1CLONE(2) Linux Programmer's Manual CLONE(2)
2
6 clone, __clone2, clone3 - create a child process
7
9 /* Prototype for the glibc wrapper function */
10 #define _GNU_SOURCE
12 #include <sched.h>
13 int clone(int (*fn)(void *), void *stack, int flags, void *arg, ...
15 /* pid_t *parent_tid, void *tls, pid_t *child_tid */ );
16 /* For the prototype of the raw clone() system call, see NOTES */
18 long clone3(struct clone_args *cl_args, size_t size);
20 Note: There is not yet a glibc wrapper for clone3(); see NOTES.
22
24 These system calls create a new ("child") process, in a manner similar
25 to fork(2).
26 By contrast with fork(2), these system calls provide more precise con‐
28 trol over what pieces of execution context are shared between the call‐
29 ing process and the child process. For example, using these system
30 calls, the caller can control whether or not the two processes share
31 the virtual address space, the table of file descriptors, and the table
32 of signal handlers. These system calls also allow the new child
33 process to be placed in separate namespaces(7).
34 Note that in this manual page, "calling process" normally corresponds
36 to "parent process". But see the descriptions of CLONE_PARENT and
37 CLONE_THREAD below.
38 This page describes the following interfaces:
40 * The glibc clone() wrapper function and the underlying system call on
42 which it is based. The main text describes the wrapper function;
43 the differences for the raw system call are described toward the end
44 of this page.
45 * The newer clone3() system call.
47 In the remainder of this page, the terminology "the clone call" is used
49 when noting details that apply to all of these interfaces,
50 The clone() wrapper function
52 When the child process is created with the clone() wrapper function, it
53 commences execution by calling the function pointed to by the argument
54 fn. (This differs from fork(2), where execution continues in the child
55 from the point of the fork(2) call.) The arg argument is passed as the
56 argument of the function fn.
57 When the fn(arg) function returns, the child process terminates. The
59 integer returned by fn is the exit status for the child process. The
60 child process may also terminate explicitly by calling exit(2) or after
61 receiving a fatal signal.
62 The stack argument specifies the location of the stack used by the
64 child process. Since the child and calling process may share memory,
65 it is not possible for the child process to execute in the same stack
66 as the calling process. The calling process must therefore set up mem‐
67 ory space for the child stack and pass a pointer to this space to
68 clone(). Stacks grow downward on all processors that run Linux (except
69 the HP PA processors), so stack usually points to the topmost address
70 of the memory space set up for the child stack. Note that clone() does
71 not provide a means whereby the caller can inform the kernel of the
72 size of the stack area.
73 The remaining arguments to clone() are discussed below.
75 clone3()
77 The clone3() system call provides a superset of the functionality of
78 the older clone() interface. It also provides a number of API improve‐
79 ments, including: space for additional flags bits; cleaner separation
80 in the use of various arguments; and the ability to specify the size of
81 the child's stack area.
82 As with fork(2), clone3() returns in both the parent and the child. It
84 returns 0 in the child process and returns the PID of the child in the
85 parent.
86 The cl_args argument of clone3() is a structure of the following form:
88 struct clone_args {
90 u64 flags; /* Flags bit mask */
91 u64 pidfd; /* Where to store PID file descriptor
92 (pid_t *) */
93 u64 child_tid; /* Where to store child TID,
94 in child's memory (pid_t *) */
95 u64 parent_tid; /* Where to store child TID,
96 in parent's memory (int *) */
97 u64 exit_signal; /* Signal to deliver to parent on
98 child termination */
99 u64 stack; /* Pointer to lowest byte of stack */
100 u64 stack_size; /* Size of stack */
101 u64 tls; /* Location of new TLS */
102 u64 set_tid; /* Pointer to a pid_t array
103 (since Linux 5.5) */
104 u64 set_tid_size; /* Number of elements in set_tid
105 (since Linux 5.5) */
106 u64 cgroup; /* File descriptor for target cgroup
107 of child (since Linux 5.7) */
108 };
109 The size argument that is supplied to clone3() should be initialized to
111 the size of this structure. (The existence of the size argument per‐
112 mits future extensions to the clone_args structure.)
113 The stack for the child process is specified via cl_args.stack, which
115 points to the lowest byte of the stack area, and cl_args.stack_size,
116 which specifies the size of the stack in bytes. In the case where the
117 CLONE_VM flag (see below) is specified, a stack must be explicitly al‐
118 located and specified. Otherwise, these two fields can be specified as
119 NULL and 0, which causes the child to use the same stack area as the
120 parent (in the child's own virtual address space).
121 The remaining fields in the cl_args argument are discussed below.
123 Equivalence between clone() and clone3() arguments
125 Unlike the older clone() interface, where arguments are passed individ‐
126 ually, in the newer clone3() interface the arguments are packaged into
127 the clone_args structure shown above. This structure allows for a su‐
128 perset of the information passed via the clone() arguments.
129 The following table shows the equivalence between the arguments of
131 clone() and the fields in the clone_args argument supplied to clone3():
132 clone() clone3() Notes
134 cl_args field
135 flags & ~0xff flags For most flags; details below
136 parent_tid pidfd See CLONE_PIDFD
137 child_tid child_tid See CLONE_CHILD_SETTID
138 parent_tid parent_tid See CLONE_PARENT_SETTID
139 flags & 0xff exit_signal
140 stack stack
141 --- stack_size
142 tls tls See CLONE_SETTLS
143 --- set_tid See below for details
144 --- set_tid_size
145 --- cgroup See CLONE_INTO_CGROUP
146 The child termination signal
148 When the child process terminates, a signal may be sent to the parent.
149 The termination signal is specified in the low byte of flags (clone())
150 or in cl_args.exit_signal (clone3()). If this signal is specified as
151 anything other than SIGCHLD, then the parent process must specify the
152 __WALL or __WCLONE options when waiting for the child with wait(2). If
153 no signal (i.e., zero) is specified, then the parent process is not
154 signaled when the child terminates.
