1SVIPC(7) Linux Programmer's Manual SVIPC(7)
2
6 svipc - System V interprocess communication mechanisms
7
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>
14
16 This manual page refers to the Linux implementation of the System V
17 interprocess communication (IPC) mechanisms: message queues, semaphore
18 sets, and shared memory segments. In the following, the word resource
19 means an instantiation of one among such mechanisms.
20 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:
26 struct ipc_perm {
28 uid_t cuid; /* creator user ID */
29 gid_t cgid; /* creator group ID */
30 uid_t uid; /* owner user ID */
31 gid_t gid; /* owner group ID */
32 unsigned short mode; /* r/w permissions */
33 };
34 The mode member of the ipc_perm structure defines, with its lower 9
36 bits, the access permissions to the resource for a process executing an
37 IPC system call. The permissions are interpreted as follows:
38 0400 Read by user.
40 0200 Write by user.
41 0040 Read by group.
42 0020 Write by group.
43 0004 Read by others.
44 0002 Write by others.
45 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
47 Furthermore, "write" effectively means "alter" for a semaphore set.
48 The same system header file also defines the following symbolic con‐
50 stants:
51 IPC_CREAT Create entry if key doesn't exist.
53 IPC_EXCL Fail if key exists.
55 IPC_NOWAIT Error if request must wait.
57 IPC_PRIVATE Private key.
59 IPC_RMID Remove resource.
61 IPC_SET Set resource options.
63 IPC_STAT Get resource options.
65 Note that IPC_PRIVATE is a key_t type, while all the other symbolic
67 constants are flag fields and can be OR'ed into an int type variable.
68 Message Queues
70 A message queue is uniquely identified by a positive integer (its
71 msqid) and has an associated data structure of type struct msqid_ds,
72 defined in <sys/msg.h>, containing the following members:
73 struct msqid_ds {
75 struct ipc_perm msg_perm;
76 msgqnum_t msg_qnum; /* no of messages on queue */
77 msglen_t msg_qbytes; /* bytes max on a queue */
78 pid_t msg_lspid; /* PID of last msgsnd(2) call */
79 pid_t msg_lrpid; /* PID of last msgrcv(2) call */
80 time_t msg_stime; /* last msgsnd(2) time */
81 time_t msg_rtime; /* last msgrcv(2) time */
82 time_t msg_ctime; /* last change time */
83 };
84 msg_perm ipc_perm structure that specifies the access permissions on
86 the message queue.
87 msg_qnum Number of messages currently on the message queue.
89 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
91 sage queue.
92 msg_lspid ID of the process that performed the last msgsnd(2) system
94 call.
95 msg_lrpid ID of the process that performed the last msgrcv(2) system
97 call.
98 msg_stime Time of the last msgsnd(2) system call.
100 msg_rtime Time of the last msgrcv(2) system call.
102 msg_ctime Time of the last system call that changed a member of the
104 msqid_ds structure.
105 Semaphore Sets
107 A semaphore set is uniquely identified by a positive integer (its
108 semid) and has an associated data structure of type struct semid_ds,
109 defined in <sys/sem.h>, containing the following members:
110 struct semid_ds {
112 struct ipc_perm sem_perm;
113 time_t sem_otime; /* last operation time */
114 time_t sem_ctime; /* last change time */
115 unsigned long sem_nsems; /* count of sems in set */
116 };
117 sem_perm ipc_perm structure that specifies the access permissions on
119 the semaphore set.
120 sem_otime Time of last semop(2) system call.
122 sem_ctime Time of last semctl(2) system call that changed a member of
124 the above structure or of one semaphore belonging to the
125 set.
126 sem_nsems Number of semaphores in the set. Each semaphore of the set
128 is referenced by a nonnegative integer ranging from 0 to
129 sem_nsems-1.
130 A semaphore is a data structure of type struct sem containing the fol‐
132 lowing members:
133 struct sem {
135 int semval; /* semaphore value */
136 int sempid; /* PID for last operation */
137 };
138 semval Semaphore value: a nonnegative integer.
140 sempid ID of the last process that performed a semaphore operation
142 on this semaphore.
143 Shared Memory Segments
145 A shared memory segment is uniquely identified by a positive integer
146 (its shmid) and has an associated data structure of type struct
147 shmid_ds, defined in <sys/shm.h>, containing the following members:
148 struct shmid_ds {
150 struct ipc_perm shm_perm;
151 size_t shm_segsz; /* size of segment */
152 pid_t shm_cpid; /* PID of creator */
153 pid_t shm_lpid; /* PID, last operation */
154 shmatt_t shm_nattch; /* no. of current attaches */
155 time_t shm_atime; /* time of last attach */
156 time_t shm_dtime; /* time of last detach */
157 time_t shm_ctime; /* time of last change */
158 };
159 shm_perm ipc_perm structure that specifies the access permissions on
161 the shared memory segment.
162 shm_segsz Size in bytes of the shared memory segment.
164 shm_cpid ID of the process that created the shared memory segment.
166 shm_lpid ID of the last process that executed a shmat(2) or shmdt(2)
168 system call.
169 shm_nattch Number of current alive attaches for this shared memory seg‐
171 ment.
172 shm_atime Time of the last shmat(2) system call.
174 shm_dtime Time of the last shmdt(2) system call.
176 shm_ctime Time of the last shmctl(2) system call that changed
178 shmid_ds.
179
181 ipc(2), msgctl(2), msgget(2), msgrcv(2), msgsnd(2), semctl(2),
182 semget(2), semop(2), shmat(2), shmctl(2), shmdt(2), shmget(2), ftok(3)
183
185 This page is part of release 3.25 of the Linux man-pages project. A
186 description of the project, and information about reporting bugs, can
187 be found at http://www.kernel.org/doc/man-pages/.
188Linux 2009-01-26 SVIPC(7)