SHMCTL(2)

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() allows the user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory segment, or destroy a segment. The information about the segment identified by shmid is returned in a shmid_ds structure:

struct shmid_ds {
    struct ipc_perm shm_perm;  /* operation perms */
    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 */
    unsigned short shm_cpid;   /* pid of creator */
    unsigned short shm_lpid;   /* pid of last operator */
    short shm_nattch;          /* no. of current attaches */
    ...
};

The highlighted fields in the member shm_perm can be set:

struct ipc_perm {
    key_t  key;
    ushort uid;   /* owner euid and egid */
    ushort gid;
    ushort cuid;  /* creator euid and egid */
    ushort cgid;
    ushort mode;  /* lower 9 bits of access modes */
    ushort seq;   /* sequence number */
};

The following cmds are available:

IPC_STAT
    is used to copy the information about the shared memory segment into the buffer buf. The user must have read access to the shared memory segment.

IPC_SET
    is used to apply the changes the user has made to the uid, gid, or mode members of the shm_perms field. Only the lowest 9 bits of mode are used. The shm_ctime member is also updated. The user must be the owner, creator, or the super-user.

IPC_RMID
    is used to mark the segment as destroyed. It will actually be destroyed after the last detach. (I.e., when the shm_nattch member of the associated structure shmid_ds is zero.) The user must be the owner, creator, or the super-user.

The user must ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap.

In addition, processes with appropriate privileges can prevent or allow swapping of a shared memory segment with the following cmds: (Linux only)

SHM_LOCK
    prevents swapping of a shared memory segment. The user must fault in any pages that are required to be present after locking is enabled.

SHM_UNLOCK
    allows the shared memory segment to be swapped out.

Processes are permitted to use SHM_LOCK and SHM_UNLOCK if they running with the CAP_IPC_LOCK capability (normally only true for root) or if their current RLIMIT_MEMLOCK resource limit is non-zero.

The IPC_INFO, SHM_STAT and SHM_INFO control calls are used by the ipcs(8) program to provide information on allocated resources. In the future, these may be modified as needed or moved to a proc file system interface.

RETURN VALUE
0 is returned on success, -1 on error.

ERRORS
On error, errno will be set to one of the following:

EACCES
    is returned if IPC_STAT is requested and shm_perm.modes does not allow read access for shmid.

EFAULT
    The argument cmd has value IPC_SET or IPC_STAT but the address pointed to by buf isn't accessible.

EINVAL
    is returned if shmid is not a valid identifier, or cmd is not a valid command.

EIDRM
    is returned if shmid points to a removed identifier.

EPERM
    is returned if IPC_SET or IPC_RMID is attempted, and the effective user ID of the calling process is not the creator (as found in shm_perm.cuid), the owner (as found in shm_perm.uid), or the super-user.

EOVERFLOW
    is returned if IPC_STAT is attempted, and the gid or uid value is too large to be stored in the structure pointed to by buf.

NOTE
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 a IPC_64 flag in cmd.)

CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EINVAL, ENOENT, ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents an EIDRM error condition.

SEE ALSO
shmget(2), shmop(2)