man2/clone.2

   1 .\" Copyright (c) 1992 Drew Eckhardt <drew@cs.colorado.edu>, March 28, 1992
   2 .\" and Copyright (c) Michael Kerrisk, 2001, 2002, 2005, 2013
   3 .\"
   4 .\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
   5 .\" May be distributed under the GNU General Public License.
   6 .\" %%%LICENSE_END
   7 .\"
   8 .\" Modified by Michael Haardt <michael@moria.de>
   9 .\" Modified 24 Jul 1993 by Rik Faith <faith@cs.unc.edu>
  10 .\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
  11 .\"   New man page (copied from 'fork.2').
  12 .\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
  13 .\" Modified 25 April 1998 by Xavier Leroy <Xavier.Leroy@inria.fr>
  14 .\" Modified 26 Jun 2001 by Michael Kerrisk
  15 .\"     Mostly upgraded to 2.4.x
  16 .\"     Added prototype for sys_clone() plus description
  17 .\"     Added CLONE_THREAD with a brief description of thread groups
  18 .\"     Added CLONE_PARENT and revised entire page remove ambiguity
  19 .\"             between "calling process" and "parent process"
  20 .\"     Added CLONE_PTRACE and CLONE_VFORK
  21 .\"     Added EPERM and EINVAL error codes
  22 .\"     Renamed "__clone" to "clone" (which is the prototype in <sched.h>)
  23 .\"     various other minor tidy ups and clarifications.
  24 .\" Modified 26 Jun 2001 by Michael Kerrisk <mtk.manpages@gmail.com>
  25 .\"     Updated notes for 2.4.7+ behavior of CLONE_THREAD
  26 .\" Modified 15 Oct 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
  27 .\"     Added description for CLONE_NEWNS, which was added in 2.4.19
  28 .\" Slightly rephrased, aeb.
  29 .\" Modified 1 Feb 2003 - added CLONE_SIGHAND restriction, aeb.
  30 .\" Modified 1 Jan 2004 - various updates, aeb
  31 .\" Modified 2004-09-10 - added CLONE_PARENT_SETTID etc. - aeb.
  32 .\" 2005-04-12, mtk, noted the PID caching behavior of NPTL's getpid()
  33 .\"     wrapper under BUGS.
  34 .\" 2005-05-10, mtk, added CLONE_SYSVSEM, CLONE_UNTRACED, CLONE_STOPPED.
  35 .\" 2005-05-17, mtk, Substantially enhanced discussion of CLONE_THREAD.
  36 .\" 2008-11-18, mtk, order CLONE_* flags alphabetically
  37 .\" 2008-11-18, mtk, document CLONE_NEWPID
  38 .\" 2008-11-19, mtk, document CLONE_NEWUTS
  39 .\" 2008-11-19, mtk, document CLONE_NEWIPC
  40 .\" 2008-11-19, Jens Axboe, mtk, document CLONE_IO
  41 .\"
  42 .TH CLONE 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  43 .SH NAME
  44 clone, __clone2 \- create a child process
  45 .SH SYNOPSIS
  46 .nf
  47 /* Prototype for the glibc wrapper function */
  48 .PP
  49 .B #define _GNU_SOURCE
  50 .B #include <sched.h>
  51 .PP
  52 .BI "int clone(int (*" "fn" ")(void *), void *" child_stack ,
  53 .BI "          int " flags ", void *" "arg" ", ... "
  54 .BI "          /* pid_t *" ptid ", void *" newtls \
  55 ", pid_t *" ctid " */ );"
  56 .PP
  57 /* For the prototype of the raw system call, see NOTES */
  58 .fi
  59 .SH DESCRIPTION
  60 .BR clone ()
  61 creates a new process, in a manner similar to
  62 .BR fork (2).
  63 .PP
  64 This page describes both the glibc
  65 .BR clone ()
  66 wrapper function and the underlying system call on which it is based.
  67 The main text describes the wrapper function;
  68 the differences for the raw system call
  69 are described toward the end of this page.
  70 .PP
  71 Unlike
  72 .BR fork (2),
  73 .BR clone ()
  74 allows the child process to share parts of its execution context with
  75 the calling process, such as the virtual address space, the table of file
  76 descriptors, and the table of signal handlers.
  77 (Note that on this manual
  78 page, "calling process" normally corresponds to "parent process".
  79 But see the description of
  80 .B CLONE_PARENT
  81 below.)
  82 .PP
  83 One use of
  84 .BR clone ()
  85 is to implement threads: multiple flows of control in a program that
  86 run concurrently in a shared address space.
  87 .PP
  88 When the child process is created with
  89 .BR clone (),
  90 it commences execution by calling the function pointed to by the argument
  91 .IR fn .
  92 (This differs from
  93 .BR fork (2),
  94 where execution continues in the child from the point
  95 of the
  96 .BR fork (2)
  97 call.)
  98 The
  99 .I arg
 100 argument is passed as the argument of the function
 101 .IR fn .
 102 .PP
 103 When the
 104 .IR fn ( arg )
 105 function returns, the child process terminates.
 106 The integer returned by
 107 .I fn
 108 is the exit status for the child process.
 109 The child process may also terminate explicitly by calling
 110 .BR exit (2)
 111 or after receiving a fatal signal.
 112 .PP
 113 The
 114 .I child_stack
 115 argument specifies the location of the stack used by the child process.
 116 Since the child and calling process may share memory,
 117 it is not possible for the child process to execute in the
 118 same stack as the calling process.
 119 The calling process must therefore
 120 set up memory space for the child stack and pass a pointer to this
 121 space to
 122 .BR clone ().
 123 Stacks grow downward on all processors that run Linux
 124 (except the HP PA processors), so
 125 .I child_stack
 126 usually points to the topmost address of the memory space set up for
 127 the child stack.
 128 .PP
 129 The low byte of
 130 .I flags
 131 contains the number of the
 132 .I "termination signal"
 133 sent to the parent when the child dies.
 134 If this signal is specified as anything other than
 135 .BR SIGCHLD ,
 136 then the parent process must specify the
 137 .B __WALL
 138 or
 139 .B __WCLONE
 140 options when waiting for the child with
 141 .BR wait (2).
 142 If no signal is specified, then the parent process is not signaled
 143 when the child terminates.