155 The set_tid array
157 By default, the kernel chooses the next sequential PID for the new
158 process in each of the PID namespaces where it is present. When creat‐
159 ing a process with clone3(), the set_tid array (available since Linux
160 5.5) can be used to select specific PIDs for the process in some or all
161 of the PID namespaces where it is present. If the PID of the newly
162 created process should be set only for the current PID namespace or in
163 the newly created PID namespace (if flags contains CLONE_NEWPID) then
164 the first element in the set_tid array has to be the desired PID and
165 set_tid_size needs to be 1.
166 If the PID of the newly created process should have a certain value in
168 multiple PID namespaces, then the set_tid array can have multiple en‐
169 tries. The first entry defines the PID in the most deeply nested PID
170 namespace and each of the following entries contains the PID in the
171 corresponding ancestor PID namespace. The number of PID namespaces in
172 which a PID should be set is defined by set_tid_size which cannot be
173 larger than the number of currently nested PID namespaces.
174 To create a process with the following PIDs in a PID namespace hierar‐
176 chy:
177 PID NS level Requested PID Notes
179 0 31496 Outermost PID namespace
180 1 42
181 2 7 Innermost PID namespace
182 Set the array to:
184 set_tid[0] = 7;
186 set_tid[1] = 42;
187 set_tid[2] = 31496;
188 set_tid_size = 3;
189 If only the PIDs in the two innermost PID namespaces need to be speci‐
191 fied, set the array to:
192 set_tid[0] = 7;
194 set_tid[1] = 42;
195 set_tid_size = 2;
196 The PID in the PID namespaces outside the two innermost PID namespaces
198 will be selected the same way as any other PID is selected.
199 The set_tid feature requires CAP_SYS_ADMIN or (since Linux 5.9)
201 CAP_CHECKPOINT_RESTORE in all owning user namespaces of the target PID
202 namespaces.
203 Callers may only choose a PID greater than 1 in a given PID namespace
205 if an init process (i.e., a process with PID 1) already exists in that
206 namespace. Otherwise the PID entry for this PID namespace must be 1.
207 The flags mask
209 Both clone() and clone3() allow a flags bit mask that modifies their
210 behavior and allows the caller to specify what is shared between the
211 calling process and the child process. This bit mask—the flags argu‐
212 ment of clone() or the cl_args.flags field passed to clone3()—is re‐
213 ferred to as the flags mask in the remainder of this page.
214 The flags mask is specified as a bitwise-OR of zero or more of the con‐
216 stants listed below. Except as noted below, these flags are available
217 (and have the same effect) in both clone() and clone3().
218 CLONE_CHILD_CLEARTID (since Linux 2.5.49)
220 Clear (zero) the child thread ID at the location pointed to by
221 child_tid (clone()) or cl_args.child_tid (clone3()) in child
222 memory when the child exits, and do a wakeup on the futex at
223 that address. The address involved may be changed by the
224 set_tid_address(2) system call. This is used by threading li‐
225 braries.
226 CLONE_CHILD_SETTID (since Linux 2.5.49)
228 Store the child thread ID at the location pointed to by
229 child_tid (clone()) or cl_args.child_tid (clone3()) in the
230 child's memory. The store operation completes before the clone
231 call returns control to user space in the child process. (Note
232 that the store operation may not have completed before the clone
233 call returns in the parent process, which will be relevant if
234 the CLONE_VM flag is also employed.)
235 CLONE_CLEAR_SIGHAND (since Linux 5.5)
237 By default, signal dispositions in the child thread are the same
238 as in the parent. If this flag is specified, then all signals
239 that are handled in the parent are reset to their default dispo‐
240 sitions (SIG_DFL) in the child.
241 Specifying this flag together with CLONE_SIGHAND is nonsensical
243 and disallowed.
244 CLONE_DETACHED (historical)
246 For a while (during the Linux 2.5 development series) there was
247 a CLONE_DETACHED flag, which caused the parent not to receive a
248 signal when the child terminated. Ultimately, the effect of
249 this flag was subsumed under the CLONE_THREAD flag and by the
250 time Linux 2.6.0 was released, this flag had no effect. Start‐
251 ing in Linux 2.6.2, the need to give this flag together with
252 CLONE_THREAD disappeared.
253 This flag is still defined, but it is usually ignored when call‐
255 ing clone(). However, see the description of CLONE_PIDFD for
256 some exceptions.
257 CLONE_FILES (since Linux 2.0)
259 If CLONE_FILES is set, the calling process and the child process
260 share the same file descriptor table. Any file descriptor cre‐
261 ated by the calling process or by the child process is also
262 valid in the other process. Similarly, if one of the processes
263 closes a file descriptor, or changes its associated flags (using
264 the fcntl(2) F_SETFD operation), the other process is also af‐
265 fected. If a process sharing a file descriptor table calls ex‐
266 ecve(2), its file descriptor table is duplicated (unshared).
267 If CLONE_FILES is not set, the child process inherits a copy of
269 all file descriptors opened in the calling process at the time
270 of the clone call. Subsequent operations that open or close
271 file descriptors, or change file descriptor flags, performed by
272 either the calling process or the child process do not affect
273 the other process. Note, however, that the duplicated file de‐
274 scriptors in the child refer to the same open file descriptions
275 as the corresponding file descriptors in the calling process,
276 and thus share file offsets and file status flags (see open(2)).
