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NAME
clone, __clone2 - create a child process
SYNOPSIS
/* Prototype for the glibc wrapper function */
#define _GNU_SOURCE
#include <sched.h>
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, void *newtls, pid_t *ctid */ );
/* For the prototype of the raw system call, see NOTES */
DESCRIPTION
clone() creates a new process, in a manner similar to fork(2).
This page describes both the glibc clone() wrapper function and the underlying system call
on which it is based. The main text describes the wrapper function; the differences for
the raw system call are described toward the end of this page.
Unlike fork(2), clone() allows the child process to share parts of its execution context
with the calling process, such as the virtual address space, the table of file
descriptors, and the table of signal handlers. (Note that on this manual page, "calling
process" normally corresponds to "parent process". But see the description of
CLONE_PARENT below.)
One use of clone() is to implement threads: multiple flows of control in a program that
run concurrently in a shared address space.
When the child process is created with clone(), it commences execution by calling the
function pointed to by the argument fn. (This differs from fork(2), where execution
continues in the child from the point of the fork(2) call.) The arg argument is passed as
the argument of the function fn.
When the fn(arg) function returns, the child process terminates. The integer returned by
fn is the exit status for the child process. The child process may also terminate
explicitly by calling exit(2) or after receiving a fatal signal.
The child_stack argument specifies the location of the stack used by the child process.
Since the child and calling process may share memory, it is not possible for the child
process to execute in the same stack as the calling process. The calling process must
therefore set up memory space for the child stack and pass a pointer to this space to
clone(). Stacks grow downward on all processors that run Linux (except the HP PA
processors), so child_stack usually points to the topmost address of the memory space set
up for the child stack.
The low byte of flags contains the number of the termination signal sent to the parent
when the child dies. If this signal is specified as anything other than SIGCHLD, then the
parent process must specify the __WALL or __WCLONE options when waiting for the child with
wait(2). If no signal is specified, then the parent process is not signaled when the
child terminates.
flags may also be bitwise-ORed with zero or more of the following constants, in order to
specify what is shared between the calling process and the child process:
CLONE_CHILD_CLEARTID (since Linux 2.5.49)
Clear (zero) the child thread ID at the location ctid 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 set_tid_address(2) system call. This is used by threading
libraries.
CLONE_CHILD_SETTID (since Linux 2.5.49)
Store the child thread ID at the location ctid in the child's memory. The store
operation completes before clone() returns control to user space.
CLONE_FILES (since Linux 2.0)
If CLONE_FILES is set, the calling process and the child process share the same
file descriptor table. 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
fcntl(2) F_SETFD operation), the other process is also affected. If a process
sharing a file descriptor table calls execve(2), its file descriptor table is
duplicated (unshared).
If CLONE_FILES is not set, the child process inherits a copy of all file
descriptors opened in the calling process at the time of clone(). Subsequent
operations that open or close file descriptors, or change file descriptor flags,
performed by either the calling process or the child process do not affect the
other process. Note, however, 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
open(2)).
CLONE_FS (since Linux 2.0)
If CLONE_FS is set, the caller and the child process share the same filesystem
information. This includes the root of the filesystem, the current working
directory, and the umask. Any call to chroot(2), chdir(2), or umask(2) performed
by the calling process or the child process also affects the other process.
If CLONE_FS 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
chroot(2), chdir(2), or umask(2) performed later by one of the processes do not
affect the other process.
CLONE_IO (since Linux 2.6.25)
If CLONE_IO is set, then the new process shares an I/O context with the calling
process. If this flag is not set, then (as with fork(2)) the new process has its
own I/O context.
The I/O context is the I/O scope of the disk scheduler (i.e., what the I/O
scheduler uses to model scheduling of a process's I/O). If processes share the
same I/O context, they are treated as one by the I/O scheduler. As a consequence,
they get to share disk time. 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 (aio_read(3), for instance),
they should employ CLONE_IO to get better I/O performance.
If the kernel is not configured with the CONFIG_BLOCK option, this flag is a no-op.
