Section (2) clone
Name
clone, __clone2, clone3 — create a child process
Synopsis
/* Prototype for the glibc wrapper function */ #define _GNU_SOURCE #include <sched.h>
int
clone( |
int (*fn)( void
*) , |
void *stack, | |
int flags, | |
void *arg, | |
... /* pid_t *parent_tid, void
*tls, pid_t *child_tid */) ; |
/* For the prototype of the raw clone() system call, see NOTES */
long
clone3( |
struct clone_args *cl_args, |
size_t size) ; |
![]() |
Note |
---|---|
There is not yet a glibc wrapper for |
.BR clone3 (); see NOTES.
DESCRIPTION
These system calls create a new (child) process, in a manner similar to fork(2).
By contrast with 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 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 namespaces(7).
Note that in this manual page, calling process normally
corresponds to parent process. But see the description of
CLONE_PARENT
below.
This page describes the following interfaces:
-
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. -
The newer
clone3
() system call.
In the remainder of this page, the terminology the clone call is used when noting details that apply to all of these interfaces,
The clone() wrapper function
When the child process is created with the clone
() wrapper function, 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 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 stack
usually points to the
topmost address of the memory space set up for the child
stack. Note that clone
() does
not provide a means whereby the caller can inform the
kernel of the size of the stack area.
The remaining arguments to clone
() are discussed below.
clone3()
The clone3
() system call
provides a superset of the functionality of the older
clone
() interface. It also
provides a number of API improvements, including: space for
additional flags bits; cleaner separation in the use of
various arguments; and the ability to specify the size of
the child_zsingle_quotesz_s stack area.
As with fork(2), clone3
() returns in both the parent and
the child. It returns 0 in the child process and returns
the PID of the child in the parent.
The cl_args
argument of clone3
() is a
structure of the following form:
struct clone_args { u64 flags
; /* Flags bit mask */u64 pidfd
; /* Where to store PID file descriptor
(pid_t *) */u64 child_tid
; /* Where to store child TID,
in child_zsingle_quotesz_s memory (pid_t *) */u64 parent_tid
; /* Where to store child TID,
in parent_zsingle_quotesz_s memory (int *) */u64 exit_signal
; /* Signal to deliver to parent on
child termination */u64 stack
; /* Pointer to lowest byte of stack */u64 stack_size
; /* Size of stack */u64 tls
; /* Location of new TLS */};
The size
argument that is supplied to clone3
() should be initialized to the
size of this structure. (The existence of the size
argument permits future
extensions to the clone_args structure.)
The stack for the child process is specified via
cl_args.stack
,
which points to the lowest byte of the stack area, and
cl_args.stack_size
, which
specifies the size of the stack in bytes. In the case where
the CLONE_VM
flag (see below)
is specified, a stack must be explicitly allocated and
specified. Otherwise, these two fields can be specified as
NULL and 0, which causes the child to use the same stack
area as the parent (in the child_zsingle_quotesz_s own virtual address
space).
The remaining fields in the cl_args
argument are
discussed below.
Equivalence between clone() and clone3() arguments
Unlike the older clone
()
interface, where arguments are passed individually, in the
newer clone3
() interface the
arguments are packaged into the clone_args structure shown above. This
structure allows for a superset of the information passed
via the clone
()
arguments.
The following table shows the equivalence between the
arguments of clone
() and the
fields in the clone_args
argument supplied to clone3
():
clone() clone(3) cl_args
fieldNotes flags & ~0xff flags For most flags; details below parent_tid pidfd See CLONE_PIDFD child_tid child_tid See CLONE_CHILD_SETTID parent_tid parent_tid See CLONE_PARENT_SETTID flags & 0xff exit_signal stack stack −−−
stack_size tls tls See CLONE_SETTLS
The child termination signal
When the child process terminates, a signal may be sent
to the parent. The termination signal is specified in the
low byte of flags
(clone
()) or in cl_args.exit_signal
(clone3
()). 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
(i.e., zero) is specified, then the parent process is not
signaled when the child terminates.