277 CLONE_FS (since Linux 2.0)
279 If CLONE_FS is set, the caller and the child process share the
280 same filesystem information. This includes the root of the
281 filesystem, the current working directory, and the umask. Any
282 call to chroot(2), chdir(2), or umask(2) performed by the call‐
283 ing process or the child process also affects the other process.
284 If CLONE_FS is not set, the child process works on a copy of the
286 filesystem information of the calling process at the time of the
287 clone call. Calls to chroot(2), chdir(2), or umask(2) performed
288 later by one of the processes do not affect the other process.
289 CLONE_INTO_CGROUP (since Linux 5.7)
291 By default, a child process is placed in the same version 2
292 cgroup as its parent. The CLONE_INTO_CGROUP flag allows the
293 child process to be created in a different version 2 cgroup.
294 (Note that CLONE_INTO_CGROUP has effect only for version 2
295 cgroups.)
296 In order to place the child process in a different cgroup, the
298 caller specifies CLONE_INTO_CGROUP in cl_args.flags and passes a
299 file descriptor that refers to a version 2 cgroup in the
300 cl_args.cgroup field. (This file descriptor can be obtained by
301 opening a cgroup v2 directory using either the O_RDONLY or the
302 O_PATH flag.) Note that all of the usual restrictions (de‐
303 scribed in cgroups(7)) on placing a process into a version 2
304 cgroup apply.
305 Among the possible use cases for CLONE_INTO_CGROUP are the fol‐
307 lowing:
308 * Spawning a process into a cgroup different from the parent's
310 cgroup makes it possible for a service manager to directly
311 spawn new services into dedicated cgroups. This eliminates
312 the accounting jitter that would be caused if the child
313 process was first created in the same cgroup as the parent
314 and then moved into the target cgroup. Furthermore, spawning
315 the child process directly into a target cgroup is signifi‐
316 cantly cheaper than moving the child process into the target
317 cgroup after it has been created.
318 * The CLONE_INTO_CGROUP flag also allows the creation of frozen
320 child processes by spawning them into a frozen cgroup. (See
321 cgroups(7) for a description of the freezer controller.)
322 * For threaded applications (or even thread implementations
324 which make use of cgroups to limit individual threads), it is
325 possible to establish a fixed cgroup layout before spawning
326 each thread directly into its target cgroup.
327 CLONE_IO (since Linux 2.6.25)
329 If CLONE_IO is set, then the new process shares an I/O context
330 with the calling process. If this flag is not set, then (as
331 with fork(2)) the new process has its own I/O context.
332 The I/O context is the I/O scope of the disk scheduler (i.e.,
334 what the I/O scheduler uses to model scheduling of a process's
335 I/O). If processes share the same I/O context, they are treated
336 as one by the I/O scheduler. As a consequence, they get to
337 share disk time. For some I/O schedulers, if two processes
338 share an I/O context, they will be allowed to interleave their
339 disk access. If several threads are doing I/O on behalf of the
340 same process (aio_read(3), for instance), they should employ
341 CLONE_IO to get better I/O performance.
342 If the kernel is not configured with the CONFIG_BLOCK option,
344 this flag is a no-op.
345 CLONE_NEWCGROUP (since Linux 4.6)
347 Create the process in a new cgroup namespace. If this flag is
348 not set, then (as with fork(2)) the process is created in the
349 same cgroup namespaces as the calling process.
350 For further information on cgroup namespaces, see cgroup_name‐
352 spaces(7).
353 Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC‐
355 GROUP.
356 CLONE_NEWIPC (since Linux 2.6.19)
358 If CLONE_NEWIPC is set, then create the process in a new IPC
359 namespace. If this flag is not set, then (as with fork(2)), the
360 process is created in the same IPC namespace as the calling
361 process.
362 For further information on IPC namespaces, see ipc_name‐
364 spaces(7).
365 Only a privileged process (CAP_SYS_ADMIN) can employ
367 CLONE_NEWIPC. This flag can't be specified in conjunction with
368 CLONE_SYSVSEM.
369 CLONE_NEWNET (since Linux 2.6.24)
371 (The implementation of this flag was completed only by about
372 kernel version 2.6.29.)
373 If CLONE_NEWNET is set, then create the process in a new network
375 namespace. If this flag is not set, then (as with fork(2)) the
376 process is created in the same network namespace as the calling
377 process.
378 For further information on network namespaces, see network_name‐
380 spaces(7).
381 Only a privileged process (CAP_SYS_ADMIN) can employ
383 CLONE_NEWNET.
384 CLONE_NEWNS (since Linux 2.4.19)
386 If CLONE_NEWNS is set, the cloned child is started in a new
387 mount namespace, initialized with a copy of the namespace of the
388 parent. If CLONE_NEWNS is not set, the child lives in the same
389 mount namespace as the parent.
390 For further information on mount namespaces, see namespaces(7)
392 and mount_namespaces(7).
393 Only a privileged process (CAP_SYS_ADMIN) can employ
395 CLONE_NEWNS. It is not permitted to specify both CLONE_NEWNS
396 and CLONE_FS in the same clone call.
397 CLONE_NEWPID (since Linux 2.6.24)
399 If CLONE_NEWPID is set, then create the process in a new PID
400 namespace. If this flag is not set, then (as with fork(2)) the
401 process is created in the same PID namespace as the calling
402 process.