CLONE_NEWCGROUP (since Linux 4.6)
Create the process in a new cgroup namespace. If this flag is not set, then (as
with fork(2)) the process is created in the same cgroup namespaces as the calling
process. This flag is intended for the implementation of containers.
For further information on cgroup namespaces, see cgroup_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.
CLONE_NEWIPC (since Linux 2.6.19)
If CLONE_NEWIPC is set, then create the process in a new IPC namespace. If this
flag is not set, then (as with fork(2)), the process is created in the same IPC
namespace as the calling process. This flag is intended for the implementation of
containers.
An IPC namespace provides an isolated view of System V IPC objects (see svipc(7))
and (since Linux 2.6.30) POSIX message queues (see mq_overview(7)). The common
characteristic of these IPC mechanisms is that IPC objects are identified by
mechanisms other than filesystem pathnames.
Objects created in an IPC namespace are visible to all other processes that are
members of that namespace, but are not visible to processes in other IPC
namespaces.
When an IPC namespace is destroyed (i.e., when the last process that is a member of
the namespace terminates), all IPC objects in the namespace are automatically
destroyed.
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWIPC. This flag can't
be specified in conjunction with CLONE_SYSVSEM.
For further information on IPC namespaces, see namespaces(7).
CLONE_NEWNET (since Linux 2.6.24)
(The implementation of this flag was completed only by about kernel version
2.6.29.)
If CLONE_NEWNET is set, then create the process in a new network namespace. If
this flag is not set, then (as with fork(2)) the process is created in the same
network namespace as the calling process. This flag is intended for the
implementation of containers.
A network namespace provides an isolated view of the networking stack (network
device interfaces, IPv4 and IPv6 protocol stacks, IP routing tables, firewall
rules, the /proc/net and /sys/class/net directory trees, sockets, etc.). A
physical network device can live in exactly one network namespace. A virtual
network (veth(4)) device pair provides a pipe-like abstraction that can be used to
create tunnels between network namespaces, and can be used to create a bridge to a
physical network device in another namespace.
When a network namespace is freed (i.e., when the last process in the namespace
terminates), its physical network devices are moved back to the initial network
namespace (not to the parent of the process). For further information on network
namespaces, see namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNET.
CLONE_NEWNS (since Linux 2.4.19)
If CLONE_NEWNS is set, the cloned child is started in a new mount namespace,
initialized with a copy of the namespace of the parent. If CLONE_NEWNS is not set,
the child lives in the same mount namespace as the parent.
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNS. It is not
permitted to specify both CLONE_NEWNS and CLONE_FS in the same clone() call.
For further information on mount namespaces, see namespaces(7) and
mount_namespaces(7).
CLONE_NEWPID (since Linux 2.6.24)
If CLONE_NEWPID is set, then create the process in a new PID namespace. If this
flag is not set, then (as with fork(2)) the process is created in the same PID
namespace as the calling process. This flag is intended for the implementation of
containers.
For further information on PID namespaces, see namespaces(7) and pid_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWPID. This flag can't
be specified in conjunction with CLONE_THREAD or CLONE_PARENT.
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.23, the current clone()
semantics were merged in Linux 3.5, and the final pieces to make the user
namespaces completely usable were merged in Linux 3.8.)
If CLONE_NEWUSER is set, then create the process in a new user namespace. If this
flag is not set, then (as with fork(2)) the process is created in the same user
namespace as the calling process.
Before Linux 3.8, use of CLONE_NEWUSER required that the caller have three
capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SETGID. Starting with Linux 3.8,
no privileges are needed to create a user namespace.
This flag can't be specified in conjunction with CLONE_THREAD or CLONE_PARENT. For
security reasons, CLONE_NEWUSER cannot be specified in conjunction with CLONE_FS.
For further information on user namespaces, see namespaces(7) and
user_namespaces(7).
CLONE_NEWUTS (since Linux 2.6.19)
If CLONE_NEWUTS 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 fork(2)) the
process is created in the same UTS namespace as the calling process. This flag is
intended for the implementation of containers.
