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SHMCTL(2) Linux Programmer's Manual SHMCTL(2)
shmctl - 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 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 in <sys/ipc.h> 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 only actually be
destroyed 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, or be privileged. 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.
IPC_INFO (Linux-specific)
Returns 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)
Returns 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)
Returns 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.
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.)
SVr4, POSIX.1-2001.
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 file system 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 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-08-07 SHMCTL(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface