1FORK(2)                    Linux Programmer's Manual                   FORK(2)
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NAME

6       fork - create a child process
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SYNOPSIS

9       #include <unistd.h>
10
11       pid_t fork(void);
12

DESCRIPTION

14       fork()  creates  a new process by duplicating the calling process.  The
15       new process, referred to as the child, is an  exact  duplicate  of  the
16       calling  process,  referred  to as the parent, except for the following
17       points:
18
19       *  The child has its own unique process ID, and this PID does not match
20          the ID of any existing process group (setpgid(2)).
21
22       *  The  child's  parent  process ID is the same as the parent's process
23          ID.
24
25       *  The child does not inherit  its  parent's  memory  locks  (mlock(2),
26          mlockall(2)).
27
28       *  Process  resource  utilizations (getrusage(2)) and CPU time counters
29          (times(2)) are reset to zero in the child.
30
31       *  The child's set of pending  signals  is  initially  empty  (sigpend‐
32          ing(2)).
33
34       *  The  child  does  not  inherit semaphore adjustments from its parent
35          (semop(2)).
36
37       *  The child does not inherit record locks from its parent (fcntl(2)).
38
39       *  The child does not inherit timers  from  its  parent  (setitimer(2),
40          alarm(2), timer_create(2)).
41
42       *  The  child  does not inherit outstanding asynchronous I/O operations
43          from its parent (aio_read(3), aio_write(3)), nor does it inherit any
44          asynchronous I/O contexts from its parent (see io_setup(2)).
45
46       The  process  attributes  in  the  preceding  list are all specified in
47       POSIX.1-2001.  The parent and child also differ  with  respect  to  the
48       following Linux-specific process attributes:
49
50       *  The  child does not inherit directory change notifications (dnotify)
51          from its parent (see the description of F_NOTIFY in fcntl(2)).
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53       *  The prctl(2) PR_SET_PDEATHSIG setting is reset  so  that  the  child
54          does not receive a signal when its parent terminates.
55
56       *  The  default  timer slack value is set to the parent's current timer
57          slack value.  See the description of PR_SET_TIMERSLACK in prctl(2).
58
59       *  Memory mappings that have been marked with the madvise(2) MADV_DONT‐
60          FORK flag are not inherited across a fork().
61
62       *  The   termination  signal  of  the  child  is  always  SIGCHLD  (see
63          clone(2)).
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65       *  The port access permission bits set by ioperm(2) are  not  inherited
66          by the child; the child must turn on any bits that it requires using
67          ioperm(2).
68
69       Note the following further points:
70
71       *  The child process is created  with  a  single  thread—the  one  that
72          called  fork().   The  entire virtual address space of the parent is
73          replicated in the child, including the states of mutexes,  condition
74          variables,  and other pthreads objects; the use of pthread_atfork(3)
75          may be helpful for dealing with problems that this can cause.
76
77       *  The child inherits copies of the parent's set of open file  descrip‐
78          tors.   Each  file  descriptor  in the child refers to the same open
79          file description (see open(2)) as the corresponding file  descriptor
80          in  the parent.  This means that the two descriptors share open file
81          status flags, current file offset, and signal-driven I/O  attributes
82          (see the description of F_SETOWN and F_SETSIG in fcntl(2)).
83
84       *  The  child inherits copies of the parent's set of open message queue
85          descriptors (see mq_overview(7)).   Each  descriptor  in  the  child
86          refers to the same open message queue description as the correspond‐
87          ing descriptor in the parent.  This means that the  two  descriptors
88          share the same flags (mq_flags).
89
90       *  The  child  inherits  copies  of  the parent's set of open directory
91          streams (see opendir(3)).  POSIX.1-2001 says that the  corresponding
92          directory  streams  in  the parent and child may share the directory
93          stream positioning; on Linux/glibc they do not.
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RETURN VALUE

96       On success, the PID of the child process is returned in the parent, and
97       0  is returned in the child.  On failure, -1 is returned in the parent,
98       no child process is created, and errno is set appropriately.
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ERRORS

101       EAGAIN fork() cannot allocate sufficient memory to  copy  the  parent's
102              page tables and allocate a task structure for the child.
103
104       EAGAIN It was not possible to create a new process because the caller's
105              RLIMIT_NPROC resource limit was  encountered.   To  exceed  this
106              limit,  the  process  must  have either the CAP_SYS_ADMIN or the
107              CAP_SYS_RESOURCE capability.
108
109       ENOMEM fork()  failed  to  allocate  the  necessary  kernel  structures
110              because memory is tight.
111
112       ENOSYS fork()  is not supported on this platform (for example, hardware
113              without a Memory-Management Unit).
114

CONFORMING TO

116       SVr4, 4.3BSD, POSIX.1-2001.
117

NOTES

119       Under Linux, fork() is implemented using copy-on-write  pages,  so  the
120       only  penalty  that it incurs is the time and memory required to dupli‐
121       cate the parent's page tables, and to create a  unique  task  structure
122       for the child.
123
124       Since  version  2.3.3,  rather than invoking the kernel's fork() system
125       call, the glibc fork() wrapper that is provided as  part  of  the  NPTL
126       threading  implementation  invokes clone(2) with flags that provide the
127       same effect as the traditional system  call.   (A  call  to  fork()  is
128       equivalent  to  a  call  to clone(2) specifying flags as just SIGCHLD.)
129       The glibc wrapper invokes any fork handlers that have been  established
130       using pthread_atfork(3).
131

EXAMPLE

133       See pipe(2) and wait(2).
134

SEE ALSO

136       clone(2),   execve(2),  exit(2),  setrlimit(2),  unshare(2),  vfork(2),
137       wait(2), daemon(3), capabilities(7), credentials(7)
138

COLOPHON

140       This page is part of release 3.53 of the Linux  man-pages  project.   A
141       description  of  the project, and information about reporting bugs, can
142       be found at http://www.kernel.org/doc/man-pages/.
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146Linux                             2013-03-12                           FORK(2)
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