A UTS namespace is the set of identifiers returned by uname(2); among these, the
domain name and the hostname can be modified by setdomainname(2) and
sethostname(2), respectively. Changes made to the identifiers in a UTS namespace
are visible to all other processes in the same namespace, but are not visible to
processes in other UTS namespaces.
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWUTS.
For further information on UTS namespaces, see namespaces(7).
CLONE_PARENT (since Linux 2.3.12)
If CLONE_PARENT is set, then the parent of the new child (as returned by
getppid(2)) will be the same as that of the calling process.
If CLONE_PARENT is not set, then (as with fork(2)) the child's parent is the
calling process.
Note that it is the parent process, as returned by getppid(2), which is signaled
when the child terminates, so that if CLONE_PARENT is set, then the parent of the
calling process, rather than the calling process itself, will be signaled.
CLONE_PARENT_SETTID (since Linux 2.5.49)
Store the child thread ID at the location ptid in the parent's memory. (In Linux
2.5.32-2.5.48 there was a flag CLONE_SETTID that did this.) The store operation
completes before clone() returns control to user space.
CLONE_PID (Linux 2.0 to 2.5.15)
If CLONE_PID is set, the child process is created with the same process ID as the
calling process. This is good for hacking the system, but otherwise of not much
use. From Linux 2.3.21 onward, this flag could be specified only by the system
boot process (PID 0). The flag disappeared completely from the kernel sources in
Linux 2.5.16. Since then, the kernel silently ignores this bit if it is specified
in flags.
CLONE_PTRACE (since Linux 2.2)
If CLONE_PTRACE is specified, and the calling process is being traced, then trace
the child also (see ptrace(2)).
CLONE_SETTLS (since Linux 2.5.32)
The TLS (Thread Local Storage) descriptor is set to newtls.
The interpretation of newtls and the resulting effect is architecture dependent.
On x86, newtls is interpreted as a struct user_desc * (see set_thread_area(2)). On
x86-64 it is the new value to be set for the %fs base register (see the ARCH_SET_FS
argument to arch_prctl(2)). On architectures with a dedicated TLS register, it is
the new value of that register.
CLONE_SIGHAND (since Linux 2.0)
If CLONE_SIGHAND is set, the calling process and the child process share the same
table of signal handlers. If the calling process or child process calls
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 sigprocmask(2) without affecting the other
process.
If CLONE_SIGHAND is not set, the child process inherits a copy of the signal
handlers of the calling process at the time clone() is called. Calls to
sigaction(2) performed later by one of the processes have no effect on the other
process.
Since Linux 2.6.0-test6, flags must also include CLONE_VM if CLONE_SIGHAND is
specified
CLONE_STOPPED (since Linux 2.6.0-test2)
If CLONE_STOPPED is set, then the child is initially stopped (as though it was sent
a SIGSTOP signal), and must be resumed by sending it a SIGCONT signal.
This flag was deprecated from Linux 2.6.25 onward, and was removed altogether in
Linux 2.6.38. Since then, the kernel silently ignores it without error. Starting
with Linux 4.6, the same bit was reused for the CLONE_NEWCGROUP flag.
CLONE_SYSVSEM (since Linux 2.5.10)
If CLONE_SYSVSEM is set, then the child and the calling process share a single list
of System V semaphore adjustment (semadj) values (see semop(2)). In this case, the
shared list accumulates 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 unshare(2)). If this flag is not
set, then the child has a separate semadj list that is initially empty.
CLONE_THREAD (since Linux 2.4.0-test8)
If CLONE_THREAD is set, the child is placed in the same thread group as the calling
process. To make the remainder of the discussion of CLONE_THREAD more readable,
the term "thread" is used to refer to the processes within a thread group.
Thread groups were a feature added in Linux 2.4 to support the 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 getpid(2) return the TGID of the caller.
The threads within a group can be distinguished by their (system-wide) unique
thread IDs (TID). A new thread's TID is available as the function result returned
to the caller of clone(), and a thread can obtain its own TID using gettid(2).
When a call is made to clone() without specifying CLONE_THREAD, then the resulting
thread is placed in a new thread group whose TGID is the same as the thread's TID.
