|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
SHMCTL(2) Linux Programmer's Manual SHMCTL(2)
shmctl - System V shared memory control
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
shmctl() performs the control operation specified by cmd on the
System V shared memory segment whose identifier is given in shmid.
The buf argument is a pointer to a shmid_ds structure, defined in
<sys/shm.h> as follows:
struct shmid_ds {
struct ipc_perm shm_perm; /* Ownership and permissions */
size_t shm_segsz; /* Size of segment (bytes) */
time_t shm_atime; /* Last attach time */
time_t shm_dtime; /* Last detach time */
time_t shm_ctime; /* Last change time */
pid_t shm_cpid; /* PID of creator */
pid_t shm_lpid; /* PID of last shmat(2)/shmdt(2) */
shmatt_t shm_nattch; /* No. of current attaches */
...
};
The ipc_perm structure is defined as follows (the highlighted fields
are settable using IPC_SET):
struct ipc_perm {
key_t __key; /* Key supplied to shmget(2) */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions + SHM_DEST and
SHM_LOCKED flags */
unsigned short __seq; /* Sequence number */
};
Valid values for cmd are:
IPC_STAT Copy information from the kernel data structure associated
with shmid into the shmid_ds structure pointed to by buf.
The caller must have read permission on the shared memory
segment.
IPC_SET Write the values of some members of the shmid_ds structure
pointed to by buf to the kernel data structure associated
with this shared memory segment, updating also its
shm_ctime member. The following fields can be changed:
shm_perm.uid, shm_perm.gid, and (the least significant 9
bits of) shm_perm.mode. The effective UID of the calling
process must match the owner (shm_perm.uid) or creator
(shm_perm.cuid) of the shared memory segment, or the caller
must be privileged.
IPC_RMID Mark the segment to be destroyed. The segment will
actually be destroyed only after the last process detaches
it (i.e., when the shm_nattch member of the associated
structure shmid_ds is zero). The caller must be the owner
or creator of the segment, or be privileged. The buf
argument is ignored.
If a segment has been marked for destruction, then the
(nonstandard) SHM_DEST flag of the shm_perm.mode field in
the associated data structure retrieved by IPC_STAT will be
set.
The caller must ensure that a segment is eventually
destroyed; otherwise its pages that were faulted in will
remain in memory or swap.
See also the description of
/proc/sys/kernel/shm_rmid_forced in proc(5).
IPC_INFO (Linux-specific)
Return information about system-wide shared memory limits
and parameters in the structure pointed to by buf. This
structure is of type shminfo (thus, a cast is required),
defined in <sys/shm.h> if the _GNU_SOURCE feature test
macro is defined:
struct shminfo {
unsigned long shmmax; /* Maximum segment size */
unsigned long shmmin; /* Minimum segment size;
always 1 */
unsigned long shmmni; /* Maximum number of segments */
unsigned long shmseg; /* Maximum number of segments
that a process can attach;
unused within kernel */
unsigned long shmall; /* Maximum number of pages of
shared memory, system-wide */
};
The shmmni, shmmax, and shmall settings can be changed via
/proc files of the same name; see proc(5) for details.
SHM_INFO (Linux-specific)
Return a shm_info structure whose fields contain
information about system resources consumed by shared
memory. This structure is defined in <sys/shm.h> if the
_GNU_SOURCE feature test macro is defined:
struct shm_info {
int used_ids; /* # of currently existing
segments */
unsigned long shm_tot; /* Total number of shared
memory pages */
unsigned long shm_rss; /* # of resident shared
memory pages */
unsigned long shm_swp; /* # of swapped shared
memory pages */
unsigned long swap_attempts;
/* Unused since Linux 2.4 */
unsigned long swap_successes;
/* Unused since Linux 2.4 */
};
SHM_STAT (Linux-specific)
Return a shmid_ds structure as for IPC_STAT. However, the
shmid argument is not a segment identifier, but instead an
index into the kernel's internal array that maintains
information about all shared memory segments on the system.
