SHMCTL

Section: Linux Programmer's Manual (2)
Updated: 2001-02-18
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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 following are private */
        unsigned short   shm_npages;  /* size of segment (pages) */
        unsigned long   *shm_pages;   
        struct shm_desc *attaches;    /* descriptors for attaches */
};

The 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 man be modified as needed or moved to a proc file system interface.

SYSTEM CALLS

fork(): After a fork() the child inherits the attached shared memory segments.
exec(): After an exec() all attached shared memory segments are detached (not destroyed).
exit(): Upon exit() all attached shared memory segments are detached (not destroyed).

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 msqid.
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 user is not the creator, the owner, or the super-user, and the user does not have permission granted to their group or to the world.

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.

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Time: 04:58:55 GMT, April 26, 2024