 144 .PP
 145 .I flags
 146 may also be bitwise-ORed with zero or more of the following constants,
 147 in order to specify what is shared between the calling process
 148 and the child process:
 149 .TP
 150 .BR CLONE_CHILD_CLEARTID " (since Linux 2.5.49)"
 151 Clear (zero) the child thread ID at the location
 152 .I ctid
 153 in child memory when the child exits, and do a wakeup on the futex
 154 at that address.
 155 The address involved may be changed by the
 156 .BR set_tid_address (2)
 157 system call.
 158 This is used by threading libraries.
 159 .TP
 160 .BR CLONE_CHILD_SETTID " (since Linux 2.5.49)"
 161 Store the child thread ID at the location
 162 .I ctid
 163 in the child's memory.
 164 The store operation completes before
 165 .BR clone ()
 166 returns control to user space.
 167 .TP
 168 .BR CLONE_FILES " (since Linux 2.0)"
 169 If
 170 .B CLONE_FILES
 171 is set, the calling process and the child process share the same file
 172 descriptor table.
 173 Any file descriptor created by the calling process or by the child
 174 process is also valid in the other process.
 175 Similarly, if one of the processes closes a file descriptor,
 176 or changes its associated flags (using the
 177 .BR fcntl (2)
 178 .B F_SETFD
 179 operation), the other process is also affected.
 180 If a process sharing a file descriptor table calls
 181 .BR execve (2),
 182 its file descriptor table is duplicated (unshared).
 183 .IP
 184 If
 185 .B CLONE_FILES
 186 is not set, the child process inherits a copy of all file descriptors
 187 opened in the calling process at the time of
 188 .BR clone ().
 189 Subsequent operations that open or close file descriptors,
 190 or change file descriptor flags,
 191 performed by either the calling
 192 process or the child process do not affect the other process.
 193 Note, however,
 194 that the duplicated file descriptors in the child refer to the same open file
 195 descriptions as the corresponding file descriptors in the calling process,
 196 and thus share file offsets and file status flags (see
 197 .BR open (2)).
 198 .TP
 199 .BR CLONE_FS " (since Linux 2.0)"
 200 If
 201 .B CLONE_FS
 202 is set, the caller and the child process share the same filesystem
 203 information.
 204 This includes the root of the filesystem, the current
 205 working directory, and the umask.
 206 Any call to
 207 .BR chroot (2),
 208 .BR chdir (2),
 209 or
 210 .BR umask (2)
 211 performed by the calling process or the child process also affects the
 212 other process.
 213 .IP
 214 If
 215 .B CLONE_FS
 216 is not set, the child process works on a copy of the filesystem
 217 information of the calling process at the time of the
 218 .BR clone ()
 219 call.
 220 Calls to
 221 .BR chroot (2),
 222 .BR chdir (2),
 223 or
 224 .BR umask (2)
 225 performed later by one of the processes do not affect the other process.
 226 .TP
 227 .BR CLONE_IO " (since Linux 2.6.25)"
 228 If
 229 .B CLONE_IO
 230 is set, then the new process shares an I/O context with
 231 the calling process.
 232 If this flag is not set, then (as with
 233 .BR fork (2))
 234 the new process has its own I/O context.
 235 .IP
 236 .\" The following based on text from Jens Axboe
 237 The I/O context is the I/O scope of the disk scheduler (i.e.,
 238 what the I/O scheduler uses to model scheduling of a process's I/O).
 239 If processes share the same I/O context,
 240 they are treated as one by the I/O scheduler.
 241 As a consequence, they get to share disk time.
 242 For some I/O schedulers,
 243 .\" the anticipatory and CFQ scheduler
 244 if two processes share an I/O context,
 245 they will be allowed to interleave their disk access.
 246 If several threads are doing I/O on behalf of the same process
 247 .RB ( aio_read (3),
 248 for instance), they should employ
 249 .BR CLONE_IO
 250 to get better I/O performance.
 251 .\" with CFQ and AS.
 252 .IP
 253 If the kernel is not configured with the
 254 .B CONFIG_BLOCK
 255 option, this flag is a no-op.
 256 .TP
 257 .BR CLONE_NEWCGROUP " (since Linux 4.6)"
 258 Create the process in a new cgroup namespace.
 259 If this flag is not set, then (as with
 260 .BR fork (2))
 261 the process is created in the same cgroup namespaces as the calling process.
 262 This flag is intended for the implementation of containers.
 263 .IP
 264 For further information on cgroup namespaces, see
 265 .BR cgroup_namespaces (7).
 266 .IP
 267 Only a privileged process
 268 .RB ( CAP_SYS_ADMIN )
 269 can employ
 270 .BR CLONE_NEWCGROUP .
 271 .\"
 272 .TP
 273 .BR CLONE_NEWIPC " (since Linux 2.6.19)"
 274 If
 275 .B CLONE_NEWIPC
 276 is set, then create the process in a new IPC namespace.
 277 If this flag is not set, then (as with
 278 .BR fork (2)),
 279 the process is created in the same IPC namespace as
 280 the calling process.
 281 This flag is intended for the implementation of containers.
 282 .IP
 283 An IPC namespace provides an isolated view of System\ V IPC objects (see
 284 .BR svipc (7))
 285 and (since Linux 2.6.30)
 286 .\" commit 7eafd7c74c3f2e67c27621b987b28397110d643f
 287 .\" https://lwn.net/Articles/312232/
 288 POSIX message queues
 289 (see
 290 .BR mq_overview (7)).
 291 The common characteristic of these IPC mechanisms is that IPC
 292 objects are identified by mechanisms other than filesystem
 293 pathnames.
 294 .IP
 295 Objects created in an IPC namespace are visible to all other processes
 296 that are members of that namespace,
 297 but are not visible to processes in other IPC namespaces.
 298 .IP
 299 When an IPC namespace is destroyed
 300 (i.e., when the last process that is a member of the namespace terminates),
 301 all IPC objects in the namespace are automatically destroyed.
 302 .IP
 303 Only a privileged process
 304 .RB ( CAP_SYS_ADMIN )
 305 can employ
 306 .BR CLONE_NEWIPC .
 307 This flag can't be specified in conjunction with
 308 .BR CLONE_SYSVSEM .
 309 .IP
 310 For further information on IPC namespaces, see
 311 .BR namespaces (7).
 312 .TP
 313 .BR CLONE_NEWNET " (since Linux 2.6.24)"
 314 (The implementation of this flag was completed only
 315 by about kernel version 2.6.29.)