The caller can prevent or allow swapping of a shared memory segment
with the following cmd values:
SHM_LOCK (Linux-specific)
Prevent swapping of the shared memory segment. The caller
must fault in any pages that are required to be present
after locking is enabled. If a segment has been locked,
then the (nonstandard) SHM_LOCKED flag of the shm_perm.mode
field in the associated data structure retrieved by
IPC_STAT will be set.
SHM_UNLOCK (Linux-specific)
Unlock the segment, allowing it to be swapped out.
In kernels before 2.6.10, only a privileged process could employ
SHM_LOCK and SHM_UNLOCK. Since kernel 2.6.10, an unprivileged
process can employ these operations if its effective UID matches the
owner or creator UID of the segment, and (for SHM_LOCK) the amount of
memory to be locked falls within the RLIMIT_MEMLOCK resource limit
(see setrlimit(2)).
A successful IPC_INFO or SHM_INFO operation returns the index of the
highest used entry in the kernel's internal array recording
information about all shared memory segments. (This information can
be used with repeated SHM_STAT operations to obtain information about
all shared memory segments on the system.) A successful SHM_STAT
operation returns the identifier of the shared memory segment whose
index was given in shmid. Other operations return 0 on success.
On error, -1 is returned, and errno is set appropriately.
EACCES IPC_STAT or SHM_STAT is requested and shm_perm.mode does not
allow read access for shmid, and the calling process does not
have the CAP_IPC_OWNER capability in the user namespace that
governs its IPC namespace.
EFAULT The argument cmd has value IPC_SET or IPC_STAT but the address
pointed to by buf isn't accessible.
EIDRM shmid points to a removed identifier.
EINVAL shmid is not a valid identifier, or cmd is not a valid
command. Or: for a SHM_STAT operation, the index value
specified in shmid referred to an array slot that is currently
unused.
ENOMEM (In kernels since 2.6.9), SHM_LOCK was specified and the size
of the to-be-locked segment would mean that the total bytes in
locked shared memory segments would exceed the limit for the
real user ID of the calling process. This limit is defined by
the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).
EOVERFLOW
IPC_STAT is attempted, and the GID or UID value is too large
to be stored in the structure pointed to by buf.
EPERM IPC_SET or IPC_RMID is attempted, and the effective user ID of
the calling process is not that of the creator (found in
shm_perm.cuid), or the owner (found in shm_perm.uid), and the
process was not privileged (Linux: did not have the
CAP_SYS_ADMIN capability).
Or (in kernels before 2.6.9), SHM_LOCK or SHM_UNLOCK was
specified, but the process was not privileged (Linux: did not
have the CAP_IPC_LOCK capability). (Since Linux 2.6.9, this
error can also occur if the RLIMIT_MEMLOCK is 0 and the caller
is not privileged.)
POSIX.1-2001, POSIX.1-2008, SVr4.
The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on
Linux or by any version of POSIX. However, some old implementations
required the inclusion of these header files, and the SVID also
documented their inclusion. Applications intended to be portable to
such old systems may need to include these header files.
The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the
ipcs(1) program to provide information on allocated resources. In
the future, these may modified or moved to a /proc filesystem
interface.
Linux permits a process to attach (shmat(2)) a shared memory segment
that has already been marked for deletion using shmctl(IPC_RMID).
This feature is not available on other UNIX implementations; portable
applications should avoid relying on it.
Various fields in a struct shmid_ds were typed as short under Linux
2.2 and have become long under Linux 2.4. To take advantage of this,
a recompilation under glibc-2.1.91 or later should suffice. (The
kernel distinguishes old and new calls by an IPC_64 flag in cmd.)
mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7),
svipc(7)
This page is part of release 4.12 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/.
Linux 2016-10-08 SHMCTL(2)
Pages that refer to this page: ipcrm(1), getrlimit(2), ipc(2), mlock(2), shmget(2), shmop(2), syscalls(2), proc(5), capabilities(7), svipc(7)