# vfork

## NAME



           vfork - create a child process and block parent
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#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 <= 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), 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.
    


## VERSIONS



           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_.”  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.
    
       **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
    


## STANDARDS



           None.
    


## 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.
    


## BUGS



           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
           _ioctl_ s are allowed and input attempts result in an end-of-file
           indication."
    


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