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# shmctl() - Unix, Linux System Call

# NAME

shmctl - 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 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 highlighted 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:

Tag | Description
--- | ---
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 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 (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; otherwise its pages that were faulted in will remain in memory or swap.

Tag | Description
--- | ---
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 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:

Tag | Description
--- | ---
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 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 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

Tag | Description
--- | ---
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.)

# 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 applications 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 recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by an IPC_64 flag in cmd.)

# CONFORMING TO

SVr4, POSIX.1-2001.

# SEE ALSO

- mlock (2)

- setrlimit (2)

- shmget (2)

- shmop (2)

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