SHMCTL

Section: Linux Programmer's Manual (2)
Updated: 2001-02-18
Index
Return to Main Contents

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.

This document was created by man2html, using the manual pages.
Time: 02:08:15 GMT, January 21, 2025