This thread is the leader of the new thread group.
A new thread created with CLONE_THREAD has the same parent process as the caller of
clone() (i.e., like CLONE_PARENT), so that calls to getppid(2) return the same
value for all of the threads in a thread group. When a CLONE_THREAD thread
terminates, the thread that created it using clone() is not sent a SIGCHLD (or
other termination) signal; nor can the status of such a thread be obtained using
wait(2). (The thread is said to be detached.)
After all of the threads in a thread group terminate the parent process of the
thread group is sent a SIGCHLD (or other termination) signal.
If any of the threads in a thread group performs an execve(2), then all threads
other than the thread group leader are terminated, and the new program is executed
in the thread group leader.
If one of the threads in a thread group creates a child using fork(2), then any
thread in the group can wait(2) for that child.
Since Linux 2.5.35, flags must also include CLONE_SIGHAND if CLONE_THREAD is
specified (and note that, since Linux 2.6.0-test6, CLONE_SIGHAND also requires
CLONE_VM to be included).
Signals may be sent to a thread group as a whole (i.e., a TGID) using kill(2), or
to a specific thread (i.e., TID) using tgkill(2).
Signal dispositions and actions are process-wide: if an unhandled signal is
delivered to a thread, then it will affect (terminate, stop, continue, be ignored
in) all members of the thread group.
Each thread has its own signal mask, as set by sigprocmask(2), but signals can be
pending either: for the whole process (i.e., deliverable to any member of the
thread group), when sent with kill(2); or for an individual thread, when sent with
tgkill(2). A call to sigpending(2) returns a signal set that is the union of the
signals pending for the whole process and the signals that are pending for the
calling thread.
If kill(2) is used to send a signal to a thread group, and the thread group has
installed a handler for the signal, then the handler will be 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
sigwaitinfo(2), the kernel will arbitrarily select one of these threads to receive
a signal sent using kill(2).
CLONE_UNTRACED (since Linux 2.5.46)
If CLONE_UNTRACED is specified, then a tracing process cannot force CLONE_PTRACE on
this child process.
CLONE_VFORK (since Linux 2.2)
If CLONE_VFORK is set, the execution of the calling process is suspended until the
child releases its virtual memory resources via a call to execve(2) or _exit(2) (as
with vfork(2)).
If CLONE_VFORK is not set, then both the calling process and the child are
schedulable after the call, and an application should not rely on execution
occurring in any particular order.
CLONE_VM (since Linux 2.0)
If CLONE_VM is set, the calling process and the child process run in the same
memory 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 mmap(2) or munmap(2) by the child or calling
process also affects the other process.
If CLONE_VM is not set, the child process runs in a separate copy of the memory
space of the calling process at the time of clone(). Memory writes or file
mappings/unmappings performed by one of the processes do not affect the other, as
with fork(2).
NOTES
Note that the glibc clone() wrapper function makes some changes in the memory pointed to
by child_stack (changes required to set the stack up correctly for the child) before
invoking the clone() system call. So, in cases where clone() is used to recursively
create children, do not use the buffer employed for the parent's stack as the stack of the
child.
C library/kernel differences
The raw clone() system call corresponds more closely to fork(2) in that execution in the
child continues from the point of the call. As such, the fn and arg arguments of the
clone() wrapper function are omitted.
Another difference for the raw clone() system call is that the child_stack argument may be
zero, in which case the child uses a duplicate of the parent's stack. (Copy-on-write
semantics ensure that the child gets separate copies of stack pages when either process
modifies the stack.) In this case, for correct operation, the CLONE_VM option should not
be specified. (If the child shares the parent's memory because of the use of the CLONE_VM
flag, then no copy-on-write duplication occurs and chaos is likely to result.)
The order of the arguments also differs in the raw system call, and there are variations
in the arguments across architectures, as detailed in the following paragraphs.
