man2/fork.2

   1 .\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" A few fragments remain from an earlier (1992) page by
   3 .\" Drew Eckhardt (drew@cs.colorado.edu),
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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@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 2016-03-15 "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 is referred to as the
  50 .I child
  51 process.
  52 The calling process is referred to as the
  53 .I parent
  54 process.
  55
  56 The child process and the parent process run in separate memory spaces.
  57 At the time of
  58 .BR fork ()
  59 both memory spaces have the same content.
  60 Memory writes, file mappings
  61 .RB ( mmap (2)),
  62 and unmappings
  63 .RB ( munmap (2))
  64 performed by one of the processes do not affect the other.
  65
  66 The child process is an exact duplicate of the parent
  67 process except for the following points:
  68 .IP * 3
  69 The child has its own unique process ID,
  70 and this PID does not match the ID of any existing process group
  71 .RB ( setpgid (2)).
  72 .IP *
  73 The child's parent process ID is the same as the parent's process ID.
  74 .IP *
  75 The child does not inherit its parent's memory locks
  76 .RB ( mlock (2),
  77 .BR mlockall (2)).
  78 .IP *
  79 Process resource utilizations
  80 .RB ( getrusage (2))
  81 and CPU time counters
  82 .RB ( times (2))
  83 are reset to zero in the child.
  84 .IP *
  85 The child's set of pending signals is initially empty
  86 .RB ( sigpending (2)).
  87 .IP *
  88 The child does not inherit semaphore adjustments from its parent
  89 .RB ( semop (2)).
  90 .IP *
  91 The child does not inherit process-associated record locks from its parent
  92 .RB ( fcntl (2)).
  93 (On the other hand, it does inherit
  94 .BR fcntl (2)
  95 open file description locks and
  96 .BR flock (2)
  97 locks from its parent.)
  98 .IP *
  99 The child does not inherit timers from its parent
 100 .RB ( setitimer (2),
 101 .BR alarm (2),
 102 .BR timer_create (2)).
 103 .IP *
 104 The child does not inherit outstanding asynchronous I/O operations
 105 from its parent
 106 .RB ( aio_read (3),
 107 .BR aio_write (3)),
 108 nor does it inherit any asynchronous I/O contexts from its parent (see
 109 .BR io_setup (2)).
 110 .PP
 111 The process attributes in the preceding list are all specified
 112 in POSIX.1.
 113 The parent and child also differ with respect to the following
 114 Linux-specific process attributes:
 115 .IP * 3
 116 The child does not inherit directory change notifications (dnotify)
 117 from its parent
 118 (see the description of
 119 .B F_NOTIFY
 120 in
 121 .BR fcntl (2)).
 122 .IP *
 123 The
 124 .BR prctl (2)
 125 .B PR_SET_PDEATHSIG
 126 setting is reset so that the child does not receive a signal
 127 when its parent terminates.
 128 .IP *
 129 The default timer slack value is set to the parent's
 130 current timer slack value.
 131 See the description of
 132 .BR PR_SET_TIMERSLACK
 133 in
 134 .BR prctl (2).
 135 .IP *
 136 Memory mappings that have been marked with the
 137 .BR madvise (2)
 138 .B MADV_DONTFORK
 139 flag are not inherited across a
 140 .BR fork ().
 141 .IP *
 142 The termination signal of the child is always
 143 .B SIGCHLD
 144 (see
 145 .BR clone (2)).
 146 .IP *
 147 The port access permission bits set by
 148 .BR ioperm (2)
 149 are not inherited by the child;
 150 the child must turn on any bits that it requires using
 151 .BR ioperm (2).
 152 .PP
 153 Note the following further points:
 154 .IP * 3
 155 The child process is created with a single thread\(emthe
 156 one that called
 157 .BR fork ().
 158 The entire virtual address space of the parent is replicated in the child,
 159 including the states of mutexes, condition variables,
 160 and other pthreads objects; the use of
 161 .BR pthread_atfork (3)
 162 may be helpful for dealing with problems that this can cause.
 163 .IP *
 164 After a
 165 .BR fork (2)
 166 in a multithreaded program,
 167 the child can safely call only async-signal-safe functions (see
 168 .BR signal (7))
 169 until such time as it calls
 170 .BR execve (2).
 171 .IP *
 172 The child inherits copies of the parent's set of open file descriptors.
 173 Each file descriptor in the child refers to the same
 174 open file description (see
 175 .BR open (2))
 176 as the corresponding file descriptor in the parent.
