1VFORK(2)                   Linux Programmer's Manual                  VFORK(2)
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3
4

NAME

6       vfork - create a child process and block parent
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
11
12       pid_t vfork(void);
13
14   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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16       vfork(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
17

DESCRIPTION

19   Standard Description
20       (From  POSIX.1)  The  vfork()  function has the same effect as fork(2),
21       except that the behavior is undefined if the process created by vfork()
22       either  modifies  any  data other than a variable of type pid_t used to
23       store the return value from vfork(), or returns from  the  function  in
24       which  vfork()  was called, or calls any other function before success‐
25       fully calling _exit(2) or one of the exec(3) family of functions.
26
27   Linux Description
28       vfork(), just like fork(2), creates a  child  process  of  the  calling
29       process.  For details and return value and errors, see fork(2).
30
31       vfork()  is  a special case of clone(2).  It is used to create new pro‐
32       cesses without copying the page tables of the parent process.   It  may
33       be  useful  in performance-sensitive applications where a child is cre‐
34       ated which then immediately issues an execve(2).
35
36       vfork() differs from fork(2) in that the parent is suspended until  the
37       child  terminates (either normally, by calling _exit(2), or abnormally,
38       after delivery of a fatal signal), or it makes  a  call  to  execve(2).
39       Until  that point, the child shares all memory with its parent, includ‐
40       ing the stack.  The child must not return from the current function  or
41       call exit(3), but may call _exit(2).
42
43       Signal  handlers  are inherited, but not shared.  Signals to the parent
44       arrive after the child releases the parent's memory  (i.e.,  after  the
45       child terminates or calls execve(2)).
46
47   Historic Description
48       Under  Linux,  fork(2) is implemented using copy-on-write pages, so the
49       only penalty incurred by fork(2) is the time  and  memory  required  to
50       duplicate  the parent's page tables, and to create a unique task struc‐
51       ture for the child.  However, in the  bad  old  days  a  fork(2)  would
52       require  making a complete copy of the caller's data space, often need‐
53       lessly, since usually immediately afterwards an exec(3) is done.  Thus,
54       for  greater  efficiency, BSD introduced the vfork() system call, which
55       did not fully copy the address space of the parent  process,  but  bor‐
56       rowed  the  parent's  memory  and  thread  of  control  until a call to
57       execve(2) or an exit occurred.  The parent process was suspended  while
58       the  child was using its resources.  The use of vfork() was tricky: for
59       example, not modifying data in the parent process depended  on  knowing
60       which variables were held in a register.
61

CONFORMING TO

63       4.3BSD,   POSIX.1-2001.   POSIX.1-2008  removes  the  specification  of
64       vfork().  The requirements put on vfork() by the standards  are  weaker
65       than  those put on fork(2), so an implementation where the two are syn‐
66       onymous is compliant.  In particular, the programmer cannot rely on the
67       parent  remaining  blocked  until  the child either terminates or calls
68       execve(2), and cannot rely on any specific  behavior  with  respect  to
69       shared memory.
70

NOTES

72   Linux Notes
73       Fork handlers established using pthread_atfork(3) are not called when a
74       multithreaded  program  employing  the  NPTL  threading  library  calls
75       vfork().   Fork handlers are called in this case in a program using the
76       LinuxThreads threading library.  (See pthreads(7) for a description  of
77       Linux threading libraries.)
78
79   History
80       The vfork() system call appeared in 3.0BSD.  In 4.4BSD it was made syn‐
81       onymous to fork(2) but NetBSD introduced it again, cf.  http://www.net
82       bsd.org/Documentation/kernel/vfork.html .  In Linux, it has been equiv‐
83       alent to fork(2) until 2.2.0-pre6 or so.  Since  2.2.0-pre9  (on  i386,
84       somewhat  later  on  other  architectures)  it is an independent system
85       call.  Support was added in glibc 2.0.112.
86

BUGS

88       It is rather unfortunate that Linux revived this specter from the past.
89       The  BSD  man  page  states:  "This system call will be eliminated when
90       proper system sharing mechanisms are  implemented.   Users  should  not
91       depend  on  the memory sharing semantics of vfork() as it will, in that
92       case, be made synonymous to fork(2)."
93
94       Details of the signal handling are obscure and differ between  systems.
95       The  BSD man page states: "To avoid a possible deadlock situation, pro‐
96       cesses that are children in the middle of  a  vfork()  are  never  sent
97       SIGTTOU  or  SIGTTIN  signals; rather, output or ioctls are allowed and
98       input attempts result in an end-of-file indication."
99

SEE ALSO

101       clone(2), execve(2), fork(2), unshare(2), wait(2)
102

COLOPHON

104       This page is part of release 3.25 of the Linux  man-pages  project.   A
105       description  of  the project, and information about reporting bugs, can
106       be found at http://www.kernel.org/doc/man-pages/.
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110Linux                             2009-06-21                          VFORK(2)
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