NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

VFORK(2)                  Linux Programmer's Manual                 VFORK(2)

NAME         top

       vfork - create a child process and block parent

SYNOPSIS         top

       #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         top

   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 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) (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 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.

CONFORMING TO         top

       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.

NOTES         top

       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.)

       *  On systems where memory is constrained, vfork() avoids the need to
          temporarily commit memory (see the description of
          /proc/sys/vm/overcommit_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 overcommitting 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
       unintended ways, since such changes will be seen by the parent
       process once the child terminates or executes another program.  In
       this regard, signal handlers can be especially problematic: if a
       signal handler that is invoked in the child of vfork() changes
       memory, those changes may result in an inconsistent process state
       from the perspective of the parent 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
       program.  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 multithreaded program running as root creates a child
       using vfork().  After 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 executed 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
       specified as:

            CLONE_VM | CLONE_VFORK | SIGCHLD

   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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.11 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                            2017-03-13                         VFORK(2)

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