 316 .IP
 317 If
 318 .B CLONE_NEWNET
 319 is set, then create the process in a new network namespace.
 320 If this flag is not set, then (as with
 321 .BR fork (2))
 322 the process is created in the same network namespace as
 323 the calling process.
 324 This flag is intended for the implementation of containers.
 325 .IP
 326 A network namespace provides an isolated view of the networking stack
 327 (network device interfaces, IPv4 and IPv6 protocol stacks,
 328 IP routing tables, firewall rules, the
 329 .I /proc/net
 330 and
 331 .I /sys/class/net
 332 directory trees, sockets, etc.).
 333 A physical network device can live in exactly one
 334 network namespace.
 335 A virtual network
 336 .RB ( veth (4))
 337 device pair provides a pipe-like abstraction
 338 that can be used to create tunnels between network namespaces,
 339 and can be used to create a bridge to a physical network device
 340 in another namespace.
 341 .IP
 342 When a network namespace is freed
 343 (i.e., when the last process in the namespace terminates),
 344 its physical network devices are moved back to the
 345 initial network namespace (not to the parent of the process).
 346 For further information on network namespaces, see
 347 .BR namespaces (7).
 348 .IP
 349 Only a privileged process
 350 .RB ( CAP_SYS_ADMIN )
 351 can employ
 352 .BR CLONE_NEWNET .
 353 .TP
 354 .BR CLONE_NEWNS " (since Linux 2.4.19)"
 355 If
 356 .B CLONE_NEWNS
 357 is set, the cloned child is started in a new mount namespace,
 358 initialized with a copy of the namespace of the parent.
 359 If
 360 .B CLONE_NEWNS
 361 is not set, the child lives in the same mount
 362 namespace as the parent.
 363 .IP
 364 Only a privileged process
 365 .RB ( CAP_SYS_ADMIN )
 366 can employ
 367 .BR CLONE_NEWNS .
 368 It is not permitted to specify both
 369 .B CLONE_NEWNS
 370 and
 371 .B CLONE_FS
 372 .\" See https://lwn.net/Articles/543273/
 373 in the same
 374 .BR clone ()
 375 call.
 376 .IP
 377 For further information on mount namespaces, see
 378 .BR namespaces (7)
 379 and
 380 .BR mount_namespaces (7).
 381 .TP
 382 .BR CLONE_NEWPID " (since Linux 2.6.24)"
 383 .\" This explanation draws a lot of details from
 384 .\" http://lwn.net/Articles/259217/
 385 .\" Authors: Pavel Emelyanov <xemul@openvz.org>
 386 .\" and Kir Kolyshkin <kir@openvz.org>
 387 .\"
 388 .\" The primary kernel commit is 30e49c263e36341b60b735cbef5ca37912549264
 389 .\" Author: Pavel Emelyanov <xemul@openvz.org>
 390 If
 391 .B CLONE_NEWPID
 392 is set, then create the process in a new PID namespace.
 393 If this flag is not set, then (as with
 394 .BR fork (2))
 395 the process is created in the same PID namespace as
 396 the calling process.
 397 This flag is intended for the implementation of containers.
 398 .IP
 399 For further information on PID namespaces, see
 400 .BR namespaces (7)
 401 and
 402 .BR pid_namespaces (7).
 403 .IP
 404 Only a privileged process
 405 .RB ( CAP_SYS_ADMIN )
 406 can employ
 407 .BR CLONE_NEWPID .
 408 This flag can't be specified in conjunction with
 409 .BR CLONE_THREAD
 410 or
 411 .BR CLONE_PARENT .
 412 .TP
 413 .BR CLONE_NEWUSER
 414 (This flag first became meaningful for
 415 .BR clone ()
 416 in Linux 2.6.23,
 417 the current
 418 .BR clone ()
 419 semantics were merged in Linux 3.5,
 420 and the final pieces to make the user namespaces completely usable were
 421 merged in Linux 3.8.)
 422 .IP
 423 If
 424 .B CLONE_NEWUSER
 425 is set, then create the process in a new user namespace.
 426 If this flag is not set, then (as with
 427 .BR fork (2))
 428 the process is created in the same user namespace as the calling process.
 429 .IP
 430 Before Linux 3.8, use of
 431 .BR CLONE_NEWUSER
 432 required that the caller have three capabilities:
 433 .BR CAP_SYS_ADMIN ,
 434 .BR CAP_SETUID ,
 435 and
 436 .BR CAP_SETGID .
 437 .\" Before Linux 2.6.29, it appears that only CAP_SYS_ADMIN was needed
 438 Starting with Linux 3.8,
 439 no privileges are needed to create a user namespace.
 440 .IP
 441 This flag can't be specified in conjunction with
 442 .BR CLONE_THREAD
 443 or
 444 .BR CLONE_PARENT .
 445 For security reasons,
 446 .\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
 447 .\" https://lwn.net/Articles/543273/
 448 .\" The fix actually went into 3.9 and into 3.8.3. However, user namespaces
 449 .\" were, for practical purposes, unusable in earlier 3.8.x because of the
 450 .\" various filesystems that didn't support userns.
 451 .BR CLONE_NEWUSER
 452 cannot be specified in conjunction with
 453 .BR CLONE_FS .
 454 .IP
 455 For further information on user namespaces, see
 456 .BR namespaces (7)
 457 and
 458 .BR user_namespaces (7).
 459 .TP
 460 .BR CLONE_NEWUTS " (since Linux 2.6.19)"
 461 If
 462 .B CLONE_NEWUTS
 463 is set, then create the process in a new UTS namespace,
 464 whose identifiers are initialized by duplicating the identifiers
 465 from the UTS namespace of the calling process.
 466 If this flag is not set, then (as with
 467 .BR fork (2))
 468 the process is created in the same UTS namespace as
 469 the calling process.
 470 This flag is intended for the implementation of containers.
 471 .IP
 472 A UTS namespace is the set of identifiers returned by
 473 .BR uname (2);
 474 among these, the domain name and the hostname can be modified by
 475 .BR setdomainname (2)
 476 and
 477 .BR sethostname (2),
 478 respectively.
 479 Changes made to the identifiers in a UTS namespace
 480 are visible to all other processes in the same namespace,
 481 but are not visible to processes in other UTS namespaces.
 482 .IP
 483 Only a privileged process
 484 .RB ( CAP_SYS_ADMIN )
 485 can employ
 486 .BR CLONE_NEWUTS .