403 For further information on PID namespaces, see namespaces(7) and
405 pid_namespaces(7).
406 Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEW‐
408 PID. This flag can't be specified in conjunction with
409 CLONE_THREAD or CLONE_PARENT.
410 CLONE_NEWUSER
412 (This flag first became meaningful for clone() in Linux 2.6.23,
413 the current clone() semantics were merged in Linux 3.5, and the
414 final pieces to make the user namespaces completely usable were
415 merged in Linux 3.8.)
416 If CLONE_NEWUSER is set, then create the process in a new user
418 namespace. If this flag is not set, then (as with fork(2)) the
419 process is created in the same user namespace as the calling
420 process.
421 For further information on user namespaces, see namespaces(7)
423 and user_namespaces(7).
424 Before Linux 3.8, use of CLONE_NEWUSER required that the caller
426 have three capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SET‐
427 GID. Starting with Linux 3.8, no privileges are needed to cre‐
428 ate a user namespace.
429 This flag can't be specified in conjunction with CLONE_THREAD or
431 CLONE_PARENT. For security reasons, CLONE_NEWUSER cannot be
432 specified in conjunction with CLONE_FS.
433 CLONE_NEWUTS (since Linux 2.6.19)
435 If CLONE_NEWUTS is set, then create the process in a new UTS
436 namespace, whose identifiers are initialized by duplicating the
437 identifiers from the UTS namespace of the calling process. If
438 this flag is not set, then (as with fork(2)) the process is cre‐
439 ated in the same UTS namespace as the calling process.
440 For further information on UTS namespaces, see uts_name‐
442 spaces(7).
443 Only a privileged process (CAP_SYS_ADMIN) can employ
445 CLONE_NEWUTS.
446 CLONE_PARENT (since Linux 2.3.12)
448 If CLONE_PARENT is set, then the parent of the new child (as re‐
449 turned by getppid(2)) will be the same as that of the calling
450 process.
451 If CLONE_PARENT is not set, then (as with fork(2)) the child's
453 parent is the calling process.
454 Note that it is the parent process, as returned by getppid(2),
456 which is signaled when the child terminates, so that if
457 CLONE_PARENT is set, then the parent of the calling process,
458 rather than the calling process itself, will be signaled.
459 The CLONE_PARENT flag can't be used in clone calls by the global
461 init process (PID 1 in the initial PID namespace) and init pro‐
462 cesses in other PID namespaces. This restriction prevents the
463 creation of multi-rooted process trees as well as the creation
464 of unreapable zombies in the initial PID namespace.
465 CLONE_PARENT_SETTID (since Linux 2.5.49)
467 Store the child thread ID at the location pointed to by par‐
468 ent_tid (clone()) or cl_args.parent_tid (clone3()) in the par‐
469 ent's memory. (In Linux 2.5.32-2.5.48 there was a flag
470 CLONE_SETTID that did this.) The store operation completes be‐
471 fore the clone call returns control to user space.
472 CLONE_PID (Linux 2.0 to 2.5.15)
474 If CLONE_PID is set, the child process is created with the same
475 process ID as the calling process. This is good for hacking the
476 system, but otherwise of not much use. From Linux 2.3.21 on‐
477 ward, this flag could be specified only by the system boot
478 process (PID 0). The flag disappeared completely from the ker‐
479 nel sources in Linux 2.5.16. Subsequently, the kernel silently
480 ignored this bit if it was specified in the flags mask. Much
481 later, the same bit was recycled for use as the CLONE_PIDFD
482 flag.
483 CLONE_PIDFD (since Linux 5.2)
485 If this flag is specified, a PID file descriptor referring to
486 the child process is allocated and placed at a specified loca‐
487 tion in the parent's memory. The close-on-exec flag is set on
488 this new file descriptor. PID file descriptors can be used for
489 the purposes described in pidfd_open(2).
490 * When using clone3(), the PID file descriptor is placed at the
492 location pointed to by cl_args.pidfd.
493 * When using clone(), the PID file descriptor is placed at the
495 location pointed to by parent_tid. Since the parent_tid ar‐
496 gument is used to return the PID file descriptor, CLONE_PIDFD
497 cannot be used with CLONE_PARENT_SETTID when calling clone().
498 It is currently not possible to use this flag together with
500 CLONE_THREAD. This means that the process identified by the PID
501 file descriptor will always be a thread group leader.
502 If the obsolete CLONE_DETACHED flag is specified alongside
504 CLONE_PIDFD when calling clone(), an error is returned. An er‐
505 ror also results if CLONE_DETACHED is specified when calling
506 clone3(). This error behavior ensures that the bit correspond‐
507 ing to CLONE_DETACHED can be reused for further PID file de‐
508 scriptor features in the future.
509 CLONE_PTRACE (since Linux 2.2)
511 If CLONE_PTRACE is specified, and the calling process is being
512 traced, then trace the child also (see ptrace(2)).
513 CLONE_SETTLS (since Linux 2.5.32)
515 The TLS (Thread Local Storage) descriptor is set to tls.
516 The interpretation of tls and the resulting effect is architec‐
518 ture dependent. On x86, tls is interpreted as a struct
519 user_desc * (see set_thread_area(2)). On x86-64 it is the new
520 value to be set for the %fs base register (see the ARCH_SET_FS
521 argument to arch_prctl(2)). On architectures with a dedicated
522 TLS register, it is the new value of that register.
523 Use of this flag requires detailed knowledge and generally it
525 should not be used except in libraries implementing threading.
