diff options
Diffstat (limited to 'man2/clone.2')
| -rw-r--r-- | man2/clone.2 | 217 |
1 files changed, 109 insertions, 108 deletions
diff --git a/man2/clone.2 b/man2/clone.2 index 8e5eae806..f0c60b9e3 100644 --- a/man2/clone.2 +++ b/man2/clone.2 @@ -72,14 +72,14 @@ Standard C library glibc provides no wrapper for .BR clone3 (), necessitating the use of -.BR syscall (2). +.MR syscall 2 . .SH DESCRIPTION These system calls create a new ("child") process, in a manner similar to -.BR fork (2). +.MR fork 2 . .P By contrast with -.BR fork (2), +.MR fork 2 , these system calls provide more precise control over what pieces of execution context are shared between the calling process and the child process. For example, using these system calls, the caller can control whether @@ -87,7 +87,7 @@ or not the two processes share the virtual address space, the table of file descriptors, and the table of signal handlers. These system calls also allow the new child process to be placed in separate -.BR namespaces (7). +.MR namespaces 7 . .P Note that in this manual page, "calling process" normally corresponds to "parent process". @@ -120,10 +120,10 @@ wrapper function, it commences execution by calling the function pointed to by the argument .IR fn . (This differs from -.BR fork (2), +.MR fork 2 , where execution continues in the child from the point of the -.BR fork (2) +.MR fork 2 call.) The .I arg @@ -137,7 +137,7 @@ The integer returned by .I fn is the exit status for the child process. The child process may also terminate explicitly by calling -.BR exit (2) +.MR exit 2 or after receiving a fatal signal. .P The @@ -176,7 +176,7 @@ cleaner separation in the use of various arguments; and the ability to specify the size of the child's stack area. .P As with -.BR fork (2), +.MR fork 2 , .BR clone3 () returns in both the parent and the child. It returns 0 in the child process and returns the PID of the child @@ -298,7 +298,7 @@ then the parent process must specify the or .B __WCLONE options when waiting for the child with -.BR wait (2). +.MR wait 2 . If no signal (i.e., zero) is specified, then the parent process is not signaled when the child terminates. .\" @@ -431,7 +431,7 @@ or in child memory when the child exits, and do a wakeup on the futex at that address. The address involved may be changed by the -.BR set_tid_address (2) +.MR set_tid_address 2 system call. This is used by threading libraries. .TP @@ -495,11 +495,11 @@ Any file descriptor created by the calling process or by the child process is also valid in the other process. Similarly, if one of the processes closes a file descriptor, or changes its associated flags (using the -.BR fcntl (2) +.MR fcntl 2 .B F_SETFD operation), the other process is also affected. If a process sharing a file descriptor table calls -.BR execve (2), +.MR execve 2 , its file descriptor table is duplicated (unshared). .IP If @@ -515,7 +515,7 @@ that the duplicated file descriptors in the child refer to the same open file descriptions as the corresponding file descriptors in the calling process, and thus share file offsets and file status flags (see -.BR open (2)). +.MR open 2 ). .TP .BR CLONE_FS " (since Linux 2.0)" If @@ -525,10 +525,10 @@ information. This includes the root of the filesystem, the current working directory, and the umask. Any call to -.BR chroot (2), -.BR chdir (2), +.MR chroot 2 , +.MR chdir 2 , or -.BR umask (2) +.MR umask 2 performed by the calling process or the child process also affects the other process. .IP @@ -537,10 +537,10 @@ If is not set, the child process works on a copy of the filesystem information of the calling process at the time of the clone call. Calls to -.BR chroot (2), -.BR chdir (2), +.MR chroot 2 , +.MR chdir 2 , or -.BR umask (2) +.MR umask 2 performed later by one of the processes do not affect the other process. .TP .BR CLONE_INTO_CGROUP " (since Linux 5.7)" @@ -569,7 +569,7 @@ or the .B O_PATH flag.) Note that all of the usual restrictions (described in -.BR cgroups (7)) +.MR cgroups 7 ) on placing a process into a version 2 cgroup apply. .IP Among the possible use cases for @@ -593,7 +593,7 @@ The flag also allows the creation of frozen child processes by spawning them into a frozen cgroup. (See -.BR cgroups (7) +.MR cgroups 7 for a description of the freezer controller.) .IP \[bu] For threaded applications (or even thread implementations which @@ -608,7 +608,7 @@ If is set, then the new process shares an I/O context with the calling process. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the new process has its own I/O context. .IP .\" The following based on text from Jens Axboe @@ -622,7 +622,8 @@ For some I/O schedulers, if two processes share an I/O context, they will be allowed to interleave their disk access. If several threads are doing I/O on behalf of the same process -.RB ( aio_read (3), +\%(\c +.MR aio_read 3 , for instance), they should employ .B CLONE_IO to get better I/O performance. @@ -635,11 +636,11 @@ option, this flag is a no-op. .BR CLONE_NEWCGROUP " (since Linux 4.6)" Create the process in a new cgroup namespace. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the process is created in the same cgroup namespaces as the calling process. .IP For further information on cgroup namespaces, see -.BR cgroup_namespaces (7). +.MR cgroup_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -652,12 +653,12 @@ If .B CLONE_NEWIPC is set, then create the process in a new IPC namespace. If this flag is not set, then (as with -.BR fork (2)), +.MR fork 2 ), the process is created in the same IPC namespace as the calling process. .IP For further information on IPC namespaces, see -.BR ipc_namespaces (7). +.MR ipc_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -674,12 +675,12 @@ If .B CLONE_NEWNET is set, then create the process in a new network namespace. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the process is created in the same network namespace as the calling process. .IP For further information on network namespaces, see -.BR network_namespaces (7). +.MR network_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -697,9 +698,9 @@ is not set, the child lives in the same mount namespace as the parent. .IP For further information on mount namespaces, see -.BR namespaces (7) +.MR namespaces 7 and -.BR mount_namespaces (7). +.MR mount_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -724,14 +725,14 @@ If .B CLONE_NEWPID is set, then create the process in a new PID namespace. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the process is created in the same PID namespace as the calling process. .IP For further information on PID namespaces, see -.BR namespaces (7) +.MR namespaces 7 and -.BR pid_namespaces (7). +.MR pid_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -754,13 +755,13 @@ If .B CLONE_NEWUSER is set, then create the process in a new user namespace. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the process is created in the same user namespace as the calling process. .IP For further information on user namespaces, see -.BR namespaces (7) +.MR namespaces 7 and -.BR user_namespaces (7). +.MR user_namespaces 7 . .IP Before Linux 3.8, use of .B CLONE_NEWUSER @@ -794,12 +795,12 @@ is set, then create the process in a new UTS namespace, whose identifiers are initialized by duplicating the identifiers from the UTS namespace of the calling process. If this flag is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the process is created in the same UTS namespace as the calling process. .IP For further information on UTS namespaces, see -.BR uts_namespaces (7). +.MR uts_namespaces 7 . .IP Only a privileged process .RB ( CAP_SYS_ADMIN ) @@ -810,17 +811,17 @@ can employ If .B CLONE_PARENT is set, then the parent of the new child (as returned by -.BR getppid (2)) +.MR getppid 2 ) will be the same as that of the calling process. .IP If .B CLONE_PARENT is not set, then (as with -.BR fork (2)) +.MR fork 2 ) the child's parent is the calling process. .IP Note that it is