 487 .IP
 488 For further information on UTS namespaces, see
 489 .BR namespaces (7).
 490 .TP
 491 .BR CLONE_PARENT " (since Linux 2.3.12)"
 492 If
 493 .B CLONE_PARENT
 494 is set, then the parent of the new child (as returned by
 495 .BR getppid (2))
 496 will be the same as that of the calling process.
 497 .IP
 498 If
 499 .B CLONE_PARENT
 500 is not set, then (as with
 501 .BR fork (2))
 502 the child's parent is the calling process.
 503 .IP
 504 Note that it is the parent process, as returned by
 505 .BR getppid (2),
 506 which is signaled when the child terminates, so that
 507 if
 508 .B CLONE_PARENT
 509 is set, then the parent of the calling process, rather than the
 510 calling process itself, will be signaled.
 511 .TP
 512 .BR CLONE_PARENT_SETTID " (since Linux 2.5.49)"
 513 Store the child thread ID at the location
 514 .I ptid
 515 in the parent's memory.
 516 (In Linux 2.5.32-2.5.48 there was a flag
 517 .B CLONE_SETTID
 518 that did this.)
 519 The store operation completes before
 520 .BR clone ()
 521 returns control to user space.
 522 .TP
 523 .BR CLONE_PID " (Linux 2.0 to 2.5.15)"
 524 If
 525 .B CLONE_PID
 526 is set, the child process is created with the same process ID as
 527 the calling process.
 528 This is good for hacking the system, but otherwise
 529 of not much use.
 530 From Linux 2.3.21 onward, this flag could be
 531 specified only by the system boot process (PID 0).
 532 The flag disappeared completely from the kernel sources in Linux 2.5.16.
 533 Since then, the kernel silently ignores this bit if it is specified in
 534 .IR flags .
 535 .TP
 536 .BR CLONE_PTRACE " (since Linux 2.2)"
 537 If
 538 .B CLONE_PTRACE
 539 is specified, and the calling process is being traced,
 540 then trace the child also (see
 541 .BR ptrace (2)).
 542 .TP
 543 .BR CLONE_SETTLS " (since Linux 2.5.32)"
 544 The TLS (Thread Local Storage) descriptor is set to
 545 .IR newtls .
 546 .IP
 547 The interpretation of
 548 .I newtls
 549 and the resulting effect is architecture dependent.
 550 On x86,
 551 .I newtls
 552 is interpreted as a
 553 .IR "struct user_desc\ *"
 554 (see
 555 .BR set_thread_area (2)).
 556 On x86-64 it is the new value to be set for the %fs base register
 557 (see the
 558 .B ARCH_SET_FS
 559 argument to
 560 .BR arch_prctl (2)).
 561 On architectures with a dedicated TLS register, it is the new value
 562 of that register.
 563 .TP
 564 .BR CLONE_SIGHAND " (since Linux 2.0)"
 565 If
 566 .B CLONE_SIGHAND
 567 is set, the calling process and the child process share the same table of
 568 signal handlers.
 569 If the calling process or child process calls
 570 .BR sigaction (2)
 571 to change the behavior associated with a signal, the behavior is
 572 changed in the other process as well.
 573 However, the calling process and child
 574 processes still have distinct signal masks and sets of pending
 575 signals.
 576 So, one of them may block or unblock signals using
 577 .BR sigprocmask (2)
 578 without affecting the other process.
 579 .IP
 580 If
 581 .B CLONE_SIGHAND
 582 is not set, the child process inherits a copy of the signal handlers
 583 of the calling process at the time
 584 .BR clone ()
 585 is called.
 586 Calls to
 587 .BR sigaction (2)
 588 performed later by one of the processes have no effect on the other
 589 process.
 590 .IP
 591 Since Linux 2.6.0,
 592 .\" Precisely: Linux 2.6.0-test6
 593 .I flags
 594 must also include
 595 .B CLONE_VM
 596 if
 597 .B CLONE_SIGHAND
 598 is specified
 599 .TP
 600 .BR CLONE_STOPPED " (since Linux 2.6.0)"
 601 .\" Precisely: Linux 2.6.0-test2
 602 If
 603 .B CLONE_STOPPED
 604 is set, then the child is initially stopped (as though it was sent a
 605 .B SIGSTOP
 606 signal), and must be resumed by sending it a
 607 .B SIGCONT
 608 signal.
 609 .IP
 610 This flag was
 611 .I deprecated
 612 from Linux 2.6.25 onward,
 613 and was
 614 .I removed
 615 altogether in Linux 2.6.38.
 616 Since then, the kernel silently ignores it without error.
 617 .\" glibc 2.8 removed this defn from bits/sched.h
 618 Starting with Linux 4.6, the same bit was reused for the
 619 .BR CLONE_NEWCGROUP
 620 flag.
 621 .TP
 622 .BR CLONE_SYSVSEM " (since Linux 2.5.10)"
 623 If
 624 .B CLONE_SYSVSEM
 625 is set, then the child and the calling process share
 626 a single list of System V semaphore adjustment
 627 .RI ( semadj )
 628 values (see
 629 .BR semop (2)).
 630 In this case, the shared list accumulates
 631 .I semadj
 632 values across all processes sharing the list,
 633 and semaphore adjustments are performed only when the last process
 634 that is sharing the list terminates (or ceases sharing the list using
 635 .BR unshare (2)).
 636 If this flag is not set, then the child has a separate
 637 .I semadj
 638 list that is initially empty.
 639 .TP
 640 .BR CLONE_THREAD " (since Linux 2.4.0)"
 641 .\" Precisely: Linux 2.6.0-test8
 642 If
 643 .B CLONE_THREAD
 644 is set, the child is placed in the same thread group as the calling process.
 645 To make the remainder of the discussion of
 646 .B CLONE_THREAD
 647 more readable, the term "thread" is used to refer to the
 648 processes within a thread group.
 649 .IP
 650 Thread groups were a feature added in Linux 2.4 to support the
 651 POSIX threads notion of a set of threads that share a single PID.
 652 Internally, this shared PID is the so-called
 653 thread group identifier (TGID) for the thread group.
 654 Since Linux 2.4, calls to
 655 .BR getpid (2)
 656 return the TGID of the caller.
 657 .IP
 658 The threads within a group can be distinguished by their (system-wide)
 659 unique thread IDs (TID).