526 CLONE_SIGHAND (since Linux 2.0)
528 If CLONE_SIGHAND is set, the calling process and the child
529 process share the same table of signal handlers. If the calling
530 process or child process calls sigaction(2) to change the behav‐
531 ior associated with a signal, the behavior is changed in the
532 other process as well. However, the calling process and child
533 processes still have distinct signal masks and sets of pending
534 signals. So, one of them may block or unblock signals using
535 sigprocmask(2) without affecting the other process.
536 If CLONE_SIGHAND is not set, the child process inherits a copy
538 of the signal handlers of the calling process at the time of the
539 clone call. Calls to sigaction(2) performed later by one of the
540 processes have no effect on the other process.
541 Since Linux 2.6.0, the flags mask must also include CLONE_VM if
543 CLONE_SIGHAND is specified
544 CLONE_STOPPED (since Linux 2.6.0)
546 If CLONE_STOPPED is set, then the child is initially stopped (as
547 though it was sent a SIGSTOP signal), and must be resumed by
548 sending it a SIGCONT signal.
549 This flag was deprecated from Linux 2.6.25 onward, and was re‐
551 moved altogether in Linux 2.6.38. Since then, the kernel
552 silently ignores it without error. Starting with Linux 4.6, the
553 same bit was reused for the CLONE_NEWCGROUP flag.
554 CLONE_SYSVSEM (since Linux 2.5.10)
556 If CLONE_SYSVSEM is set, then the child and the calling process
557 share a single list of System V semaphore adjustment (semadj)
558 values (see semop(2)). In this case, the shared list accumu‐
559 lates semadj values across all processes sharing the list, and
560 semaphore adjustments are performed only when the last process
561 that is sharing the list terminates (or ceases sharing the list
562 using unshare(2)). If this flag is not set, then the child has
563 a separate semadj list that is initially empty.
564 CLONE_THREAD (since Linux 2.4.0)
566 If CLONE_THREAD is set, the child is placed in the same thread
567 group as the calling process. To make the remainder of the dis‐
568 cussion of CLONE_THREAD more readable, the term "thread" is used
569 to refer to the processes within a thread group.
570 Thread groups were a feature added in Linux 2.4 to support the
572 POSIX threads notion of a set of threads that share a single
573 PID. Internally, this shared PID is the so-called thread group
574 identifier (TGID) for the thread group. Since Linux 2.4, calls
575 to getpid(2) return the TGID of the caller.
576 The threads within a group can be distinguished by their (sys‐
578 tem-wide) unique thread IDs (TID). A new thread's TID is avail‐
579 able as the function result returned to the caller, and a thread
580 can obtain its own TID using gettid(2).
581 When a clone call is made without specifying CLONE_THREAD, then
583 the resulting thread is placed in a new thread group whose TGID
584 is the same as the thread's TID. This thread is the leader of
585 the new thread group.
586 A new thread created with CLONE_THREAD has the same parent
588 process as the process that made the clone call (i.e., like
589 CLONE_PARENT), so that calls to getppid(2) return the same value
590 for all of the threads in a thread group. When a CLONE_THREAD
591 thread terminates, the thread that created it is not sent a
592 SIGCHLD (or other termination) signal; nor can the status of
593 such a thread be obtained using wait(2). (The thread is said to
594 be detached.)
595 After all of the threads in a thread group terminate the parent
597 process of the thread group is sent a SIGCHLD (or other termina‐
598 tion) signal.
599 If any of the threads in a thread group performs an execve(2),
601 then all threads other than the thread group leader are termi‐
602 nated, and the new program is executed in the thread group
603 leader.
604 If one of the threads in a thread group creates a child using
606 fork(2), then any thread in the group can wait(2) for that
607 child.
608 Since Linux 2.5.35, the flags mask must also include CLONE_SIG‐
610 HAND if CLONE_THREAD is specified (and note that, since Linux
611 2.6.0, CLONE_SIGHAND also requires CLONE_VM to be included).
612 Signal dispositions and actions are process-wide: if an unhan‐
614 dled signal is delivered to a thread, then it will affect (ter‐
615 minate, stop, continue, be ignored in) all members of the thread
616 group.
617 Each thread has its own signal mask, as set by sigprocmask(2).
619 A signal may be process-directed or thread-directed. A process-
621 directed signal is targeted at a thread group (i.e., a TGID),
622 and is delivered to an arbitrarily selected thread from among
623 those that are not blocking the signal. A signal may be
624 process-directed because it was generated by the kernel for rea‐
625 sons other than a hardware exception, or because it was sent us‐
626 ing kill(2) or sigqueue(3). A thread-directed signal is tar‐
627 geted at (i.e., delivered to) a specific thread. A signal may
628 be thread directed because it was sent using tgkill(2) or
629 pthread_sigqueue(3), or because the thread executed a machine
630 language instruction that triggered a hardware exception (e.g.,
631 invalid memory access triggering SIGSEGV or a floating-point ex‐
632 ception triggering SIGFPE).
633 A call to sigpending(2) returns a signal set that is the union
635 of the pending process-directed signals and the signals that are
636 pending for the calling thread.
637 If a process-directed signal is delivered to a thread group, and
639 the thread group has installed a handler for the signal, then
640 the handler will be invoked in exactly one, arbitrarily selected
641 member of the thread group that has not blocked the signal. If
642 multiple threads in a group are waiting to accept the same sig‐
643 nal using sigwaitinfo(2), the kernel will arbitrarily select one
644 of these threads to receive the signal.
645 CLONE_UNTRACED (since Linux 2.5.46)
647 If CLONE_UNTRACED is specified, then a tracing process cannot
648 force CLONE_PTRACE on this child process.
