Table of Contentsshmctl - shared memory control #include <sys/ipc.h> #include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
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(1) program to provide information on allocated resources. In the future, these man be modified as needed or moved to a proc file system interface.
- 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).
0 is returned on success, -1 on error. On error, errno will be set to one of the following: - EACCESS
- 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.
shmget(2), shmop(2)
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