The flags mask
Both clone
() and
clone3
() allow a flags bit
mask that modifies their behavior and allows the caller to
specify what is shared between the calling process and the
child process. This bit mask—the flags
argument of
clone
() or the cl_args.flags
field passed
to clone3
()(emis referred to
as the flags
mask
in the remainder of this page.
The flags
mask
is specified as a bitwise-OR of zero or more of the
constants listed below. Except as noted below, these flags
are available (and have the same effect) in both
clone
() and clone3
().
CLONE_CHILD_CLEARTID
(since Linux 2.5.49)-
Clear (zero) the child thread ID at the location pointed to by
child_tid
(clone
()) orcl_args.child_tid
(clone3
()) 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 pointed to by
child_tid
(clone
()) orcl_args.child_tid
(clone3
()) in the child_zsingle_quotesz_s memory. The store operation completes before the clone call returns control to user space in the child process. (Note that the store operation may not have completed before the clone call returns in the parent process, which will be relevant if theCLONE_VM
flag is also employed.) CLONE_DETACHED
(historical)-
For a while (during the Linux 2.5 development series) there was a
CLONE_DETACHED
flag, which caused the parent not to receive a signal when the child terminated. Ultimately, the effect of this flag was subsumed under theCLONE_THREAD
flag and by the time Linux 2.6.0 was released, this flag had no effect. Starting in Linux 2.6.2, the need to give this flag together withCLONE_THREAD
disappeared.This flag is still defined, but it is usually ignored when calling
clone
(). However, see the description ofCLONE_PIDFD
for some exceptions. 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 the clone call. 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_zsingle_quotesz_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.
For further information on cgroup namespaces, see cgroup_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_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.For further information on IPC namespaces, see ipc_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_NEWIPC
. This flag can_zsingle_quotesz_t be specified in conjunction withCLONE_SYSVSEM
. 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.For further information on network namespaces, see network_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_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. IfCLONE_NEWNS
is not set, the child lives in the same mount namespace as the parent.For further information on mount namespaces, see namespaces(7) and mount_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_NEWNS
. It is not permitted to specify bothCLONE_NEWNS
andCLONE_FS
in the same clone call. 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.For further information on PID namespaces, see namespaces(7) and pid_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_NEWPID
. This flag can_zsingle_quotesz_t be specified in conjunction withCLONE_THREAD
orCLONE_PARENT
. CLONE_NEWUSER
-
(This flag first became meaningful for
clone
() in Linux 2.6.23, the currentclone
() 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.For further information on user namespaces, see namespaces(7) and user_namespaces(7).
Before Linux 3.8, use of
CLONE_NEWUSER
required that the caller have three capabilities:CAP_SYS_ADMIN
,CAP_SETUID
, andCAP_SETGID
. Starting with Linux 3.8, no privileges are needed to create a user namespace.This flag can_zsingle_quotesz_t be specified in conjunction with
CLONE_THREAD
orCLONE_PARENT
. For security reasons,CLONE_NEWUSER
cannot be specified in conjunction withCLONE_FS
. 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.For further information on UTS namespaces, see uts_namespaces(7).
Only a privileged process (
CAP_SYS_ADMIN
) can employCLONE_NEWUTS
. 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_zsingle_quotesz_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 pointed to by
parent_tid
(clone
()) orcl_args.child_tid
(clone3
()) in the parent_zsingle_quotesz_s memory. (In Linux 2.5.32-2.5.48 there was a flagCLONE_SETTID
that did this.) The store operation completes before the clone call 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. Subsequently, the kernel silently ignored this bit if it was specified in theflags
mask. Much later, the same bit was recycled for use as theCLONE_PIDFD
flag. CLONE_PIDFD
(since Linux 5.2)-
If this flag is specified, a PID file descriptor referring to the child process is allocated and placed at a specified location in the parent_zsingle_quotesz_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 pidfd_open(2).
-
When using
clone3
(), the PID file descriptor is placed at the location pointed to bycl_args.pidfd
. -
When using
clone
(), the PID file descriptor is placed at the location pointed to byparent_tid
. Since theparent_tid
argument is used to return the PID file descriptor,CLONE_PIDFD
cannot be used withCLONE_PARENT_SETTID
when callingclone
().