649 CLONE_VFORK (since Linux 2.2)
651 If CLONE_VFORK is set, the execution of the calling process is
652 suspended until the child releases its virtual memory resources
653 via a call to execve(2) or _exit(2) (as with vfork(2)).
654 If CLONE_VFORK is not set, then both the calling process and the
656 child are schedulable after the call, and an application should
657 not rely on execution occurring in any particular order.
658 CLONE_VM (since Linux 2.0)
660 If CLONE_VM is set, the calling process and the child process
661 run in the same memory space. In particular, memory writes per‐
662 formed by the calling process or by the child process are also
663 visible in the other process. Moreover, any memory mapping or
664 unmapping performed with mmap(2) or munmap(2) by the child or
665 calling process also affects the other process.
666 If CLONE_VM is not set, the child process runs in a separate
668 copy of the memory space of the calling process at the time of
669 the clone call. Memory writes or file mappings/unmappings per‐
670 formed by one of the processes do not affect the other, as with
671 fork(2).
672 If the CLONE_VM flag is specified and the CLONE_VM flag is not
674 specified, then any alternate signal stack that was established
675 by sigaltstack(2) is cleared in the child process.
676
678 On success, the thread ID of the child process is returned in the
679 caller's thread of execution. On failure, -1 is returned in the
680 caller's context, no child process will be created, and errno will be
681 set appropriately.
682
684 EAGAIN Too many processes are already running; see fork(2).
685 EBUSY (clone3() only)
687 CLONE_INTO_CGROUP was specified in cl_args.flags, but the file
688 descriptor specified in cl_args.cgroup refers to a version 2
689 cgroup in which a domain controller is enabled.
690 EEXIST (clone3() only)
692 One (or more) of the PIDs specified in set_tid already exists in
693 the corresponding PID namespace.
694 EINVAL Both CLONE_SIGHAND and CLONE_CLEAR_SIGHAND were specified in the
696 flags mask.
697 EINVAL CLONE_SIGHAND was specified in the flags mask, but CLONE_VM was
699 not. (Since Linux 2.6.0.)
700 EINVAL CLONE_THREAD was specified in the flags mask, but CLONE_SIGHAND
702 was not. (Since Linux 2.5.35.)
703 EINVAL CLONE_THREAD was specified in the flags mask, but the current
705 process previously called unshare(2) with the CLONE_NEWPID flag
706 or used setns(2) to reassociate itself with a PID namespace.
707 EINVAL Both CLONE_FS and CLONE_NEWNS were specified in the flags mask.
709 EINVAL (since Linux 3.9)
711 Both CLONE_NEWUSER and CLONE_FS were specified in the flags
712 mask.
713 EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in the flags
715 mask.
716 EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both)
718 of CLONE_THREAD or CLONE_PARENT were specified in the flags
719 mask.
720 EINVAL (since Linux 2.6.32)
722 CLONE_PARENT was specified, and the caller is an init process.
723 EINVAL Returned by the glibc clone() wrapper function when fn or stack
725 is specified as NULL.
726 EINVAL CLONE_NEWIPC was specified in the flags mask, but the kernel was
728 not configured with the CONFIG_SYSVIPC and CONFIG_IPC_NS op‐
729 tions.
730 EINVAL CLONE_NEWNET was specified in the flags mask, but the kernel was
732 not configured with the CONFIG_NET_NS option.
733 EINVAL CLONE_NEWPID was specified in the flags mask, but the kernel was
735 not configured with the CONFIG_PID_NS option.
736 EINVAL CLONE_NEWUSER was specified in the flags mask, but the kernel
738 was not configured with the CONFIG_USER_NS option.
739 EINVAL CLONE_NEWUTS was specified in the flags mask, but the kernel was
741 not configured with the CONFIG_UTS_NS option.
742 EINVAL stack is not aligned to a suitable boundary for this architec‐
744 ture. For example, on aarch64, stack must be a multiple of 16.
745 EINVAL (clone3() only)
747 CLONE_DETACHED was specified in the flags mask.
748 EINVAL (clone() only)
750 CLONE_PIDFD was specified together with CLONE_DETACHED in the
751 flags mask.
752 EINVAL CLONE_PIDFD was specified together with CLONE_THREAD in the
754 flags mask.
755 EINVAL (clone() only)
757 CLONE_PIDFD was specified together with CLONE_PARENT_SETTID in
758 the flags mask.
759 EINVAL (clone3() only)
761 set_tid_size is greater than the number of nested PID name‐
762 spaces.
763 EINVAL (clone3() only)
765 One of the PIDs specified in set_tid was an invalid.
766 EINVAL (AArch64 only, Linux 4.6 and earlier)
768 stack was not aligned to a 126-bit boundary.
769 ENOMEM Cannot allocate sufficient memory to allocate a task structure
771 for the child, or to copy those parts of the caller's context
772 that need to be copied.
773 ENOSPC (since Linux 3.7)
775 CLONE_NEWPID was specified in the flags mask, but the limit on
776 the nesting depth of PID namespaces would have been exceeded;
777 see pid_namespaces(7).
778 ENOSPC (since Linux 4.9; beforehand EUSERS)
780 CLONE_NEWUSER was specified in the flags mask, and the call
781 would cause the limit on the number of nested user namespaces to
782 be exceeded. See user_namespaces(7).
783 From Linux 3.11 to Linux 4.8, the error diagnosed in this case
785 was EUSERS.
786 ENOSPC (since Linux 4.9)
788 One of the values in the flags mask specified the creation of a
789 new user namespace, but doing so would have caused the limit de‐
790 fined by the corresponding file in /proc/sys/user to be ex‐
791 ceeded. For further details, see namespaces(7).