 177 This means that the two file descriptors share open file status flags,
 178 file offset,
 179 and signal-driven I/O attributes (see the description of
 180 .B F_SETOWN
 181 and
 182 .B F_SETSIG
 183 in
 184 .BR fcntl (2)).
 185 .IP *
 186 The child inherits copies of the parent's set of open message
 187 queue descriptors (see
 188 .BR mq_overview (7)).
 189 Each file descriptor in the child refers to the same
 190 open message queue description
 191 as the corresponding file descriptor in the parent.
 192 This means that the two file descriptors share the same flags
 193 .RI ( mq_flags ).
 194 .IP *
 195 The child inherits copies of the parent's set of open directory streams (see
 196 .BR opendir (3)).
 197 POSIX.1 says that the corresponding directory streams
 198 in the parent and child
 199 .I may
 200 share the directory stream positioning;
 201 on Linux/glibc they do not.
 202 .SH RETURN VALUE
 203 On success, the PID of the child process is returned in the parent,
 204 and 0 is returned in the child.
 205 On failure, \-1 is returned in the parent,
 206 no child process is created, and
 207 .I errno
 208 is set appropriately.
 209 .SH ERRORS
 210 .TP
 211 .B EAGAIN
 212 \" NOTE! The following should match the description in pthread_create(3)
 213 A system-imposed limit on the number of threads was encountered.
 214 There are a number of limits that may trigger this error: the
 215 .BR RLIMIT_NPROC
 216 soft resource limit (set via
 217 .BR setrlimit (2)),
 218 which limits the number of processes and threads for a real user ID,
 219 was reached;
 220 the kernel's system-wide limit on the number of processes and threads,
 221 .IR /proc/sys/kernel/threads-max ,
 222 was reached (see
 223 .BR proc (5));
 224 or the maximum number of PIDs,
 225 .IR /proc/sys/kernel/pid_max ,
 226 was reached (see
 227 .BR proc (5)).
 228 .TP
 229 .B EAGAIN
 230 The caller is operating under the
 231 .BR SCHED_DEADLINE
 232 scheduling policy and does not have the reset-on-fork flag set.
 233 See
 234 .BR sched (7).
 235 .TP
 236 .B ENOMEM
 237 .BR fork ()
 238 failed to allocate the necessary kernel structures because memory is tight.
 239 .TP
 240 .B ENOMEM
 241 An attempt was made to create a child process in a PID namespace
 242 whose "init" process has terminated.
 243 See
 244 .BR pid_namespaces (7).
 245 .TP
 246 .B ENOSYS
 247 .BR fork ()
 248 is not supported on this platform (for example,
 249 .\" e.g., arm (optionally), blackfin, c6x, frv, h8300, microblaze, xtensa
 250 hardware without a Memory-Management Unit).
 251 .TP
 252 .BR ERESTARTNOINTR " (since Linux 2.6.17)"
 253 .\" commit 4a2c7a7837da1b91468e50426066d988050e4d56
 254 System call was interrupted by a signal and will be restarted.
 255 (This can be seen only during a trace.)
 256 .SH CONFORMING TO
 257 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 258 .SH NOTES
 259 .PP
 260 Under Linux,
 261 .BR fork ()
 262 is implemented using copy-on-write pages, so the only penalty that it incurs
 263 is the time and memory required to duplicate the parent's page tables,
 264 and to create a unique task structure for the child.
 265 .SS C library/kernel differences
 266 Since version 2.3.3,
 267 .\" nptl/sysdeps/unix/sysv/linux/fork.c
 268 rather than invoking the kernel's
 269 .BR fork ()
 270 system call,
 271 the glibc
 272 .BR fork ()
 273 wrapper that is provided as part of the
 274 NPTL threading implementation invokes
 275 .BR clone (2)
 276 with flags that provide the same effect as the traditional system call.
 277 (A call to
 278 .BR fork ()
 279 is equivalent to a call to
 280 .BR clone (2)
 281 specifying
 282 .I flags
 283 as just
 284 .BR SIGCHLD .)
 285 The glibc wrapper invokes any fork handlers that have been
 286 established using
 287 .BR pthread_atfork (3).
 288 .\" and does some magic to ensure that getpid(2) returns the right value.
 289 .SH EXAMPLE
 290 See
 291 .BR pipe (2)
 292 and
 293 .BR wait (2).
 294 .SH SEE ALSO
 295 .BR clone (2),
 296 .BR execve (2),
 297 .BR exit (2),
 298 .BR setrlimit (2),
 299 .BR unshare (2),
 300 .BR vfork (2),
 301 .BR wait (2),
 302 .BR daemon (3),
 303 .BR capabilities (7),
 304 .BR credentials (7)