man2/fork.2

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   3 .\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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  27 .\" Modified by Michael Haardt (michael@moria.de)
  28 .\" Modified Sat Jul 24 13:22:07 1993 by Rik Faith (faith@cs.unc.edu)
  29 .\" Modified 21 Aug 1994 by Michael Chastain (mec@shell.portal.com):
  30 .\"   Referenced 'clone(2)'.
  31 .\" Modified 1995-06-10, 1996-04-18, 1999-11-01, 2000-12-24
  32 .\"   by Andries Brouwer (aeb@cwi.nl)
  33 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Added notes on capability requirements
  35 .\" 2006-09-04, Michael Kerrisk
  36 .\"     Greatly expanded, to describe all attributes that differ
  37 .\"     parent and child.
  38 .\"
  39 .TH FORK 2 2009-04-27 "Linux" "Linux Programmer's Manual"
  40 .SH NAME
  41 fork \- create a child process
  42 .SH SYNOPSIS
  43 .B #include <unistd.h>
  44 .sp
  45 .B pid_t fork(void);
  46 .SH DESCRIPTION
  47 .BR fork ()
  48 creates a new process by duplicating the calling process.
  49 The new process, referred to as the \fIchild\fP,
  50 is an exact duplicate of the calling process,
  51 referred to as the \fIparent\fP, except for the following points:
  52 .IP * 3
  53 The child has its own unique process ID,
  54 and this PID does not match the ID of any existing process group
  55 .RB ( setpgid (2)).
  56 .IP *
  57 The child's parent process ID is the same as the parent's process ID.
  58 .IP *
  59 The child does not inherit its parent's memory locks
  60 .RB ( mlock (2),
  61 .BR mlockall (2)).
  62 .IP *
  63 Process resource utilizations
  64 .RB ( getrusage (2))
  65 and CPU time counters
  66 .RB ( times (2))
  67 are reset to zero in the child.
  68 .IP *
  69 The child's set of pending signals is initially empty
  70 .RB ( sigpending (2)).
  71 .IP *
  72 The child does not inherit semaphore adjustments from its parent
  73 .RB ( semop (2)).
  74 .IP *
  75 The child does not inherit record locks from its parent
  76 .RB ( fcntl (2)).
  77 .IP *
  78 The child does not inherit timers from its parent
  79 .RB ( setitimer (2),
  80 .BR alarm (2),
  81 .BR timer_create (2)).
  82 .IP *
  83 The child does not inherit outstanding asynchronous I/O operations
  84 from its parent
  85 .RB ( aio_read (3),
  86 .BR aio_write (3)),
  87 nor does it inherit any asynchronous I/O contexts from its parent (see
  88 .BR io_setup (2)).
  89 .PP
  90 The process attributes in the preceding list are all specified
  91 in POSIX.1-2001.
  92 The parent and child also differ with respect to the following
  93 Linux-specific process attributes:
  94 .IP * 3
  95 The child does not inherit directory change notifications (dnotify)
  96 from its parent
  97 (see the description of
  98 .B F_NOTIFY
  99 in
 100 .BR fcntl (2)).
 101 .IP *
 102 The
 103 .BR prctl (2)
 104 .B PR_SET_PDEATHSIG
 105 setting is reset so that the child does not receive a signal
 106 when its parent terminates.
 107 .IP *
 108 Memory mappings that have been marked with the
 109 .BR madvise (2)
 110 .B MADV_DONTFORK
 111 flag are not inherited across a
 112 .BR fork ().
 113 .IP *
 114 The termination signal of the child is always
 115 .B SIGCHLD
 116 (see
 117 .BR clone (2)).
 118 .PP
 119 Note the following further points:
 120 .IP * 3
 121 The child process is created with a single thread\(emthe
 122 one that called
 123 .BR fork ().
 124 The entire virtual address space of the parent is replicated in the child,
 125 including the states of mutexes, condition variables,
 126 and other pthreads objects; the use of
 127 .BR pthread_atfork (3)
 128 may be helpful for dealing with problems that this can cause.
 129 .IP *
 130 The child inherits copies of the parent's set of open file descriptors.
 131 Each file descriptor in the child refers to the same
 132 open file description (see
 133 .BR open (2))
 134 as the corresponding file descriptor in the parent.
 135 This means that the two descriptors share open file status flags,
 136 current file offset,
 137 and signal-driven I/O attributes (see the description of
 138 .B F_SETOWN
 139 and
 140 .B F_SETSIG
 141 in
 142 .BR fcntl (2)).
 143 .IP *
 144 The child inherits copies of the parent's set of open message
 145 queue descriptors (see
 146 .BR mq_overview (7)).
 147 Each descriptor in the child refers to the same
 148 open message queue description
 149 as the corresponding descriptor in the parent.
 150 This means that the two descriptors share the same flags
 151 .RI ( mq_flags ).
 152 .IP *
 153 The child inherits copies of the parent's set of open directory streams (see
 154 .BR opendir (3)).
 155 POSIX.1-2001 says that the corresponding directory streams
 156 in the parent and child
 157 .I may
 158 share the directory stream positioning;
 159 on Linux/glibc they do not.
 160 .SH "RETURN VALUE"
 161 On success, the PID of the child process is returned in the parent,
 162 and 0 is returned in the child.
 163 On failure, \-1 is returned in the parent,
 164 no child process is created, and
 165 .I errno
 166 is set appropriately.
 167 .SH ERRORS
 168 .TP
 169 .B EAGAIN
 170 .BR fork ()
 171 cannot allocate sufficient memory to copy the parent's page tables and
 172 allocate a task structure for the child.
 173 .TP
 174 .B EAGAIN
 175 It was not possible to create a new process because the caller's
 176 .B RLIMIT_NPROC
 177 resource limit was encountered.
 178 To exceed this limit, the process must have either the
 179 .B CAP_SYS_ADMIN
 180 or the
 181 .B CAP_SYS_RESOURCE
 182 capability.
 183 .TP
 184 .B ENOMEM
 185 .BR fork ()
 186 failed to allocate the necessary kernel structures because memory is tight.
 187 .SH "CONFORMING TO"
 188 SVr4, 4.3BSD, POSIX.1-2001.
 189 .SH NOTES
 190 .PP
 191 Under Linux,
 192 .BR fork ()
 193 is implemented using copy-on-write pages, so the only penalty that it incurs
 194 is the time and memory required to duplicate the parent's page tables,
 195 and to create a unique task structure for the child.
 196
 197 Since version 2.3.3,
 198 .\" nptl/sysdeps/unix/sysv/linux/fork.c
 199 rather than invoking the kernel's
 200 .BR fork ()
 201 system call,
 202 the glibc
 203 .BR fork ()
 204 wrapper that is provided as part of the
 205 NPTL threading implementation invokes
 206 .BR clone (2)
 207 with flags that provide the same effect as the traditional system call.
 208 The glibc wrapper invokes any fork handlers that have been
 209 established using
 210 .BR pthread_atfork (3).
 211 .\" and does some magic to ensure that getpid(2) returns the right value.
 212 .SH EXAMPLE
 213 See
 214 .BR pipe (2)
 215 and
 216 .BR wait (2).
 217 .SH "SEE ALSO"
 218 .BR clone (2),
 219 .BR execve (2),
 220 .BR setrlimit (2),
 221 .BR unshare (2),
 222 .BR vfork (2),
 223 .BR wait (2),
 224 .BR daemon (3),
 225 .BR capabilities (7),
 226 .BR credentials (7)