 660 A new thread's TID is available as the function result
 661 returned to the caller of
 662 .BR clone (),
 663 and a thread can obtain
 664 its own TID using
 665 .BR gettid (2).
 666 .IP
 667 When a call is made to
 668 .BR clone ()
 669 without specifying
 670 .BR CLONE_THREAD ,
 671 then the resulting thread is placed in a new thread group
 672 whose TGID is the same as the thread's TID.
 673 This thread is the
 674 .I leader
 675 of the new thread group.
 676 .IP
 677 A new thread created with
 678 .B CLONE_THREAD
 679 has the same parent process as the caller of
 680 .BR clone ()
 681 (i.e., like
 682 .BR CLONE_PARENT ),
 683 so that calls to
 684 .BR getppid (2)
 685 return the same value for all of the threads in a thread group.
 686 When a
 687 .B CLONE_THREAD
 688 thread terminates, the thread that created it using
 689 .BR clone ()
 690 is not sent a
 691 .B SIGCHLD
 692 (or other termination) signal;
 693 nor can the status of such a thread be obtained
 694 using
 695 .BR wait (2).
 696 (The thread is said to be
 697 .IR detached .)
 698 .IP
 699 After all of the threads in a thread group terminate
 700 the parent process of the thread group is sent a
 701 .B SIGCHLD
 702 (or other termination) signal.
 703 .IP
 704 If any of the threads in a thread group performs an
 705 .BR execve (2),
 706 then all threads other than the thread group leader are terminated,
 707 and the new program is executed in the thread group leader.
 708 .IP
 709 If one of the threads in a thread group creates a child using
 710 .BR fork (2),
 711 then any thread in the group can
 712 .BR wait (2)
 713 for that child.
 714 .IP
 715 Since Linux 2.5.35,
 716 .I flags
 717 must also include
 718 .B CLONE_SIGHAND
 719 if
 720 .B CLONE_THREAD
 721 is specified
 722 (and note that, since Linux 2.6.0,
 723 .\" Precisely: Linux 2.6.0-test6
 724 .BR CLONE_SIGHAND
 725 also requires
 726 .BR CLONE_VM
 727 to be included).
 728 .IP
 729 Signal dispositions and actions are process-wide:
 730 if an unhandled signal is delivered to a thread, then
 731 it will affect (terminate, stop, continue, be ignored in)
 732 all members of the thread group.
 733 .IP
 734 Each thread has its own signal mask, as set by
 735 .BR sigprocmask (2).
 736 .IP
 737 A signal may be process-directed or thread-directed.
 738 A process-directed signal is targeted at a thread group (i.e., a TGID),
 739 and is delivered to an arbitrarily selected thread from among those
 740 that are not blocking the signal.
 741 A signal may be process directed because it was generated by the kernel
 742 for reasons other than a hardware exception, or because it was sent using
 743 .BR kill (2)
 744 or
 745 .BR sigqueue (3).
 746 A thread-directed signal is targeted at (i.e., delivered to)
 747 a specific thread.
 748 A signal may be thread directed because it was sent using
 749 .BR tgkill (2)
 750 or
 751 .BR pthread_sigqueue (3),
 752 or because the thread executed a machine language instruction that triggered
 753 a hardware exception
 754 (e.g., invalid memory access triggering
 755 .BR SIGSEGV
 756 or a floating-point exception triggering
 757 .BR SIGFPE ).
 758 .IP
 759 A call to
 760 .BR sigpending (2)
 761 returns a signal set that is the union of the pending process-directed
 762 signals and the signals that are pending for the calling thread.
 763 .IP
 764 If a process-directed signal is delivered to a thread group,
 765 and the thread group has installed a handler for the signal, then
 766 the handler will be invoked in exactly one, arbitrarily selected
 767 member of the thread group that has not blocked the signal.
 768 If multiple threads in a group are waiting to accept the same signal using
 769 .BR sigwaitinfo (2),
 770 the kernel will arbitrarily select one of these threads
 771 to receive the signal.
 772 .TP
 773 .BR CLONE_UNTRACED " (since Linux 2.5.46)"
 774 If
 775 .B CLONE_UNTRACED
 776 is specified, then a tracing process cannot force
 777 .B CLONE_PTRACE
 778 on this child process.
 779 .TP
 780 .BR CLONE_VFORK " (since Linux 2.2)"
 781 If
 782 .B CLONE_VFORK
 783 is set, the execution of the calling process is suspended
 784 until the child releases its virtual memory
 785 resources via a call to
 786 .BR execve (2)
 787 or
 788 .BR _exit (2)
 789 (as with
 790 .BR vfork (2)).
 791 .IP
 792 If
 793 .B CLONE_VFORK
 794 is not set, then both the calling process and the child are schedulable
 795 after the call, and an application should not rely on execution occurring
 796 in any particular order.
 797 .TP
 798 .BR CLONE_VM " (since Linux 2.0)"
 799 If
 800 .B CLONE_VM
 801 is set, the calling process and the child process run in the same memory
 802 space.
 803 In particular, memory writes performed by the calling process
 804 or by the child process are also visible in the other process.
 805 Moreover, any memory mapping or unmapping performed with
 806 .BR mmap (2)
 807 or
 808 .BR munmap (2)
 809 by the child or calling process also affects the other process.
 810 .IP
 811 If
 812 .B CLONE_VM
 813 is not set, the child process runs in a separate copy of the memory
 814 space of the calling process at the time of
 815 .BR clone ().
 816 Memory writes or file mappings/unmappings performed by one of the
 817 processes do not affect the other, as with
 818 .BR fork (2).
 819 .SH NOTES
 820 Note that the glibc
 821 .BR clone ()
 822 wrapper function makes some changes
 823 in the memory pointed to by
 824 .I child_stack
 825 (changes required to set the stack up correctly for the child)
 826 .I before
 827 invoking the
 828 .BR clone ()
 829 system call.
 830 So, in cases where
 831 .BR clone ()
 832 is used to recursively create children,
 833 do not use the buffer employed for the parent's stack
 834 as the stack of the child.
 835 .\"
 836 .SS C library/kernel differences
 837 The raw
 838 .BR clone ()
 839 system call corresponds more closely to
 840 .BR fork (2)
 841 in that execution in the child continues from the point of the
 842 call.
 843 As such, the
 844 .I fn
 845 and
 846 .I arg
 847 arguments of the
 848 .BR clone ()
 849 wrapper function are omitted.
