IPC Section: Linux Programmer's Manual (5) Updated: November 1, 2025 Index   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 The 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 defines also the following symbolic constants: IPC_CREAT Create entry if key doesn't exists. 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 others symbolic constants are flag fields or-able 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 msquid_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;  /* no. of awaiting semval to increase */        ushort semzcnt;  /* no. of 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 shmdt (2).   Index NAME SYNOPSIS DESCRIPTION Resource Access Permissions Message Queues Semaphore Sets Shared Memory Segments SEE ALSO This document was created by using the manual pages. Time: 12:12:24 GMT, March 24, 2025