IPC(5)

NAME
ipc - System V interprocess communication mechanisms

SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
# include <sys/sem.h>
# include <sys/shm.h>

DESCRIPTION
This manual page refers to the Linux implementation of the System V interprocess communication mechanisms: message queues, semaphore sets, and shared memory segments. In the following, the word resource means an instantiation of one among such mechanisms.

  Resource Access Permissions
For each resource, the system uses a common structure of type struct ipc_perm to store information needed in determining permissions to perform an ipc operation. The ipc_perm structure, defined by the <sys/ipc.h> system header file, includes the following members:

    ushort cuid; /* creator user id */
    ushort cgid; /* creator group id */
    ushort uid; /* owner user id */
    ushort gid; /* owner group id */
    ushort mode; /* r/w permissions */

The mode member of the ipc_perm structure defines, with its lower 9 bits, the access permissions to the resource for a process executing an ipc system call. The permissions are interpreted as follows:

	0400	Read by user.
	0200	Write by user.

	0040	Read by group.
	0020	Write by group.

	0004	Read by others.
	0002	Write by others.

Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. Furthermore, "write" effectively means "alter" for a semaphore set.

The same system header file also defines the following symbolic constants:

IPC_CREAT
    Create entry if key doesn't exist.

IPC_EXCL
    Fail if key exists.

IPC_NOWAIT
    Error if request must wait.

IPC_PRIVATE
    Private key.

IPC_RMID
    Remove resource.

IPC_SET
    Set resource options.

IPC_STAT
    Get resource options.

Note that IPC_PRIVATE is a key_t type, while all the other symbolic constants are flag fields and can be OR'ed into an int type variable.

  Message Queues
A message queue is uniquely identified by a positive integer (its msqid) and has an associated data structure of type struct msqid_ds, defined in <sys/msg.h>, containing the following members:

    struct ipc_perm msg_perm;
    ushort msg_qnum; /* no of messages on queue */
    ushort msg_qbytes; /* bytes max on a queue */
    ushort msg_lspid; /* pid of last msgsnd call */
    ushort msg_lrpid; /* pid of last msgrcv call */
    time_t msg_stime; /* last msgsnd time */
    time_t msg_rtime; /* last msgrcv time */
    time_t msg_ctime; /* last change time */

msg_perm
    ipc_perm structure that specifies the access permissions on the message queue.

msg_qnum
    Number of messages currently on the message queue.

msg_qbytes
    Maximum number of bytes of message text allowed on the message queue.

msg_lspid
    ID of the process that performed the last msgsnd system call.

msg_lrpid
    ID of the process that performed the last msgrcv system call.

msg_stime
    Time of the last msgsnd system call.

msg_rtime
    Time of the last msgcv system call.

msg_ctime
    Time of the last system call that changed a member of the msqid_ds structure.

  Semaphore Sets
A semaphore set is uniquely identified by a positive integer (its semid) and has an associated data structure of type struct semid_ds, defined in <sys/sem.h>, containing the following members:

    struct ipc_perm sem_perm;
    time_t sem_otime; /* last operation time */
    time_t sem_ctime; /* last change time */
    ushort sem_nsems; /* count of sems in set */

sem_perm
    ipc_perm structure that specifies the access permissions on the semaphore set.

sem_otime
    Time of last semop system call.

sem_ctime
    Time of last semctl system call that changed a member of the above structure or of one semaphore belonging to the set.

sem_nsems
    Number of semaphores in the set. Each semaphore of the set is referenced by a non-negative integer ranging from 0 to sem_nsems-1.

A semaphore is a data structure of type struct sem containing the following members:

    ushort semval; /* semaphore value */
    short sempid; /* pid for last operation */
    ushort semncnt; /* nr awaiting semval to increase */
    ushort semzcnt; /* nr awaiting semval = 0 */

semval
    Semaphore value: a non-negative integer.

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

semncnt
    Number of processes suspended awaiting for semval to increase.

semznt
    Number of processes suspended awaiting for semval to become zero.

  Shared Memory Segments
A shared memory segment is uniquely identified by a positive integer (its shmid) and has an associated data structure of type struct shmid_ds, defined in <sys/shm.h>, containing the following members:

    struct ipc_perm shm_perm;
    int shm_segsz; /* size of segment */
    ushort shm_cpid; /* pid of creator */
    ushort shm_lpid; /* pid, last operation */
    short shm_nattch; /* no. of current attaches */
    time_t shm_atime; /* time of last attach */
    time_t shm_dtime; /* time of last detach */
    time_t shm_ctime; /* time of last change */

shm_perm
    ipc_perm structure that specifies the access permissions on the shared memory segment.

shm_segsz
    Size in bytes of the shared memory segment.

shm_cpid
    ID of the process that created the shared memory segment.

shm_lpid
    ID of the last process that executed a shmat or shmdt system call.

shm_nattch
    Number of current alive attaches for this shared memory segment.

shm_atime
    Time of the last shmat system call.

shm_dtime
    Time of the last shmdt system call.

shm_ctime
    Time of the last shmctl system call that changed shmid_ds.

SEE ALSO
msgctl(2), msgget(2), msgrcv(2), msgsnd(2), semctl(2), semget(2), semop(2), shmat(2), shmctl(2), shmdt(2), shmget(2), ftok(3)