 850 .PP
 851 Another difference for the raw
 852 .BR clone ()
 853 system call is that the
 854 .I child_stack
 855 argument may be NULL,
 856 in which case the child uses a duplicate of the parent's stack.
 857 (Copy-on-write semantics ensure that the child gets separate copies
 858 of stack pages when either process modifies the stack.)
 859 In this case, for correct operation, the
 860 .B CLONE_VM
 861 option should not be specified.
 862 (If the child
 863 .I shares
 864 the parent's memory because of the use of the
 865 .BR CLONE_VM
 866 flag,
 867 then no copy-on-write duplication occurs and chaos is likely to result.)
 868 .PP
 869 The order of the arguments also differs in the raw system call,
 870 and there are variations in the arguments across architectures,
 871 as detailed in the following paragraphs.
 872 .PP
 873 The raw system call interface on x86-64 and some other architectures
 874 (including sh, tile, ia-64, and alpha) is:
 875 .PP
 876 .in +4
 877 .EX
 878 .BI "long clone(unsigned long " flags ", void *" child_stack ,
 879 .BI "           int *" ptid ", int *" ctid ,
 880 .BI "           unsigned long " newtls );
 881 .EE
 882 .in
 883 .PP
 884 On x86-32, and several other common architectures
 885 (including score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa,
 886 and MIPS),
 887 .\" CONFIG_CLONE_BACKWARDS
 888 the order of the last two arguments is reversed:
 889 .PP
 890 .in +4
 891 .EX
 892 .BI "long clone(unsigned long " flags ", void *" child_stack ,
 893 .BI "          int *" ptid ", unsigned long " newtls ,
 894 .BI "          int *" ctid );
 895 .EE
 896 .in
 897 .PP
 898 On the cris and s390 architectures,
 899 .\" CONFIG_CLONE_BACKWARDS2
 900 the order of the first two arguments is reversed:
 901 .PP
 902 .in +4
 903 .EX
 904 .BI "long clone(void *" child_stack ", unsigned long " flags ,
 905 .BI "           int *" ptid ", int *" ctid ,
 906 .BI "           unsigned long " newtls );
 907 .EE
 908 .in
 909 .PP
 910 On the microblaze architecture,
 911 .\" CONFIG_CLONE_BACKWARDS3
 912 an additional argument is supplied:
 913 .PP
 914 .in +4
 915 .EX
 916 .BI "long clone(unsigned long " flags ", void *" child_stack ,
 917 .BI "           int " stack_size , "\fR         /* Size of stack */"
 918 .BI "           int *" ptid ", int *" ctid ,
 919 .BI "           unsigned long " newtls );
 920 .EE
 921 .in
 922 .\"
 923 .SS blackfin, m68k, and sparc
 924 .\" Mike Frysinger noted in a 2013 mail:
 925 .\"     these arches don't define __ARCH_WANT_SYS_CLONE:
 926 .\"     blackfin ia64 m68k sparc
 927 The argument-passing conventions on
 928 blackfin, m68k, and sparc are different from the descriptions above.
 929 For details, see the kernel (and glibc) source.
 930 .SS ia64
 931 On ia64, a different interface is used:
 932 .PP
 933 .in +4
 934 .EX
 935 .BI "int __clone2(int (*" "fn" ")(void *), "
 936 .BI "             void *" child_stack_base ", size_t " stack_size ,
 937 .BI "             int " flags ", void *" "arg" ", ... "
 938 .BI "          /* pid_t *" ptid ", struct user_desc *" tls \
 939 ", pid_t *" ctid " */ );"
 940 .EE
 941 .in
 942 .PP
 943 The prototype shown above is for the glibc wrapper function;
 944 for the system call itself,
 945 the prototype can be described as follows (it is identical to the
 946 .BR clone ()
 947 prototype on microblaze):
 948 .PP
 949 .in +4
 950 .EX
 951 .BI "long clone2(unsigned long " flags ", void *" child_stack_base ,
 952 .BI "            int " stack_size , "\fR         /* Size of stack */"
 953 .BI "            int *" ptid ", int *" ctid ,
 954 .BI "            unsigned long " tls );
 955 .EE
 956 .in
 957 .PP
 958 .BR __clone2 ()
 959 operates in the same way as
 960 .BR clone (),
 961 except that
 962 .I child_stack_base
 963 points to the lowest address of the child's stack area,
 964 and
 965 .I stack_size
 966 specifies the size of the stack pointed to by
 967 .IR child_stack_base .
 968 .SS Linux 2.4 and earlier
 969 In Linux 2.4 and earlier,
 970 .BR clone ()
 971 does not take arguments
 972 .IR ptid ,
 973 .IR tls ,
 974 and
 975 .IR ctid .
 976 .SH RETURN VALUE
 977 .\" gettid(2) returns current->pid;
 978 .\" getpid(2) returns current->tgid;
 979 On success, the thread ID of the child process is returned
 980 in the caller's thread of execution.
 981 On failure, \-1 is returned
 982 in the caller's context, no child process will be created, and
 983 .I errno
 984 will be set appropriately.
 985 .SH ERRORS
 986 .TP
 987 .B EAGAIN
 988 Too many processes are already running; see
 989 .BR fork (2).
 990 .TP
 991 .B EINVAL
 992 .B CLONE_SIGHAND
 993 was specified, but
 994 .B CLONE_VM
 995 was not.
 996 (Since Linux 2.6.0.)
 997 .\" Precisely: Linux 2.6.0-test6
 998 .TP
 999 .B EINVAL
1000 .B CLONE_THREAD
1001 was specified, but
1002 .B CLONE_SIGHAND
1003 was not.
1004 (Since Linux 2.5.35.)
1005 .\" .TP
1006 .\" .B EINVAL
1007 .\" Precisely one of
1008 .\" .B CLONE_DETACHED
1009 .\" and
1010 .\" .B CLONE_THREAD
1011 .\" was specified.
1012 .\" (Since Linux 2.6.0-test6.)
1013 .TP
1014 .B EINVAL
1015 .B CLONE_THREAD
1016 was specified, but the current process previously called
1017 .BR unshare (2)
1018 with the
1019 .B CLONE_NEWPID
1020 flag or used
1021 .BR setns (2)
1022 to reassociate itself with a PID namespace.
