NAME

kedef − SVID Kernel Extension definitions

DESCRIPTION

The SVID Kernel Extension provides three mechanisms for inter-process communication (IPC): messages, semaphores and shared-memory. This section defines some of the data structures employed. 

IPC Permissions

All of the IPC mechanisms use a common structure type, ipc_perm, to pass information used in determining permission to perform an IPC operation.

The ipc_perm structure is defined by the header file <sys/ipc.h> and includes the following members:

ushortcuid;/∗ creator’s user id ∗/
ushortcgid;/∗ creator’s group id ∗/
ushortuid;/∗ owner’s user id ∗/
ushortgid;/∗ owner’s group id ∗/
ushortmode;/∗ access permissions ∗/

The following symbolic constants are also defined by the <sys/ipc.h> header file:

IPC_CREAT/∗ create entry if key does not exist ∗/
IPC_EXCL/∗ fail if key exists ∗/
IPC_NOWAIT/∗ error if request must wait ∗/
IPC_RMID/∗ remove identifier ∗/
IPC_SET/∗ set options ∗/
IPC_STAT/∗ get options ∗/
IPC_PRIVATE/∗ private key ∗/

Message Queue Identifier

A message queue identifier msqid is a unique positive integer created by a call to the msgget(2) routine. Each msqid has a message queue and a data structure associated with it.  The data structure is referred to as msqid_ds and contains the following members:

structipc_perm msg_perm;/∗ operation permissions ∗/
ushortmsg_qnum;/∗ number of messages on q ∗/
ushortmsg_qbytes;/∗ max number of bytes on q ∗/
ushortmsg_lspid;/∗ pid of last msgsnd operation ∗/
ushortmsg_lrpid;/∗ pid of last msgrcv operation ∗/
time_tmsg_stime;/∗ last msgsnd time ∗/
time_tmsg_rtime;/∗ last msgrcv time ∗/
time_tmsg_ctime;/∗ last change time ∗/
/∗ Times in seconds since ∗/
/∗ 00:00:00 GMT, 1st January 1970 ∗/

msg_perm is an ipc_perm structure (see above) that specifies the message operation permission. 

msg_qnum is the number of messages currently on the queue. 

msg_qbytes
is the maximum number of bytes allowed on the queue.

msg_lspid is the process id of the last process that performed a msgsnd operation.

msg_lrpid is the process id of the last process that performed a msgrcv operation.

msg_stime is the time of the last msgsnd operation. 

msg_rtime is the time of the last msgrcv operation. 

msg_ctime is the time of the last msgctl(2) operation that changed a member of the above structure.

Message Operation Permissions

In the msgop(2) and msgctl(2) system call descriptions, the permission required for an operation is given as a bit pattern in msg_perm.mode, where the type of permission needed is interpreted as follows:

00400 Read by user

00200 Write by user

00040 Read by group

00020 Write by group

00004 Read by others

00002 Write by others

Read and Write permissions on a msqid are granted to a process if one or more of the following are true:

The effective user ID of the process is super-user. 

The effective user ID of the process matches msg_perm.cuid or msg_perm.uid in the data structure associated with msqid and the appropriate bit of the “user” portion (0600) of msg_perm.mode is set. 

The effective user ID of the process does not match msg_perm.cuid or msg_perm.uid and the effective group ID of the process matches msg_perm.cgid or msg_perm.gid and the appropriate bit of the “group” portion (0060) of msg_perm.mode is set. 

The effective user ID of the process does not match msg_perm.cuid or msg_perm.uid and the effective group ID of the process does not match msg_perm.cgid or msg_perm.gid and the appropriate bit of the “other” portion (0006) of msg_perm.mode is set. 

Otherwise, the corresponding permissions are denied. 

Semaphore Identifier

A semaphore identifier semid is a unique positive integer created by a semget(2) system call. Each semid has a set of semaphores and a data structure associated with it.  The data structure is semid_ds and contains the following members:

structipc_perm sem_perm;/∗ operation permissions ∗/
ushortsem_nsems;/∗ count of semaphores in set ∗/
time_tsem_otime;/∗ last operation time ∗/
time_tsem_ctime;/∗ last change time ∗/
/∗ Times measured in seconds since ∗/
/∗ 00:00:00 GMT, 1st January 1970 ∗/

sem_perm is an ipc_perm structure that specifies the semaphore operation permission (see above). 

sem_nsems has a value equal to the number of semaphores in the set.  Each semaphore in the set is referenced by a positive integer referred to as a sem_num. The value of sem_num runs sequentially from 0 to the value of sem_nsems-1

sem_otime is the time of the last semop(2) operation.

sem_ctime is the time of the last semctl(2) operation that changed a member of the above structure.