It is currently not possible to use this flag together with
CLONE_THREAD.
This means that the process identified by the PID file descriptor will always be a thread group leader.If the obsolete
CLONE_DETACHED
flag is specified alongsideCLONE_PIDFD
when callingclone
(), an error is returned. An error also results ifCLONE_DETACHED
is specified when callingclone3
(). This error behavior ensures that the bit corresponding toCLONE_DETACHED
can be reused for further PID file descriptor features in the future. -
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
tls
.The interpretation of
tls
and the resulting effect is architecture dependent. On x86,tls
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 theARCH_SET_FS
argument to arch_prctl(2)). On architectures with a dedicated TLS register, it is the new value of that register.Use of this flag requires detailed knowledge and generally it should not be used except in libraries implementing threading.
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 of the clone call. Calls to sigaction(2) performed later by one of the processes have no effect on the other process.Since Linux 2.6.0, the
flags
mask must also includeCLONE_VM
ifCLONE_SIGHAND
is specified CLONE_STOPPED
(since Linux 2.6.0)-
If
CLONE_STOPPED
is set, then the child is initially stopped (as though it was sent aSIGSTOP
signal), and must be resumed by sending it aSIGCONT
signal.This flag was
deprecated
from Linux 2.6.25 onward, and wasremoved
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 theCLONE_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 accumulatessemadj
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 separatesemadj
list that is initially empty. CLONE_THREAD
(since Linux 2.4.0)-
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 ofCLONE_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_zsingle_quotesz_s TID is available as the function result returned to the caller, and a thread can obtain its own TID using gettid(2).
When a clone call is made without specifying
CLONE_THREAD
, then the resulting thread is placed in a new thread group whose TGID is the same as the thread_zsingle_quotesz_s TID. This thread is theleader
of the new thread group.A new thread created with
CLONE_THREAD
has the same parent process as the process that made the clone call (i.e., likeCLONE_PARENT
), so that calls to getppid(2) return the same value for all of the threads in a thread group. When aCLONE_THREAD
thread terminates, the thread that created it is not sent aSIGCHLD
(or other termination) signal; nor can the status of such a thread be obtained using wait(2). (The thread is said to bedetached
.)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, the
flags
mask must also includeCLONE_SIGHAND
ifCLONE_THREAD
is specified (and note that, since Linux 2.6.0,CLONE_SIGHAND
also requiresCLONE_VM
to be included).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).
A signal may be process-directed or thread-directed. A process-directed signal is targeted at a thread group (i.e., a TGID), 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 kill(2) or 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 tgkill(2) or pthread_sigqueue(3), or because the thread executed a machine language instruction that triggered a hardware exception (e.g., invalid memory access triggering
SIGSEGV
or a floating-point exception triggeringSIGFPE
).A call to 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.
If a process-directed signal is delivered 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 the signal.
CLONE_UNTRACED
(since Linux 2.5.46)-
If
CLONE_UNTRACED
is specified, then a tracing process cannot forceCLONE_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 the clone call. Memory writes or file mappings/unmappings performed by one of the processes do not affect the other, as with fork(2).
NOTES
One use of these systems calls is to implement threads: multiple flows of control in a program that run concurrently in a shared address space.
Glibc does not provide a wrapper for clone3
(); call it using syscall(2).
Note that the glibc clone
()
wrapper function makes some changes in the memory pointed to
by 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_zsingle_quotesz_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.
