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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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shmctl(2) System Calls Manual shmctl(2)
shmctl - System V shared memory control
Standard C library (libc, -lc)
#include <sys/shm.h>
int shmctl(int shmid, int op, struct shmid_ds *buf);
shmctl() performs the control operation specified by op 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; /* Creation time/time of last
modification via shmctl() */
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 fields of the shmid_ds structure are as follows:
shm_perm
This is an ipc_perm structure (see below) that specifies
the access permissions on the shared memory segment.
shm_segsz
Size in bytes of the shared memory segment.
shm_atime
Time of the last shmat(2) system call that attached this
segment.
shm_dtime
Time of the last shmdt(2) system call that detached tgis
segment.
shm_ctime
Time of creation of segment or time of the last shmctl()
IPC_SET operation.
shm_cpid
ID of the process that created the shared memory segment.
shm_lpid
ID of the last process that executed a shmat(2) or
shmdt(2) system call on this segment.
shm_nattch
Number of processes that have this segment attached.
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 */
};
The least significant 9 bits of the mode field of the ipc_perm
structure define the access permissions for the shared memory
segment. The permission bits are as follows:
0400 Read by user
0200 Write by user
0040 Read by group
0020 Write by group
0004 Read by others
0002 Write by others
Bits 0100, 0010, and 0001 (the execute bits) are unused by the
system. (It is not necessary to have execute permission on a
segment in order to perform a shmat(2) call with the SHM_EXEC
flag.)
Valid values for op 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 are updated: 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.
SHM_STAT_ANY (Linux-specific, since Linux 4.17)
Return a shmid_ds structure as for SHM_STAT. However,
shm_perm.mode is not checked for read access for shmid,
meaning that any user can employ this operation (just as
any user may read /proc/sysvipc/shm to obtain the same
information).
The caller can prevent or allow swapping of a shared memory
segment with the following op 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.
Before Linux 2.6.10, only a privileged process could employ
SHM_LOCK and SHM_UNLOCK. Since Linux 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 or SHM_STAT_ANY 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 to indicate the error.
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 op 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 op is not a valid
operation. Or: for a SHM_STAT or SHM_STAT_ANY operation,
the index value specified in shmid referred to an array
slot that is currently unused.
ENOMEM (Since Linux 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 (before Linux 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.)
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.
POSIX.1-2008.
POSIX.1-2001, SVr4.
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 op.)
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.
mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7),
sysvipc(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 shmctl(2)
Pages that refer to this page: ipcrm(1), ipcs(1), getrlimit(2), ipc(2), mlock(2), shmget(2), shmop(2), syscalls(2), proc_sys_kernel(5), capabilities(7), sysvipc(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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