the parent process, as returned by -.BR getppid (2), +.MR getppid 2 , which is signaled when the child terminates, so that if .B CLONE_PARENT @@ -873,7 +874,7 @@ a PID file descriptor referring to the child process is allocated and placed at a specified location in the parent's memory. The close-on-exec flag is set on this new file descriptor. PID file descriptors can be used for the purposes described in -.BR pidfd_open (2). +.MR pidfd_open 2 . .RS .IP \[bu] 3 When using @@ -920,7 +921,7 @@ If .B CLONE_PTRACE is specified, and the calling process is being traced, then trace the child also (see -.BR ptrace (2)). +.MR ptrace 2 ). .TP .BR CLONE_SETTLS " (since Linux 2.5.32)" The TLS (Thread Local Storage) descriptor is set to @@ -934,12 +935,12 @@ On x86, is interpreted as a .I struct user_desc\~* (see -.BR set_thread_area (2)). +.MR set_thread_area 2 ). On x86-64 it is the new value to be set for the %fs base register (see the .B ARCH_SET_FS argument to -.BR arch_prctl (2)). +.MR arch_prctl 2 ). On architectures with a dedicated TLS register, it is the new value of that register. .IP @@ -952,14 +953,14 @@ If is set, the calling process and the child process share the same table of signal handlers. If the calling process or child process calls -.BR sigaction (2) +.MR sigaction 2 to change the behavior associated with a signal, the behavior is changed in the other process as well. However, the calling process and child processes still have distinct signal masks and sets of pending signals. So, one of them may block or unblock signals using -.BR sigprocmask (2) +.MR sigprocmask 2 without affecting the other process. .IP If @@ -967,7 +968,7 @@ If is not set, the child process inherits a copy of the signal handlers of the calling process at the time of the clone call. Calls to -.BR sigaction (2) +.MR sigaction 2 performed later by one of the processes have no effect on the other process. .IP @@ -1010,13 +1011,13 @@ is set, then the child and the calling process share a single list of System V semaphore adjustment .RI ( semadj ) values (see -.BR semop (2)). +.MR semop 2 ). In this case, the shared list accumulates .I semadj values across all processes sharing the list, and semaphore adjustments are performed only when the last process that is sharing the list terminates (or ceases sharing the list using -.BR unshare (2)). +.MR unshare 2 ). If this flag is not set, then the child has a separate .I semadj list that is initially empty. @@ -1036,7 +1037,7 @@ POSIX threads notion of a set of threads that share a single PID. Internally, this shared PID is the so-called thread group identifier (TGID) for the thread group. Since Linux 2.4, calls to -.BR getpid (2) +.MR getpid 2 return the TGID of the caller. .IP The threads within a group can be distinguished by their (system-wide) @@ -1045,7 +1046,7 @@ A new thread's TID is available as the function result returned to the caller, and a thread can obtain its own TID using -.BR gettid (2). +.MR gettid 2 . .IP When a clone call is made without specifying .BR CLONE_THREAD , @@ -1061,7 +1062,7 @@ has the same parent process as the process that made the clone call (i.e., like .BR CLONE_PARENT ), so that calls to -.BR getppid (2) +.MR getppid 2 return the same value for all of the threads in a thread group. When a .B CLONE_THREAD @@ -1070,7 +1071,7 @@ thread terminates, the thread that created it is not sent a (or other termination) signal; nor can the status of such a thread be obtained using -.BR wait (2). +.MR wait 2 . (The thread is said to be .IR detached .) .IP @@ -1080,14 +1081,14 @@ the parent process of the thread group is sent a (or other termination) signal. .IP If any of the threads in a thread group performs an -.BR execve (2), +.MR execve 2 , then all threads other than the thread group leader are terminated, and the new program is executed in the thread group leader. .IP If one of the threads in a thread group creates a child using -.BR fork (2), +.MR fork 2 , then any thread in the group can -.BR wait (2) +.MR wait 2 for that child. .IP Since Linux 2.5.35, the @@ -1110,7 +1111,7 @@ it will affect (terminate, stop, continue, be ignored in) all members of the thread group. .IP Each thread has its own signal mask, as set by -.BR sigprocmask (2). +.MR sigprocmask 2 . .IP A signal may be process-directed or thread-directed. A process-directed signal is targeted at a thread group (i.e., a TGID), @@ -1118,15 +1119,15 @@ and is delivered to an arbitrarily selected thread from among those that are not blocking the signal. A signal may be process-directed because it was generated by the kernel for reasons other than a hardware exception, or because it was sent using -.BR kill (2) +.MR kill 2 or -.BR sigqueue (3). +.MR sigqueue 3 . A thread-directed signal is targeted at (i.e., delivered to) a specific thread. A signal may be thread directed because it was sent using -.BR tgkill (2) +.MR tgkill 2 or -.BR pthread_sigqueue (3), +.MR pthread_sigqueue 3 , or because the thread executed a machine language instruction that triggered a hardware exception (e.g., invalid memory access triggering @@ -1135,7 +1136,7 @@ or a floating-point exception triggering .BR SIGFPE ). .IP A call to -.BR sigpending (2) +.MR sigpending 2 returns a signal set that is the union of the pending process-directed signals and the signals that are pending for the calling thread. .IP @@ -1144,7 +1145,7 @@ and the thread group has installed a handler for the signal, then the handler is invoked in exactly one, arbitrarily selected member of the thread group that has not blocked the signal. If multiple threads in a group are waiting to accept the same signal using -.BR sigwaitinfo (2), +.MR sigwaitinfo 2 , the kernel will arbitrarily select one of these threads to receive the signal. .TP @@ -1161,11 +1162,11 @@ If is set, the execution of the calling process is suspended until the child releases its virtual memory resources via a call to -.BR execve (2) +.MR execve 2 or -.BR _exit (2) +.MR _exit 2 (as with -.BR vfork (2)). +.MR vfork 2 ). .IP If .B CLONE_VFORK @@ -1181,9 +1182,9 @@ space. In particular, memory writes performed by the calling process or by the child process are also visible in the other process. Moreover, any memory mapping or unmapping performed with -.BR mmap (2) +.MR mmap 2 or -.BR munmap (2) +.MR munmap 2 by the child or calling process also affects the other process. .IP If @@ -1192,7 +1193,7 @@ is not set, the child process runs in a separate copy of the memory space of the calling process at the time of the clone call. Memory writes or file mappings/unmappings performed by one of the processes do not affect the other, as with -.BR fork (2). +.MR fork 2 . .IP If the .B CLONE_VM @@ -1200,7 +1201,7 @@ flag is specified and the .B CLONE_VFORK flag is not specified, then any alternate signal stack that was established by -.BR sigaltstack (2) +.MR sigaltstack 2 is cleared in the child process. .SH RETURN VALUE .\" gettid(2) returns current->pid; @@ -1218,14 +1219,14 @@ is set to indicate the error. was specified in .IR cl_args.flags , but the restrictions (described in -.BR cgroups (7)) +.MR cgroups 7 ) on placing the child process into the version 2 cgroup referred to by .I cl_args.cgroup are not met. .TP .B EAGAIN Too many processes are already running; see -.BR fork (2). +.MR fork 2 . .TP .BR EBUSY " (" clone3 "() only)" .B CLONE_INTO_CGROUP @@ -1281,11 +1282,11 @@ was not. was specified in the .I flags mask, but the current process previously called -.BR unshare (2) +.MR unshare 2 with the .B CLONE_NEWPID flag or used -.BR setns (2) +.MR setns 2 to reassociate itself with a PID namespace. .TP .B EINVAL @@ -1468,7 +1469,7 @@ was specified in the mask, but the limit on the nesting depth of PID namespaces would have been exceeded; see -.BR pid_namespaces (7). +.MR pid_namespaces 7 . .TP .BR ENOSPC " (since Linux 4.9; beforehand " EUSERS ) .B CLONE_NEWUSER @@ -1477,7 +1478,7 @@ was specified in the mask, and the call would cause the limit on the number of nested user