vfork(2) — Linux manual page

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; 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         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 5.10 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)

Pages that refer to this page: strace(1)clone(2)fork(2)getpid(2)ptrace(2)setns(2)syscalls(2)unshare(2)posix_spawn(3)persistent-keyring(7)pid_namespaces(7)session-keyring(7)user-keyring(7)user-session-keyring(7)