shmctl - 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 {
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struct |
ipc_perm shm_perm; |
/* operation perms */ | ||||||
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int |
shm_segsz; |
/* size of segment (bytes) */ | ||||||
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time_t |
shm_atime; |
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/* last attach time */ | ||||||||
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time_t |
shm_dtime; |
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/* last detach time */ | ||||||||
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time_t |
shm_ctime; |
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/* last change time */ | ||||||||
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unsigned short |
shm_cpid; |
/* pid of creator */ | ||||||
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unsigned short |
shm_lpid; |
/* pid of last operator */ | ||||||
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short |
shm_nattch; |
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/* no. of current attaches */ | ||||||||
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/* the following are private */ |
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unsigned short shm_npages; /* size of segment (pages) */ |
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unsigned long *shm_pages; |
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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:
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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. | ||
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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. | ||
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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)
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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. | ||
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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.
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fork() |
After a fork() the child inherits the attached shared memory segments. | ||
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exec() |
After an exec() all attached shared memory segments are detached (not destroyed). | ||
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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:
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EACCESS |
is returned if IPC_STAT is requested and shm_perm.modes does not allow read access for msqid. | ||
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EFAULT |
The argument cmd has value IPC_SET or IPC_STAT but the address pointed to by buf isn’t accessible. | ||
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EINVAL |
is returned if shmid is not a valid identifier, or cmd is not a valid command. | ||
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EIDRM |
is returned if shmid points to a removed identifier. | ||
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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)