In contrast to the glibc wrapper, the raw clone
() system call accepts NULL as a
stack
argument (and
clone3
() likewise allows
cl_args.stack
to
be NULL). In this case, the child uses a duplicate of the
parent_zsingle_quotesz_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_zsingle_quotesz_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:
long clone
(unsigned long flags
,void *stack
,int *parent_tid
,int *child_tid
,unsigned long tls
);
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 *stack
,int *parent_tid
,unsigned long tls
,int *child_tid
);
On the cris and s390 architectures, the order of the first two arguments is reversed:
long clone
(void *stack
,unsigned long flags
,int *parent_tid
,int *child_tid
,unsigned long tls
);
On the microblaze architecture, an additional argument is supplied:
long clone
(unsigned long flags
,void *stack
,int stack_size
,fR /* Size of stack */ int *parent_tid
,int *child_tid
,unsigned long tls
);
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 *stack_base, size_t stack_size, int flags, void *arg, ... /* pid_t *parent_tid, struct user_desc *tls, pid_t *child_tid */ )
;
The prototype shown above is for the glibc wrapper
function; for the system call itself, the prototype can be
described as follows (it is identical to the clone
() prototype on microblaze):
long clone2
(unsigned long flags
,void *stack_base
,int stack_size
,fR /* Size of stack */ int *parent_tid
,int *child_tid
,unsigned long tls
);
__clone2
() operates in the
same way as clone
(), except
that stack_base
points to the lowest address of the child_zsingle_quotesz_s stack area, and
stack_size
specifies the size of the stack pointed to by stack_base
.
RETURN VALUE
On success, the thread ID of the child process is returned
in the caller_zsingle_quotesz_s thread of execution. On failure, −1 is
returned in the caller_zsingle_quotesz_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 in theflags
mask, butCLONE_VM
was not. (Since Linux 2.6.0.) - EINVAL
-
CLONE_THREAD
was specified in theflags
mask, butCLONE_SIGHAND
was not. (Since Linux 2.5.35.) - EINVAL
-
CLONE_THREAD
was specified in theflags
mask, but the current process previously called unshare(2) with theCLONE_NEWPID
flag or used setns(2) to reassociate itself with a PID namespace. - EINVAL
-
Both
CLONE_FS
andCLONE_NEWNS
were specified in theflags
mask. - EINVAL (since Linux 3.9)
-
Both
CLONE_NEWUSER
andCLONE_FS
were specified in theflags
mask. - EINVAL
-
Both
CLONE_NEWIPC
andCLONE_SYSVSEM
were specified in theflags
mask. - EINVAL
-
One (or both) of
CLONE_NEWPID
orCLONE_NEWUSER
and one (or both) ofCLONE_THREAD
orCLONE_PARENT
were specified in theflags
mask. - EINVAL
-
Returned by the glibc
clone
() wrapper function whenfn
orstack
is specified as NULL. - EINVAL
-
CLONE_NEWIPC
was specified in theflags
mask, but the kernel was not configured with theCONFIG_SYSVIPC
andCONFIG_IPC_NS
options. - EINVAL
-
CLONE_NEWNET
was specified in theflags
mask, but the kernel was not configured with theCONFIG_NET_NS
option. - EINVAL
-
CLONE_NEWPID
was specified in theflags
mask, but the kernel was not configured with theCONFIG_PID_NS
option. - EINVAL
-
CLONE_NEWUSER
was specified in theflags
mask, but the kernel was not configured with theCONFIG_USER_NS
option. - EINVAL
-
CLONE_NEWUTS
was specified in theflags
mask, but the kernel was not configured with theCONFIG_UTS_NS
option. - EINVAL
-
stack
is not aligned to a suitable boundary for this architecture. For example, on aarch64,stack
must be a multiple of 16. EINVAL (clone3
() only-
CLONE_DETACHED
was specified in theflags
mask. EINVAL (clone
() only-
CLONE_PIDFD
was specified together withCLONE_DETACHED
in theflags
mask. - EINVAL
-
CLONE_PIDFD
was specified together withCLONE_THREAD
in theflags
mask. EINVAL (clone
() only)-
CLONE_PIDFD
was specified together withCLONE_PARENT_SETTID
in theflags
mask. - ENOMEM
-
Cannot allocate sufficient memory to allocate a task structure for the child, or to copy those parts of the caller_zsingle_quotesz_s context that need to be copied.
- ENOSPC (since Linux 3.7)
-
CLONE_NEWPID
was specified in theflags
mask, 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 theflags
mask, 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 the
flags
mask 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
, orCLONE_NEWUTS
was specified by an unprivileged process (process withoutCAP_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 theflags
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 user_namespaces(7)). - EPERM (since Linux 3.9)
-
CLONE_NEWUSER
was specified in theflags
mask and the caller is in a chroot environment (i.e., the caller_zsingle_quotesz_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 theflags
mask, and the limit on the number of nested user namespaces would be exceeded. See the discussion of the ENOSPC error above.
