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NAME
shmctl - System V shared memory control
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
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; /* 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 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 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 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)).
RETURN VALUE
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 appropriately.
ERRORS
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 or
SHM_STAT_ANY 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.)
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
NOTES
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.)
SEE ALSO
mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), sysvipc(7)
COLOPHON
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