFORK | SIGCHLD

   History
       The vfork() system call appeared in 3.0BSD.  In 4.4BSD it was made syn-
       onymous    to   fork(2)   but   NetBSD   introduced   it   again;   see
       <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
       Details  of the signal handling are obscure and differ between systems.
       The BSD man page 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."

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

COLOPHON
       This  page  is  part of release 4.16 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          VFORK(2)

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