man2/fork.2

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   3 .\" Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
   4 .\" and Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu),
   5 .\" March 28, 1992
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  26 .\"
  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@gmx.net>
  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 2006-09-04 "Linux 2.6.17" "Linux Programmer's Manual"
  40 .SH NAME
  41 fork \- create a child process
  42 .SH SYNOPSIS
  43 .B #include <sys/types.h>
  44 .br
  45 .B #include <unistd.h>
  46 .sp
  47 .B pid_t fork(void);
  48 .SH DESCRIPTION
  49 .BR fork ()
  50 creates a new process by duplicating the calling process.
  51 The new process, referred to as the \fIchild\fP,
  52 is an exact duplicate of the calling process,
  53 referred to as the \fIparent\fP, except for the following points:
  54 .IP * 4
  55 The child has its own unique process ID,
  56 and this PID does not match the ID of any existing process group
  57 .RB ( setpgid (2)).
  58 .IP * 4
  59 The child's parent process ID is the same as the parent's process ID.
  60 .IP * 4
  61 The child does not inherit its parent's memory locks
  62 .RB ( mlock (2),
  63 .BR mlockall (2)).
  64 .IP * 4
  65 Process resource utilisations
  66 .RB ( getrusage (2))
  67 and CPU time counters
  68 .RB ( times (2))
  69 are reset to zero in the child.
  70 .IP * 4
  71 The child set of pending signals is initially empty
  72 .RB ( sigpending (2)).
  73 .IP * 4
  74 The child does not inherit semaphore adjustments from its parent
  75 .RB ( semop (2)).
  76 .IP * 4
  77 The child does not inherit record locks from its parent
  78 .RB ( fcntl (2)).
  79 .IP * 4
  80 The parent does not inherit timers from its parent
  81 .RB ( setitimer (2)
  82 .BR alarm (3),
  83 .BR timer_create (3)).
  84 .IP * 4
  85 The child does not inherit outstanding asynchronous I/O operations
  86 from its parent
  87 .RB ( aio_read (3),
  88 .BR aio_write (3)).
  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 * 4
  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 * 4
 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 * 4
 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 (2).
 113 .IP * 4
 114 The termination signal of the child is always SIGCHLD
 115 (see
 116 .BR clone (2)).
 117 .PP
 118 Note the following further points:
 119 .IP * 4
 120 The child process is created with a single thread \(em the
 121 one that called
 122 .BR fork (2).
 123 The entire virtual address space of the parent is replicated in the child,
 124 including the states of mutexes, condition variables,
 125 and other pthreads objects; the use of
 126 .BR pthread_atfork (3)
 127 may be helpful for dealing with problems that this can cause.
 128 .IP * 4
 129 The child inherits copies of the parent's set of open file descriptors.
 130 Each file descriptor in the child refers to the same
 131 open file description (see
 132 .BR open (2))
 133 as the corresponding file descriptor in the parent.
 134 This means that the two descriptors share open file status flags,
 135 current file offset,
 136 and signal-driven I/O attributes (see the description of
 137 .B F_SETOWN
 138 and
 139 .B F_SETSIG
 140 in
 141 .BR fcntl (2)).
 142 .IP * 4
 143 The child inherits copies of the parent's set of open message
 144 queue descriptors (see
 145 .BR mq_overview (7)).
 146 Each descriptor in the child refers to the same
 147 open message queue description
 148 as the corresponding descriptor in the parent.
 149 This means that the two descriptors share the same flags
 150 .RI ( mq_flags ).
 151 .SH "RETURN VALUE"
 152 On success, the PID of the child process is returned in the parent's thread
 153 of execution, and a 0 is returned in the child's thread of execution.
 154 On failure, a \-1 will be returned in the parent's context,
 155 no child process will be created, and
 156 .I errno
 157 will be set appropriately.
 158 .SH ERRORS
 159 .TP
 160 .B EAGAIN
 161 .BR fork ()
 162 cannot allocate sufficient memory to copy the parent's page tables and
 163 allocate a task structure for the child.
 164 .TP
 165 .B EAGAIN
 166 It was not possible to create a new process because the caller's
 167 .B RLIMIT_NPROC
 168 resource limit was encountered.
 169 To exceed this limit, the process must have either the
 170 .BR CAP_SYS_ADMIN
 171 or the
 172 .BR CAP_SYS_RESOURCE
 173 capability.
 174 .TP
 175 .B ENOMEM
 176 .BR fork ()
 177 failed to allocate the necessary kernel structures because memory is tight.
 178 .SH "CONFORMING TO"
 179 SVr4, 4.3BSD, POSIX.1-2001.
 180 .SH EXAMPLE
 181 See
 182 .BR pipe (2)
 183 and
 184 .BR wait (2).
 185 .SH NOTES
 186 .PP
 187 Under Linux,
 188 .BR fork ()
 189 is implemented using copy-on-write pages, so the only penalty that it incurs
 190 is the time and memory required to duplicate the parent's page tables,
 191 and to create a unique task structure for the child.
 192 .SH "SEE ALSO"
 193 .BR clone (2),
 194 .BR execve (2),
 195 .BR setrlimit (2),
 196 .BR unshare (2),
 197 .BR vfork (2),
 198 .BR wait (2),
 199 .BR capabilities (7)