SHMCTL(2) Linux Programmer's Manual SHMCTL(2)NAMEshmctl - shared memory control
SYNOPSIS
#include <sys/ipc.h> #include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTIONshmctl() 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()/shmdt() */
shmatt_t shm_nattch; /* No. of current attaches */
...
};
The ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
lighted fields are settable using IPC_SET):
struct ipc_perm {
key_t key; /* Key supplied to shmget() */
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 arg.buf to the kernel data structure associ‐
ated 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 struc‐
ture 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 (non-standard) 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; other‐
wise 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; /* Max. segment size */
unsigned long shmmin; /* Min. segment size; always 1 */
unsigned long shmmni; /* Max. # of segments */
unsigned long shmseg; /* Max. # of segments that a
process can attach; unused */
unsigned long shmall; /* Max. # 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 struc‐
ture 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 (non-standard) 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 setr‐
limit(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 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.
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 com‐
mand. Or: for a SHM_STAT operation, the index value speci‐
fied 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.)
NOTES
The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the ipcs(8)
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()) 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 appli‐
cations should avoid relying on it.
Various fields in a struct shmid_ds were shorts under Linux 2.2 and
have become longs under Linux 2.4. To take advantage of this, a recom‐
pilation under glibc-2.1.91 or later should suffice. (The kernel dis‐
tinguishes old and new calls by an IPC_64 flag in cmd.)
CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSOmlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7)Linux 2.6.11 2005-05-30 SHMCTL(2)