namespaces to be exceeded. See -.BR user_namespaces (7). +.MR user_namespaces 7 . .IP From Linux 3.11 to Linux 4.8, the error diagnosed in this case was .BR EUSERS . @@ -1490,7 +1491,7 @@ but doing so would have caused the limit defined by the corresponding file in .I /proc/sys/user to be exceeded. For further details, see -.BR namespaces (7). +.MR namespaces 7 . .TP .BR EOPNOTSUPP " (" clone3 "() only)" .B CLONE_INTO_CGROUP @@ -1524,7 +1525,7 @@ was specified in the mask, but either the effective user ID or the effective group ID of the caller does not have a mapping in the parent namespace (see -.BR user_namespaces (7)). +.MR user_namespaces 7 ). .TP .BR EPERM " (since Linux 3.9)" .\" commit 3151527ee007b73a0ebd296010f1c0454a919c7d @@ -1582,7 +1583,7 @@ should not be called through vsyscall, but directly through The raw .BR clone () system call corresponds more closely to -.BR fork (2) +.MR fork 2 in that execution in the child continues from the point of the call. As such, the @@ -1750,18 +1751,18 @@ is to implement threads: multiple flows of control in a program that run concurrently in a shared address space. .P The -.BR kcmp (2) +.MR kcmp 2 system call can be used to test whether two processes share various resources such as a file descriptor table, System V semaphore undo operations, or a virtual address space. .P Handlers registered using -.BR pthread_atfork (3) +.MR pthread_atfork 3 are not executed during a clone call. .SH BUGS GNU C library versions 2.3.4 up to and including 2.24 contained a wrapper function for -.BR getpid (2) +.MR getpid 2 that performed caching of PIDs. This caching relied on support in the glibc wrapper for .BR clone (), @@ -1771,7 +1772,7 @@ In particular, if a signal was delivered to the child immediately after the .BR clone () call, then a call to -.BR getpid (2) +.MR getpid 2 in a handler for the signal could return the PID of the calling process ("the parent"), if the clone wrapper had not yet had a chance to update the PID @@ -1779,7 +1780,7 @@ cache in the child. (This discussion ignores the case where the child was created using .BR CLONE_THREAD , when -.BR getpid (2) +.MR getpid 2 .I should return the same value in the child and in the process that called .BR clone (), @@ -1804,7 +1805,7 @@ mypid = syscall(SYS_getpid); .\" http://sourceware.org/bugzilla/show_bug.cgi?id=6910 .P Because of the stale-cache problem, as well as other problems noted in -.BR getpid (2), +.MR getpid 2 , the PID caching feature was removed in glibc 2.25. .SH EXAMPLES The following program demonstrates the use of @@ -1815,22 +1816,22 @@ Both parent and child then display the system hostname, making it possible to see that the hostname differs in the UTS namespaces of the parent and child. For an example of the use of this program, see -.BR setns (2). +.MR setns 2 . .P Within the sample program, we allocate the memory that is to be used for the child's stack using -.BR mmap (2) +.MR mmap 2 rather than -.BR malloc (3) +.MR malloc 3 for the following reasons: .IP \[bu] 3 -.BR mmap (2) +.MR mmap 2 allocates a block of memory that starts on a page boundary and is a multiple of the page size. This is useful if we want to establish a guard page (a page with protection .BR PROT_NONE ) at the end of the stack using -.BR mprotect (2). +.MR mprotect 2 . .IP \[bu] We can specify the .B MAP_STACK @@ -1931,19 +1932,19 @@ main(int argc, char *argv[]) .EE .\" SRC END .SH SEE ALSO -.BR fork (2), -.BR futex (2), -.BR getpid (2), -.BR gettid (2), -.BR kcmp (2), -.BR mmap (2), -.BR pidfd_open (2), -.BR set_thread_area (2), -.BR set_tid_address (2), -.BR setns (2), -.BR tkill (2), -.BR unshare (2), -.BR wait (2), -.BR capabilities (7), -.BR namespaces (7), -.BR pthreads (7) +.MR fork 2 , +.MR futex 2 , +.MR getpid 2 , +.MR gettid 2 , +.MR kcmp 2 , +.MR mmap 2 , +.MR pidfd_open 2 , +.MR set_thread_area 2 , +.MR set_tid_address 2 , +.MR setns 2 , +.MR tkill 2 , +.MR unshare 2 , +.MR wait 2 , +.MR capabilities 7 , +.MR namespaces 7 , +.MR pthreads 7 |