CONFORMING TO
These system calls are 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 clone call.
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).
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).
Within the sample program, we allocate the memory that is to be used for the child_zsingle_quotesz_s stack using mmap(2) rather than malloc(3) for the following reasons:
-
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
PROT_NONE
) at the end of the stack using mprotect(2). -
We can specify the
MAP_STACK
flag to request a mapping that is suitable for a stack. For the moment, this flag is a no-op on Linux, but it exists and has effect on some other systems, so we should include it for portability.
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> #include <sys/mman.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 , 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> , argv[0]); exit(EXIT_SUCCESS); } /* Allocate memory to be used for the stack of the child */ stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, −1, 0); if (stack == MAP_FAILED) errExit(mmap); 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 , (long) pid); /* Parent falls through to here */ sleep(1); /* Give child time to change its hostname */ /* Display hostname in parent_zsingle_quotesz_s UTS namespace. This will be different from hostname in child_zsingle_quotesz_s UTS namespace. */ if (uname(&uts) == −1) errExit(uname); printf(uts.nodename in parent: %s , uts.nodename); if (waitpid(pid, NULL, 0) == −1) /* Wait for child */ errExit(waitpid); printf(child has terminated ); exit(EXIT_SUCCESS); }
SEE ALSO
fork(2), futex(2), getpid(2), gettid(2), kcmp(2), mmap(2), pidfd_open(2), set_thread_area(2), set_tid_address(2), setns(2), tkill(2), unshare(2), wait(2), capabilities(7), namespaces(7), pthreads(7)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 1992 Drew Eckhardt <drewcs.colorado.edu>, March 28, 1992 and Copyright (c) Michael Kerrisk, 2001, 2002, 2005, 2013, 2019 %%%LICENSE_START(GPL_NOVERSION_ONELINE) May be distributed under the GNU General Public License. %%%LICENSE_END Modified by Michael Haardt <michaelmoria.de> Modified 24 Jul 1993 by Rik Faith <faithcs.unc.edu> Modified 21 Aug 1994 by Michael Chastain <mecshell.portal.com>: New man page (copied from _zsingle_quotesz_fork.2_zsingle_quotesz_). Modified 10 June 1995 by Andries Brouwer <aebcwi.nl> Modified 25 April 1998 by Xavier Leroy <Xavier.Leroyinria.fr> Modified 26 Jun 2001 by Michael Kerrisk Mostly upgraded to 2.4.x Added prototype for sys_clone() plus description Added CLONE_THREAD with a brief description of thread groups Added CLONE_PARENT and revised entire page remove ambiguity between calling process and parent process Added CLONE_PTRACE and CLONE_VFORK Added EPERM and EINVAL error codes Renamed __clone to clone (which is the prototype in <sched.h>) various other minor tidy ups and clarifications. Modified 26 Jun 2001 by Michael Kerrisk <mtk.manpagesgmail.com> Updated notes for 2.4.7+ behavior of CLONE_THREAD Modified 15 Oct 2002 by Michael Kerrisk <mtk.manpagesgmail.com> Added description for CLONE_NEWNS, which was added in 2.4.19 Slightly rephrased, aeb. Modified 1 Feb 2003 - added CLONE_SIGHAND restriction, aeb. Modified 1 Jan 2004 - various updates, aeb Modified 2004-09-10 - added CLONE_PARENT_SETTID etc. - aeb. 2005-04-12, mtk, noted the PID caching behavior of NPTL_zsingle_quotesz_s getpid() wrapper under BUGS. 2005-05-10, mtk, added CLONE_SYSVSEM, CLONE_UNTRACED, CLONE_STOPPED. 2005-05-17, mtk, Substantially enhanced discussion of CLONE_THREAD. 2008-11-18, mtk, order CLONE_* flags alphabetically 2008-11-18, mtk, document CLONE_NEWPID 2008-11-19, mtk, document CLONE_NEWUTS 2008-11-19, mtk, document CLONE_NEWIPC 2008-11-19, Jens Axboe, mtk, document CLONE_IO |