A semaphore is a data structure that contains the following members:

ushortsemval;/∗ semaphore value ∗/
shortsempid;/∗ pid of last operation  ∗/
ushortsemncnt;/∗ number awaiting semval > cval ∗/
ushortsemzcnt;/∗ number awaiting semval = 0 ∗/

semval is a non-negative integer. 

sempid is equal to the process ID of the last process that performed a semaphore operation on this semaphore. 

semncnt is a count of the number of processes that are currently suspended awaiting this semaphore’s semval to become greater than its current value. 

semzcnt is a count of the number of processes that are currently suspended awaiting this semaphore’s semval to become zero. 

Semaphore Operation Permissions

In the semop(2) and semctl(2) system call descriptions, the permission required for an operation is given as a bit pattern in sem_perm.mode where the type of permission needed interpreted as follows:

00400 Read by user

00200 Alter by user

00040 Read by group

00020 Alter by group

00004 Readby others

00002 Alter by others

The Read and Alter permissions on a semid are granted to a process if one or more of the following are true:

The effective user ID of the process is super-user. 

The effective user ID of the process matches sem_perm.cuid or sem_perm.uid in the data structure associated with semid and the appropriate bit of the “user” portion (0600) of sem_perm.mode is set. 

The effective user ID of the process does not match sem_perm.cuid or sem_perm.uid and the effective group ID of the process matches sem_perm.cgid or sem_perm.gid and the appropriate bit of the “group” portion (0060) of sem_perm.mode is set. 

The effective user ID of the process does not match sem_perm.cuid or sem_perm.uid and the effective group ID of the process does not match sem_perm.cgid or sem_perm.gid and the appropriate bit of the “other” portion (0006) of sem_perm.mode is set. 

Otherwise, the corresponding permissions are denied. 

Shared Memory Identifier

A shared memory identifier shmid is a unique positive integer created by a shmget(2) system call. Each shmid has a segment of memory (referred to as a shared memory segment) and a data structure associated with it. 

The data structure is referred to as shmid_ds and contains the following members:

structipc_perm shm_perm;/∗ operation permissions ∗/
intshm_segsz;/∗ size of segment ∗/
ushortshm_cpid;/∗ creator pid ∗/
ushortshm_lpid;/∗ pid of last operation ∗/
shortshm_nattch;/∗ number of current attaches ∗/
time_tshm_atime;/∗ last attach time ∗/
time_tshm_dtime;/∗ last detach time ∗/
time_tshm_ctime;/∗ last change time ∗/
/∗ Times measured in seconds since ∗/
/∗ 00:00:00 GMT, 1st January 1970 ∗/

shm_perm is an ipc_perm structure that specifies the shared memory operation permission (see above). 

shm_segsz specifies the size of the shared memory segment. 

shm_cpid is the process id of the process that created the shared memory identifier. 

shm_lpid is the process id of the last process that performed a shmop(2) operation.

shm_nattch
is the number of processes that currently have this segment attached.

shm_atime is the time of the last shmat operation. 

shm_dtime is the time of the last shmdt operation. 

shm_ctime is the time of the last shmctl(2) operation that changed one of the members of the above structure.

Shared Memory Operation Permissions

In the shmop(2) and shmctl(2) system call descriptions, the permission required for an operation is given as a bit pattern in shm_perm.mode where the type of permission needed interpreted as follows:

00400 Read by user

00200 Write by user

00040 Read by group

00020 Write by group

00004 Read by others

00002 Write by others

Read and Write permissions on a shmid are granted to a process if one or more of the following are true:

The effective user ID of the process is super-user. 

The effective user ID of the process matches shm_perm.cuid or shm_perm.uid in the data structure associated with shmid and the appropriate bit of the “user” portion (0600) of shm_perm.mode is set. 

The effective user ID of the process does not match shm_perm.cuid or shm_perm.uid and the effective group ID of the process matches shm_perm.cgid or shm_perm.gid and the appropriate bit of the “group” portion (0060) of shm_perm.mode is set. 

The effective user ID of the process does not match shm_perm.cuid or shm_perm.uid and the effective group ID of the process does not match shm_perm.cgid or shm_perm.gid and the appropriate bit of the “other” portion (0006) of shm_perm.mode is set. 

Otherwise, the corresponding permissions are denied. 

SEE ALSO

Chapters 8-11 of the System V Interface Definition, Issue 2, Volume 1. 

7th Edition  —  Revision 1.2 of 26/05/89