man svipc (7): System V interprocess communication mechanisms

SYNOPSIS

#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 (IPC) 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 includes the following members:

struct ipc_perm {
    uid_t          cuid;   /* creator user ID */
    gid_t          cgid;   /* creator group ID */
    uid_t          uid;    /* owner user ID */
    gid_t          gid;    /* owner group ID */
    unsigned short 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 msqid_ds {
    struct ipc_perm msg_perm;
    msgqnum_t       msg_qnum;    /* no of messages on queue */
    msglen_t        msg_qbytes;  /* bytes max on a queue */
    pid_t           msg_lspid;   /* PID of last msgsnd(2) call */
    pid_t           msg_lrpid;   /* PID of last msgrcv(2) call */
    time_t          msg_stime;   /* last msgsnd(2) time */
    time_t          msg_rtime;   /* last msgrcv(2) 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(2) system call.
msg_lrpid: ID of the process that performed the last msgrcv(2) system call.
msg_stime: Time of the last msgsnd(2) system call.
msg_rtime: Time of the last msgrcv(2) 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 semid_ds {
    struct ipc_perm sem_perm;
    time_t          sem_otime;   /* last operation time */
    time_t          sem_ctime;   /* last change time */
    unsigned long   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(2) system call.
sem_ctime: Time of last semctl(2) 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 nonnegative integer ranging from 0 to sem_nsems-1.

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

struct sem {
    int semval;  /* semaphore value */
    int sempid;  /* PID of process that last modified */
};

semval: Semaphore value: a nonnegative integer.
sempid: PID of the last process that modified the value of this semaphore.

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 shmid_ds {
    struct ipc_perm shm_perm;
    size_t          shm_segsz;   /* size of segment */
    pid_t           shm_cpid;    /* PID of creator */
    pid_t           shm_lpid;    /* PID, last operation */
    shmatt_t        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(2) or shmdt(2) system call.
shm_nattch: Number of current alive attaches for this shared memory segment.
shm_atime: Time of the last shmat(2) system call.
shm_dtime: Time of the last shmdt(2) system call.
shm_ctime: Time of the last shmctl(2) system call that changed shmid_ds.

IPC namespaces

For a discussion of the interaction of System V IPC objects and IPC namespaces, see namespaces(7).

COLOPHON

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