svipc(7)

1SVIPC(7)                   Linux Programmer's Manual                  SVIPC(7)
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NAME

6       svipc - System V interprocess communication mechanisms
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SYNOPSIS

9       # include <sys/types.h>
10       # include <sys/ipc.h>
11       # include <sys/msg.h>
12       # include <sys/sem.h>
13       # include <sys/shm.h>
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DESCRIPTION

16       This  manual  page  refers  to the Linux implementation of the System V
17       interprocess communication mechanisms: message queues, semaphore  sets,
18       and  shared memory segments.  In the following, the word resource means
19       an instantiation of one among such mechanisms.
20
21   Resource Access Permissions
22       For each resource, the system uses a common structure  of  type  struct
23       ipc_perm to store information needed in determining permissions to per‐
24       form  an  ipc  operation.   The  ipc_perm  structure,  defined  by  the
25       <sys/ipc.h> system header file, includes the following members:
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27            uid_t cuid;     /* creator user ID */
28            gid_t cgid;     /* creator group ID */
29            uid_t uid;      /* owner user ID */
30            gid_t gid;      /* owner group ID */
31            ushort mode;    /* r/w permissions */
32
33       The  mode  member  of  the ipc_perm structure defines, with its lower 9
34       bits, the access permissions to the resource for a process executing an
35       ipc system call.  The permissions are interpreted as follows:
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37            0400 Read by user.
38            0200 Write by user.
39            0040 Read by group.
40            0020 Write by group.
41            0004 Read by others.
42            0002 Write by others.
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44       Bits  0100, 0010, and 0001 (the execute bits) are unused by the system.
45       Furthermore, "write" effectively means "alter" for a semaphore set.
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47       The same system header file also defines the  following  symbolic  con‐
48       stants:
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50       IPC_CREAT     Create entry if key doesn't exist.
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52       IPC_EXCL      Fail if key exists.
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54       IPC_NOWAIT    Error if request must wait.
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56       IPC_PRIVATE   Private key.
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58       IPC_RMID      Remove resource.
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60       IPC_SET       Set resource options.
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62       IPC_STAT      Get resource options.
63
64       Note  that  IPC_PRIVATE  is  a key_t type, while all the other symbolic
65       constants are flag fields and can be OR'ed into an int type variable.
66
67   Message Queues
68       A message queue is uniquely  identified  by  a  positive  integer  (its
69       msqid)  and  has  an associated data structure of type struct msqid_ds,
70       defined in <sys/msg.h>, containing the following members:
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72            struct ipc_perm msg_perm;
73            msgqnum_t msg_qnum;  /* no of messages on queue */
74            msglen_t msg_qbytes;      /* bytes max on a queue */
75            pid_t msg_lspid;     /* PID of last msgsnd() call */
76            pid_t msg_lrpid;     /* PID of last msgrcv() call */
77            time_t msg_stime;    /* last msgsnd() time */
78            time_t msg_rtime;    /* last msgrcv() time */
79            time_t msg_ctime;    /* last change time */
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81       msg_perm   ipc_perm structure that specifies the access permissions  on
82                  the message queue.
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84       msg_qnum   Number of messages currently on the message queue.
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86       msg_qbytes Maximum  number of bytes of message text allowed on the mes‐
87                  sage queue.
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89       msg_lspid  ID of the process that performed the  last  msgsnd()  system
90                  call.
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92       msg_lrpid  ID  of  the  process that performed the last msgrcv() system
93                  call.
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95       msg_stime  Time of the last msgsnd() system call.
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97       msg_rtime  Time of the last msgcv() system call.
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99       msg_ctime  Time of the last system call that changed a  member  of  the
100                  msqid_ds structure.
101
102   Semaphore Sets
103       A  semaphore  set  is  uniquely  identified  by a positive integer (its
104       semid) and has an associated data structure of  type  struct  semid_ds,
105       defined in <sys/sem.h>, containing the following members:
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107            struct ipc_perm sem_perm;
108            time_t sem_otime;    /* last operation time */
109            time_t sem_ctime;    /* last change time */
110            ulong sem_nsems;     /* count of sems in set */
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112       sem_perm   ipc_perm  structure that specifies the access permissions on
113                  the semaphore set.
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115       sem_otime  Time of last semop() system call.
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117       sem_ctime  Time of last semctl() system call that changed a  member  of
118                  the  above  structure  or  of one semaphore belonging to the
119                  set.
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121       sem_nsems  Number of semaphores in the set.  Each semaphore of the  set
122                  is  referenced  by  a non-negative integer ranging from 0 to
123                  sem_nsems-1.
124
125       A semaphore is a data structure of type struct sem containing the  fol‐
126       lowing members:
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128            int semval;     /* semaphore value */
129            int sempid;     /* PID for last operation */
130
131       semval     Semaphore value: a non-negative integer.
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133       sempid     ID  of the last process that performed a semaphore operation
134                  on this semaphore.
135
136   Shared Memory Segments
137       A shared memory segment is uniquely identified by  a  positive  integer
138       (its  shmid)  and  has  an  associated  data  structure  of type struct
139       shmid_ds, defined in <sys/shm.h>, containing the following members:
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141            struct ipc_perm shm_perm;
142            size_t shm_segsz;    /* size of segment */
143            pid_t shm_cpid;      /* PID of creator */
144            pid_t shm_lpid;      /* PID, last operation */
145            shmatt_t shm_nattch;      /* no. of current attaches */
146            time_t shm_atime;    /* time of last attach */
147            time_t shm_dtime;    /* time of last detach */
148            time_t shm_ctime;    /* time of last change */
149
150       shm_perm   ipc_perm structure that specifies the access permissions  on
151                  the shared memory segment.
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153       shm_segsz  Size in bytes of the shared memory segment.
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155       shm_cpid   ID of the process that created the shared memory segment.
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157       shm_lpid   ID  of  the  last process that executed a shmat() or shmdt()
158                  system call.
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160       shm_nattch Number of current alive attaches for this shared memory seg‐
161                  ment.
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163       shm_atime  Time of the last shmat() system call.
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165       shm_dtime  Time of the last shmdt() system call.
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167       shm_ctime  Time of the last shmctl() system call that changed shmid_ds.
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NAME

SYNOPSIS

DESCRIPTION

SEE ALSO