792 EOPNOTSUPP (clone3() only)
794 CLONE_INTO_CGROUP was specified in cl_args.flags, but the file
795 descriptor specified in cl_args.cgroup refers to a version 2
796 cgroup that is in the domain invalid state.
797 EPERM CLONE_NEWCGROUP, CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS,
799 CLONE_NEWPID, or CLONE_NEWUTS was specified by an unprivileged
800 process (process without CAP_SYS_ADMIN).
801 EPERM CLONE_PID was specified by a process other than process 0.
803 (This error occurs only on Linux 2.5.15 and earlier.)
804 EPERM CLONE_NEWUSER was specified in the flags mask, but either the
806 effective user ID or the effective group ID of the caller does
807 not have a mapping in the parent namespace (see user_name‐
808 spaces(7)).
809 EPERM (since Linux 3.9)
811 CLONE_NEWUSER was specified in the flags mask and the caller is
812 in a chroot environment (i.e., the caller's root directory does
813 not match the root directory of the mount namespace in which it
814 resides).
815 EPERM (clone3() only)
817 set_tid_size was greater than zero, and the caller lacks the
818 CAP_SYS_ADMIN capability in one or more of the user namespaces
819 that own the corresponding PID namespaces.
820 ERESTARTNOINTR (since Linux 2.6.17)
822 System call was interrupted by a signal and will be restarted.
823 (This can be seen only during a trace.)
824 EUSERS (Linux 3.11 to Linux 4.8)
826 CLONE_NEWUSER was specified in the flags mask, and the limit on
827 the number of nested user namespaces would be exceeded. See the
828 discussion of the ENOSPC error above.
829
831 The clone3() system call first appeared in Linux 5.3.
832
834 These system calls are Linux-specific and should not be used in pro‐
835 grams intended to be portable.
836
838 One use of these systems calls is to implement threads: multiple flows
839 of control in a program that run concurrently in a shared address
840 space.
841 Glibc does not provide a wrapper for clone3(); call it using
843 syscall(2).
844 Note that the glibc clone() wrapper function makes some changes in the
846 memory pointed to by stack (changes required to set the stack up cor‐
847 rectly for the child) before invoking the clone() system call. So, in
848 cases where clone() is used to recursively create children, do not use
849 the buffer employed for the parent's stack as the stack of the child.
850 The kcmp(2) system call can be used to test whether two processes share
852 various resources such as a file descriptor table, System V semaphore
853 undo operations, or a virtual address space.
854 Handlers registered using pthread_atfork(3) are not executed during a
856 clone call.
857 In the Linux 2.4.x series, CLONE_THREAD generally does not make the
859 parent of the new thread the same as the parent of the calling process.
860 However, for kernel versions 2.4.7 to 2.4.18 the CLONE_THREAD flag im‐
861 plied the CLONE_PARENT flag (as in Linux 2.6.0 and later).
862 On i386, clone() should not be called through vsyscall, but directly
864 through int $0x80.
865 C library/kernel differences
867 The raw clone() system call corresponds more closely to fork(2) in that
868 execution in the child continues from the point of the call. As such,
869 the fn and arg arguments of the clone() wrapper function are omitted.
870 In contrast to the glibc wrapper, the raw clone() system call accepts
872 NULL as a stack argument (and clone3() likewise allows cl_args.stack to
873 be NULL). In this case, the child uses a duplicate of the parent's
874 stack. (Copy-on-write semantics ensure that the child gets separate
875 copies of stack pages when either process modifies the stack.) In this
876 case, for correct operation, the CLONE_VM option should not be speci‐
877 fied. (If the child shares the parent's memory because of the use of
878 the CLONE_VM flag, then no copy-on-write duplication occurs and chaos
879 is likely to result.)
880 The order of the arguments also differs in the raw system call, and
882 there are variations in the arguments across architectures, as detailed
883 in the following paragraphs.
884 The raw system call interface on x86-64 and some other architectures
886 (including sh, tile, and alpha) is:
887 long clone(unsigned long flags, void *stack,
889 int *parent_tid, int *child_tid,
890 unsigned long tls);
891 On x86-32, and several other common architectures (including score,
893 ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and MIPS), the order of
894 the last two arguments is reversed:
895 long clone(unsigned long flags, void *stack,
897 int *parent_tid, unsigned long tls,
898 int *child_tid);
899 On the cris and s390 architectures, the order of the first two argu‐
901 ments is reversed:
902 long clone(void *stack, unsigned long flags,
904 int *parent_tid, int *child_tid,
905 unsigned long tls);
906 On the microblaze architecture, an additional argument is supplied:
908 long clone(unsigned long flags, void *stack,
910 int stack_size, /* Size of stack */
911 int *parent_tid, int *child_tid,
912 unsigned long tls);
913 blackfin, m68k, and sparc
915 The argument-passing conventions on blackfin, m68k, and sparc are dif‐
916 ferent from the descriptions above. For details, see the kernel (and
917 glibc) source.
918 ia64
920 On ia64, a different interface is used:
921 int __clone2(int (*fn)(void *),
923 void *stack_base, size_t stack_size,
924 int flags, void *arg, ...
925 /* pid_t *parent_tid, struct user_desc *tls,
926 pid_t *child_tid */ );
927 The prototype shown above is for the glibc wrapper function; for the
929 system call itself, the prototype can be described as follows (it is
930 identical to the clone() prototype on microblaze):
931 long clone2(unsigned long flags, void *stack_base,
933 int stack_size, /* Size of stack */
934 int *parent_tid, int *child_tid,
935 unsigned long tls);
936 __clone2() operates in the same way as clone(), except that stack_base
938 points to the lowest address of the child's stack area, and stack_size
939 specifies the size of the stack pointed to by stack_base.