The raw system call interface on x86-64 and some other architectures (including sh, tile,
and alpha) is roughly:
long clone(unsigned long flags, void *child_stack,
int *ptid, int *ctid,
unsigned long newtls);
On x86-32, and several other common architectures (including score, ARM, ARM 64, PA-RISC,
arc, Power PC, xtensa, and MIPS), the order of the last two arguments is reversed:
long clone(unsigned long flags, void *child_stack,
int *ptid, unsigned long newtls,
int *ctid);
On the cris and s390 architectures, the order of the first two arguments is reversed:
long clone(void *child_stack, unsigned long flags,
int *ptid, int *ctid,
unsigned long newtls);
On the microblaze architecture, an additional argument is supplied:
long clone(unsigned long flags, void *child_stack,
int stack_size, /* Size of stack */
int *ptid, int *ctid,
unsigned long newtls);
blackfin, m68k, and sparc
The argument-passing conventions on blackfin, m68k, and sparc are different from the
descriptions above. For details, see the kernel (and glibc) source.
ia64
On ia64, a different interface is used:
int __clone2(int (*fn)(void *),
void *child_stack_base, size_t stack_size,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
The prototype shown above is for the glibc wrapper function; the raw system call interface
has no fn or arg argument, and changes the order of the arguments so that flags is the
first argument, and tls is the last argument.
__clone2() operates in the same way as clone(), except that child_stack_base points to the
lowest address of the child's stack area, and stack_size specifies the size of the stack
pointed to by child_stack_base.
Linux 2.4 and earlier
In Linux 2.4 and earlier, clone() does not take arguments ptid, tls, and ctid.
RETURN VALUE
On success, the thread ID of the child process is returned in the caller's thread of
execution. On failure, -1 is returned in the caller's context, no child process will be
created, and errno will be set appropriately.
ERRORS
EAGAIN Too many processes are already running; see fork(2).
EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not. (Since Linux 2.6.0-test6.)
EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not. (Since Linux 2.5.35.)
EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.
EINVAL (since Linux 3.9)
Both CLONE_NEWUSER and CLONE_FS were specified in flags.
EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.
EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both) of CLONE_THREAD or
CLONE_PARENT were specified in flags.
EINVAL Returned by the glibc clone() wrapper function when fn or child_stack is specified
as NULL.
EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not configured with the
CONFIG_SYSVIPC and CONFIG_IPC_NS options.
EINVAL CLONE_NEWNET was specified in flags, but the kernel was not configured with the
CONFIG_NET_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the kernel was not configured with the
CONFIG_PID_NS option.
EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not configured with the
CONFIG_UTS option.
EINVAL child_stack is not aligned to a suitable boundary for this architecture. For
example, on aarch64, child_stack must be a multiple of 16.
ENOMEM Cannot allocate sufficient memory to allocate a task structure for the child, or to
copy those parts of the caller's context that need to be copied.
ENOSPC (since Linux 3.7)
CLONE_NEWPID was specified in flags, but the limit on the nesting depth of PID
namespaces would have been exceeded; see pid_namespaces(7).
ENOSPC (since Linux 4.9; beforehand EUSERS)
CLONE_NEWUSER was specified in flags, and the call would cause the limit on the
number of nested user namespaces to be exceeded. See user_namespaces(7).
From Linux 3.11 to Linux 4.8, the error diagnosed in this case was EUSERS.
ENOSPC (since Linux 4.9)
One of the values in flags specified the creation of a new user namespace, but
doing so would have caused the limit defined by the corresponding file in
/proc/sys/user to be exceeded. For further details, see namespaces(7).
EPERM CLONE_NEWCGROUP, CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS, CLONE_NEWPID, or
CLONE_NEWUTS was specified by an unprivileged process (process without
CAP_SYS_ADMIN).
EPERM CLONE_PID was specified by a process other than process 0. (This error occurs only
on Linux 2.5.15 and earlier.)
EPERM CLONE_NEWUSER was specified in flags, but either the effective user ID or the
effective group ID of the caller does not have a mapping in the parent namespace
(see user_namespaces(7)).
EPERM (since Linux 3.9)
CLONE_NEWUSER was specified in flags and the caller is in a chroot environment
(i.e., the caller's root directory does not match the root directory of the mount
namespace in which it resides).