1023 .TP
1024 .B EINVAL
1025 .\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
1026 Both
1027 .B CLONE_FS
1028 and
1029 .B CLONE_NEWNS
1030 were specified in
1031 .IR flags .
1032 .TP
1033 .BR EINVAL " (since Linux 3.9)"
1034 Both
1035 .B CLONE_NEWUSER
1036 and
1037 .B CLONE_FS
1038 were specified in
1039 .IR flags .
1040 .TP
1041 .B EINVAL
1042 Both
1043 .B CLONE_NEWIPC
1044 and
1045 .B CLONE_SYSVSEM
1046 were specified in
1047 .IR flags .
1048 .TP
1049 .B EINVAL
1050 One (or both) of
1051 .BR CLONE_NEWPID
1052 or
1053 .BR CLONE_NEWUSER
1054 and one (or both) of
1055 .BR CLONE_THREAD
1056 or
1057 .BR CLONE_PARENT
1058 were specified in
1059 .IR flags .
1060 .TP
1061 .B EINVAL
1062 Returned by the glibc
1063 .BR clone ()
1064 wrapper function when
1065 .IR fn
1066 or
1067 .IR child_stack
1068 is specified as NULL.
1069 .TP
1070 .B EINVAL
1071 .BR CLONE_NEWIPC
1072 was specified in
1073 .IR flags ,
1074 but the kernel was not configured with the
1075 .B CONFIG_SYSVIPC
1076 and
1077 .BR CONFIG_IPC_NS
1078 options.
1079 .TP
1080 .B EINVAL
1081 .BR CLONE_NEWNET
1082 was specified in
1083 .IR flags ,
1084 but the kernel was not configured with the
1085 .B CONFIG_NET_NS
1086 option.
1087 .TP
1088 .B EINVAL
1089 .BR CLONE_NEWPID
1090 was specified in
1091 .IR flags ,
1092 but the kernel was not configured with the
1093 .B CONFIG_PID_NS
1094 option.
1095 .TP
1096 .B EINVAL
1097 .BR CLONE_NEWUSER
1098 was specified in
1099 .IR flags ,
1100 but the kernel was not configured with the
1101 .B CONFIG_USER_NS
1102 option.
1103 .TP
1104 .B EINVAL
1105 .BR CLONE_NEWUTS
1106 was specified in
1107 .IR flags ,
1108 but the kernel was not configured with the
1109 .B CONFIG_UTS_NS
1110 option.
1111 .TP
1112 .B EINVAL
1113 .I child_stack
1114 is not aligned to a suitable boundary for this architecture.
1115 For example, on aarch64,
1116 .I child_stack
1117 must be a multiple of 16.
1118 .TP
1119 .B ENOMEM
1120 Cannot allocate sufficient memory to allocate a task structure for the
1121 child, or to copy those parts of the caller's context that need to be
1122 copied.
1123 .TP
1124 .BR ENOSPC " (since Linux 3.7)"
1125 .\" commit f2302505775fd13ba93f034206f1e2a587017929
1126 .B CLONE_NEWPID
1127 was specified in flags,
1128 but the limit on the nesting depth of PID namespaces
1129 would have been exceeded; see
1130 .BR pid_namespaces (7).
1131 .TP
1132 .BR ENOSPC " (since Linux 4.9; beforehand " EUSERS )
1133 .B CLONE_NEWUSER
1134 was specified in
1135 .IR flags ,
1136 and the call would cause the limit on the number of
1137 nested user namespaces to be exceeded.
1138 See
1139 .BR user_namespaces (7).
1140 .IP
1141 From Linux 3.11 to Linux 4.8, the error diagnosed in this case was
1142 .BR EUSERS .
1143 .TP
1144 .BR ENOSPC " (since Linux 4.9)"
1145 One of the values in
1146 .I flags
1147 specified the creation of a new user namespace,
1148 but doing so would have caused the limit defined by the corresponding file in
1149 .IR /proc/sys/user
1150 to be exceeded.
1151 For further details, see
1152 .BR namespaces (7).
1153 .TP
1154 .B EPERM
1155 .BR CLONE_NEWCGROUP ,
1156 .BR CLONE_NEWIPC ,
1157 .BR CLONE_NEWNET ,
1158 .BR CLONE_NEWNS ,
1159 .BR CLONE_NEWPID ,
1160 or
1161 .BR CLONE_NEWUTS
1162 was specified by an unprivileged process (process without \fBCAP_SYS_ADMIN\fP).
1163 .TP
1164 .B EPERM
1165 .B CLONE_PID
1166 was specified by a process other than process 0.
1167 (This error occurs only on Linux 2.5.15 and earlier.)
1168 .TP
1169 .B EPERM
1170 .BR CLONE_NEWUSER
1171 was specified in
1172 .IR flags ,
1173 but either the effective user ID or the effective group ID of the caller
1174 does not have a mapping in the parent namespace (see
1175 .BR user_namespaces (7)).
1176 .TP
1177 .BR EPERM " (since Linux 3.9)"
1178 .\" commit 3151527ee007b73a0ebd296010f1c0454a919c7d
1179 .B CLONE_NEWUSER
1180 was specified in
1181 .I flags
1182 and the caller is in a chroot environment
1183 .\" FIXME What is the rationale for this restriction?
1184 (i.e., the caller's root directory does not match the root directory
1185 of the mount namespace in which it resides).
1186 .TP
1187 .BR ERESTARTNOINTR " (since Linux 2.6.17)"
1188 .\" commit 4a2c7a7837da1b91468e50426066d988050e4d56
1189 System call was interrupted by a signal and will be restarted.
1190 (This can be seen only during a trace.)
1191 .TP
1192 .BR EUSERS " (Linux 3.11 to Linux 4.8)"
1193 .B CLONE_NEWUSER
1194 was specified in
1195 .IR flags ,
1196 and the limit on the number of nested user namespaces would be exceeded.
1197 See the discussion of the
1198 .BR ENOSPC
1199 error above.
1200 .\" .SH VERSIONS
1201 .\" There is no entry for
1202 .\" .BR clone ()
1203 .\" in libc5.
1204 .\" glibc2 provides
1205 .\" .BR clone ()
1206 .\" as described in this manual page.
1207 .SH CONFORMING TO
1208 .BR clone ()
1209 is Linux-specific and should not be used in programs
1210 intended to be portable.