940 Linux 2.4 and earlier
942 In Linux 2.4 and earlier, clone() does not take arguments parent_tid,
943 tls, and child_tid.
944
946 GNU C library versions 2.3.4 up to and including 2.24 contained a wrap‐
947 per function for getpid(2) that performed caching of PIDs. This
948 caching relied on support in the glibc wrapper for clone(), but limita‐
949 tions in the implementation meant that the cache was not up to date in
950 some circumstances. In particular, if a signal was delivered to the
951 child immediately after the clone() call, then a call to getpid(2) in a
952 handler for the signal could return the PID of the calling process
953 ("the parent"), if the clone wrapper had not yet had a chance to update
954 the PID cache in the child. (This discussion ignores the case where
955 the child was created using CLONE_THREAD, when getpid(2) should return
956 the same value in the child and in the process that called clone(),
957 since the caller and the child are in the same thread group. The
958 stale-cache problem also does not occur if the flags argument includes
959 CLONE_VM.) To get the truth, it was sometimes necessary to use code
960 such as the following:
961 #include <syscall.h>
963 pid_t mypid;
965 mypid = syscall(SYS_getpid);
967 Because of the stale-cache problem, as well as other problems noted in
969 getpid(2), the PID caching feature was removed in glibc 2.25.
970
972 The following program demonstrates the use of clone() to create a child
973 process that executes in a separate UTS namespace. The child changes
974 the hostname in its UTS namespace. Both parent and child then display
975 the system hostname, making it possible to see that the hostname dif‐
976 fers in the UTS namespaces of the parent and child. For an example of
977 the use of this program, see setns(2).
978 Within the sample program, we allocate the memory that is to be used
980 for the child's stack using mmap(2) rather than malloc(3) for the fol‐
981 lowing reasons:
982 * mmap(2) allocates a block of memory that starts on a page boundary
984 and is a multiple of the page size. This is useful if we want to
985 establish a guard page (a page with protection PROT_NONE) at the end
986 of the stack using mprotect(2).
987 * We can specify the MAP_STACK flag to request a mapping that is suit‐
989 able for a stack. For the moment, this flag is a no-op on Linux,
990 but it exists and has effect on some other systems, so we should in‐
991 clude it for portability.
992 Program source
994 #define _GNU_SOURCE
995 #include <sys/wait.h>
996 #include <sys/utsname.h>
997 #include <sched.h>
998 #include <string.h>
999 #include <stdint.h>
1000 #include <stdio.h>
1001 #include <stdlib.h>
1002 #include <unistd.h>
1003 #include <sys/mman.h>
1004 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
1006 } while (0)
1007 static int /* Start function for cloned child */
1009 childFunc(void *arg)
1010 {
1011 struct utsname uts;
1012 /* Change hostname in UTS namespace of child */
1014 if (sethostname(arg, strlen(arg)) == -1)
1016 errExit("sethostname");
1017 /* Retrieve and display hostname */
1019 if (uname(&uts) == -1)
1021 errExit("uname");
1022 printf("uts.nodename in child: %s\n", uts.nodename);
1023 /* Keep the namespace open for a while, by sleeping.
1025 This allows some experimentation--for example, another
1026 process might join the namespace. */
1027 sleep(200);
1029 return 0; /* Child terminates now */
1031 }
1032 #define STACK_SIZE (1024 * 1024) /* Stack size for cloned child */
1034 int
1036 main(int argc, char *argv[])
1037 {
1038 char *stack; /* Start of stack buffer */
1039 char *stackTop; /* End of stack buffer */
1040 pid_t pid;
1041 struct utsname uts;
1042 if (argc < 2) {
1044 fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
1045 exit(EXIT_SUCCESS);
1046 }
1047 /* Allocate memory to be used for the stack of the child */
1049 stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
1051 MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
1052 if (stack == MAP_FAILED)
1053 errExit("mmap");
1054 stackTop = stack + STACK_SIZE; /* Assume stack grows downward */
1056 /* Create child that has its own UTS namespace;
1058 child commences execution in childFunc() */
1059 pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
1061 if (pid == -1)
1062 errExit("clone");
1063 printf("clone() returned %jd\n", (intmax_t) pid);
1064 /* Parent falls through to here */
1066 sleep(1); /* Give child time to change its hostname */
1068 /* Display hostname in parent's UTS namespace. This will be
1070 different from hostname in child's UTS namespace. */
1071 if (uname(&uts) == -1)
1073 errExit("uname");
1074 printf("uts.nodename in parent: %s\n", uts.nodename);
1075 if (waitpid(pid, NULL, 0) == -1) /* Wait for child */
1077 errExit("waitpid");
1078 printf("child has terminated\n");
1079 exit(EXIT_SUCCESS);
1081 }
1082
1084 fork(2), futex(2), getpid(2), gettid(2), kcmp(2), mmap(2),
1085 pidfd_open(2), set_thread_area(2), set_tid_address(2), setns(2),
1086 tkill(2), unshare(2), wait(2), capabilities(7), namespaces(7),
1087 pthreads(7)
1088
1090 This page is part of release 5.10 of the Linux man-pages project. A
1091 description of the project, information about reporting bugs, and the
1092 latest version of this page, can be found at
1093 https://www.kernel.org/doc/man-pages/.
1094Linux 2020-11-01 CLONE(2)