ERESTARTNOINTR (since Linux 2.6.17)
System call was interrupted by a signal and will be restarted. (This can be seen
only during a trace.)
EUSERS (Linux 3.11 to Linux 4.8)
CLONE_NEWUSER was specified in flags, and the limit on the number of nested user
namespaces would be exceeded. See the discussion of the ENOSPC error above.
CONFORMING TO
clone() is Linux-specific and should not be used in programs intended to be portable.
NOTES
The 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.
Handlers registered using pthread_atfork(3) are not executed during a call to clone().
In the Linux 2.4.x series, CLONE_THREAD generally does not make the parent of the new
thread the same as the parent of the calling process. However, for kernel versions 2.4.7
to 2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0 and
later).
For a while there was CLONE_DETACHED (introduced in 2.5.32): parent wants no child-exit
signal. In Linux 2.6.2, the need to give this flag together with CLONE_THREAD
disappeared. This flag is still defined, but has no effect.
On i386, clone() should not be called through vsyscall, but directly through int $0x80.
BUGS
GNU C library versions 2.3.4 up to and including 2.24 contained a wrapper function for
getpid(2) that performed caching of PIDs. This caching relied on support in the glibc
wrapper for clone(), but limitations in the implementation meant that the cache was not up
to date in some circumstances. In particular, if a signal was delivered to the child
immediately after the clone() call, then a call to 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 cache in the child. (This discussion ignores the case
where the child was created using CLONE_THREAD, when getpid(2) should return the same
value in the child and in the process that called clone(), since the caller and the child
are in the same thread group. The stale-cache problem also does not occur if the flags
argument includes CLONE_VM.) To get the truth, it was sometimes necessary to use code
such as the following:
#include <syscall.h>
pid_t mypid;
mypid = syscall(SYS_getpid);
Because of the stale-cache problem, as well as other problems noted in getpid(2), the PID
caching feature was removed in glibc 2.25.
EXAMPLE
The following program demonstrates the use of clone() to create a child process that
executes in a separate UTS namespace. The child changes the hostname in its UTS
namespace. 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 setns(2).
Program source
#define _GNU_SOURCE
#include <sys/wait.h>
#include <sys/utsname.h>
#include <sched.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct utsname uts;
/* Change hostname in UTS namespace of child */
if (sethostname(arg, strlen(arg)) == -1)
errExit("sethostname");
/* Retrieve and display hostname */
if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in child: %s\n", uts.nodename);
/* Keep the namespace open for a while, by sleeping.
This allows some experimentation--for example, another
process might join the namespace. */
sleep(200);
return 0; /* Child terminates now */
}
#define STACK_SIZE (1024 * 1024) /* Stack size for cloned child */
int
main(int argc, char *argv[])
{
char *stack; /* Start of stack buffer */
char *stackTop; /* End of stack buffer */
pid_t pid;
struct utsname uts;
if (argc < 2) {
fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
exit(EXIT_SUCCESS);
}
/* Allocate stack for child */
stack = malloc(STACK_SIZE);
if (stack == NULL)
errExit("malloc");
stackTop = stack + STACK_SIZE; /* Assume stack grows downward */
/* Create child that has its own UTS namespace;
child commences execution in childFunc() */
pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
if (pid == -1)
errExit("clone");
printf("clone() returned %ld\n", (long) pid);
/* Parent falls through to here */
sleep(1); /* Give child time to change its hostname */
/* Display hostname in parent's UTS namespace. This will be
different from hostname in child's UTS namespace. */
if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in parent: %s\n", uts.nodename);
if (waitpid(pid, NULL, 0) == -1) /* Wait for child */
errExit("waitpid");
printf("child has terminated\n");
exit(EXIT_SUCCESS);
}
SEE ALSO
fork(2), futex(2), getpid(2), gettid(2), kcmp(2), set_thread_area(2), set_tid_address(2),
setns(2), tkill(2), unshare(2), wait(2), capabilities(7), namespaces(7), pthreads(7)
COLOPHON
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