1211 .SH NOTES
1212 The
1213 .BR kcmp (2)
1214 system call can be used to test whether two processes share various
1215 resources such as a file descriptor table,
1216 System V semaphore undo operations, or a virtual address space.
1217 .PP
1218 .PP
1219 Handlers registered using
1220 .BR pthread_atfork (3)
1221 are not executed during a call to
1222 .BR clone ().
1223 .PP
1224 In the Linux 2.4.x series,
1225 .B CLONE_THREAD
1226 generally does not make the parent of the new thread the same
1227 as the parent of the calling process.
1228 However, for kernel versions 2.4.7 to 2.4.18 the
1229 .B CLONE_THREAD
1230 flag implied the
1231 .B CLONE_PARENT
1232 flag (as in Linux 2.6.0 and later).
1233 .PP
1234 For a while there was
1235 .B CLONE_DETACHED
1236 (introduced in 2.5.32):
1237 parent wants no child-exit signal.
1238 In Linux 2.6.2, the need to give this flag together with
1239 .B CLONE_THREAD
1240 disappeared.
1241 This flag is still defined, but has no effect.
1242 .PP
1243 On i386,
1244 .BR clone ()
1245 should not be called through vsyscall, but directly through
1246 .IR "int $0x80" .
1247 .SH BUGS
1248 GNU C library versions 2.3.4 up to and including 2.24
1249 contained a wrapper function for
1250 .BR getpid (2)
1251 that performed caching of PIDs.
1252 This caching relied on support in the glibc wrapper for
1253 .BR clone (),
1254 but limitations in the implementation
1255 meant that the cache was not up to date in some circumstances.
1256 In particular,
1257 if a signal was delivered to the child immediately after the
1258 .BR clone ()
1259 call, then a call to
1260 .BR getpid (2)
1261 in a handler for the signal could return the PID
1262 of the calling process ("the parent"),
1263 if the clone wrapper had not yet had a chance to update the PID
1264 cache in the child.
1265 (This discussion ignores the case where the child was created using
1266 .BR CLONE_THREAD ,
1267 when
1268 .BR getpid (2)
1269 .I should
1270 return the same value in the child and in the process that called
1271 .BR clone (),
1272 since the caller and the child are in the same thread group.
1273 The stale-cache problem also does not occur if the
1274 .I flags
1275 argument includes
1276 .BR CLONE_VM .)
1277 To get the truth, it was sometimes necessary to use code such as the following:
1278 .PP
1279 .in +4n
1280 .EX
1281 #include <syscall.h>
1282
1283 pid_t mypid;
1284
1285 mypid = syscall(SYS_getpid);
1286 .EE
1287 .in
1288 .\" See also the following bug reports
1289 .\" https://bugzilla.redhat.com/show_bug.cgi?id=417521
1290 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=6910
1291 .PP
1292 Because of the stale-cache problem, as well as other problems noted in
1293 .BR getpid (2),
1294 the PID caching feature was removed in glibc 2.25.
1295 .SH EXAMPLE
1296 The following program demonstrates the use of
1297 .BR clone ()
1298 to create a child process that executes in a separate UTS namespace.
1299 The child changes the hostname in its UTS namespace.
1300 Both parent and child then display the system hostname,
1301 making it possible to see that the hostname
1302 differs in the UTS namespaces of the parent and child.
1303 For an example of the use of this program, see
1304 .BR setns (2).
1305 .SS Program source
1306 .EX
1307 #define _GNU_SOURCE
1308 #include <sys/wait.h>
1309 #include <sys/utsname.h>
1310 #include <sched.h>
1311 #include <string.h>
1312 #include <stdio.h>
1313 #include <stdlib.h>
1314 #include <unistd.h>
1315
1316 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
1317                         } while (0)
1318
1319 static int              /* Start function for cloned child */
1320 childFunc(void *arg)
1321 {
1322     struct utsname uts;
1323
1324     /* Change hostname in UTS namespace of child */
1325
1326     if (sethostname(arg, strlen(arg)) == \-1)
1327         errExit("sethostname");
1328
1329     /* Retrieve and display hostname */
1330
1331     if (uname(&uts) == \-1)
1332         errExit("uname");
1333     printf("uts.nodename in child:  %s\\n", uts.nodename);
1334
1335     /* Keep the namespace open for a while, by sleeping.
1336        This allows some experimentation\-\-for example, another
1337        process might join the namespace. */
1338
1339     sleep(200);
1340
1341     return 0;           /* Child terminates now */
1342 }
1343
1344 #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */
1345
1346 int
1347 main(int argc, char *argv[])
1348 {
1349     char *stack;                    /* Start of stack buffer */
1350     char *stackTop;                 /* End of stack buffer */
1351     pid_t pid;
1352     struct utsname uts;
1353
1354     if (argc < 2) {
1355         fprintf(stderr, "Usage: %s <child\-hostname>\\n", argv[0]);
1356         exit(EXIT_SUCCESS);
1357     }
1358
1359     /* Allocate stack for child */
1360
1361     stack = malloc(STACK_SIZE);
1362     if (stack == NULL)
1363         errExit("malloc");
1364     stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */
1365
1366     /* Create child that has its own UTS namespace;
1367        child commences execution in childFunc() */
1368
1369     pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
1370     if (pid == \-1)
1371         errExit("clone");
1372     printf("clone() returned %ld\\n", (long) pid);
1373
1374     /* Parent falls through to here */
1375
1376     sleep(1);           /* Give child time to change its hostname */
1377
1378     /* Display hostname in parent\(aqs UTS namespace. This will be
1379        different from hostname in child\(aqs UTS namespace. */
1380
1381     if (uname(&uts) == \-1)
1382         errExit("uname");
1383     printf("uts.nodename in parent: %s\\n", uts.nodename);
1384
1385     if (waitpid(pid, NULL, 0) == \-1)    /* Wait for child */
1386         errExit("waitpid");
1387     printf("child has terminated\\n");
1388
1389     exit(EXIT_SUCCESS);
1390 }
1391 .EE
1392 .SH SEE ALSO
1393 .BR fork (2),
1394 .BR futex (2),
1395 .BR getpid (2),
1396 .BR gettid (2),
1397 .BR kcmp (2),
1398 .BR set_thread_area (2),
1399 .BR set_tid_address (2),
1400 .BR setns (2),
1401 .BR tkill (2),
1402 .BR unshare (2),
1403 .BR wait (2),
1404 .BR capabilities (7),
1405 .BR namespaces (7),
1406 .BR pthreads (7)