shmctl - Linux Command - Unix Command

Linux

Filed In:

Linux

Linux / Unix Command: shmctl

Command Library

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 */
    int 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, the super-user 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.

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.