1SVIPC(7) Linux Programmer's Manual SVIPC(7)
2
6 sysvipc - System V interprocess communication mechanisms
7
9 #include <sys/msg.h>
10 #include <sys/sem.h>
11 #include <sys/shm.h>
12
14 This manual page refers to the Linux implementation of the System V
15 interprocess communication (IPC) mechanisms: message queues, semaphore
16 sets, and shared memory segments. In the following, the word resource
17 means an instantiation of one among such mechanisms.
18 Resource access permissions
20 For each resource, the system uses a common structure of type struct
21 ipc_perm to store information needed in determining permissions to per‐
22 form an IPC operation. The ipc_perm structure includes the following
23 members:
24 struct ipc_perm {
26 uid_t cuid; /* creator user ID */
27 gid_t cgid; /* creator group ID */
28 uid_t uid; /* owner user ID */
29 gid_t gid; /* owner group ID */
30 unsigned short mode; /* r/w permissions */
31 };
32 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:
36 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.
43 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
45 Furthermore, "write" effectively means "alter" for a semaphore set.
46 The same system header file also defines the following symbolic con‐
48 stants:
49 IPC_CREAT Create entry if key doesn't exist.
51 IPC_EXCL Fail if key exists.
53 IPC_NOWAIT Error if request must wait.
55 IPC_PRIVATE Private key.
57 IPC_RMID Remove resource.
59 IPC_SET Set resource options.
61 IPC_STAT Get resource options.
63 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 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:
71 struct msqid_ds {
73 struct ipc_perm msg_perm;
74 msgqnum_t msg_qnum; /* no of messages on queue */
75 msglen_t msg_qbytes; /* bytes max on a queue */
76 pid_t msg_lspid; /* PID of last msgsnd(2) call */
77 pid_t msg_lrpid; /* PID of last msgrcv(2) call */
78 time_t msg_stime; /* last msgsnd(2) time */
79 time_t msg_rtime; /* last msgrcv(2) time */
80 time_t msg_ctime; /* last change time */
81 };
82 msg_perm ipc_perm structure that specifies the access permissions on
84 the message queue.
85 msg_qnum Number of messages currently on the message queue.
87 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
89 sage queue.
90 msg_lspid ID of the process that performed the last msgsnd(2) system
92 call.
93 msg_lrpid ID of the process that performed the last msgrcv(2) system
95 call.
96 msg_stime Time of the last msgsnd(2) system call.
98 msg_rtime Time of the last msgrcv(2) system call.
100 msg_ctime Time of the last system call that changed a member of the
102 msqid_ds structure.
103 Semaphore sets
105 A semaphore set is uniquely identified by a positive integer (its
106 semid) and has an associated data structure of type struct semid_ds,
107 defined in <sys/sem.h>, containing the following members:
108 struct semid_ds {
110 struct ipc_perm sem_perm;
111 time_t sem_otime; /* last operation time */
112 time_t sem_ctime; /* last change time */
113 unsigned long sem_nsems; /* count of sems in set */
114 };
115 sem_perm ipc_perm structure that specifies the access permissions on
117 the semaphore set.
118 sem_otime Time of last semop(2) system call.
120 sem_ctime Time of last semctl(2) system call that changed a member of
122 the above structure or of one semaphore belonging to the
123 set.
124 sem_nsems Number of semaphores in the set. Each semaphore of the set
126 is referenced by a nonnegative integer ranging from 0 to
127 sem_nsems-1.
128 A semaphore is a data structure of type struct sem containing the fol‐
130 lowing members:
131 struct sem {
133 int semval; /* semaphore value */
134 int sempid; /* PID of process that last modified */
135 };
136 semval Semaphore value: a nonnegative integer.
138 sempid PID of the last process that modified the value of this sem‐
140 aphore.
141 Shared memory segments
143 A shared memory segment is uniquely identified by a positive integer
144 (its shmid) and has an associated data structure of type struct
145 shmid_ds, defined in <sys/shm.h>, containing the following members:
146 struct shmid_ds {
148 struct ipc_perm shm_perm;
149 size_t shm_segsz; /* size of segment */
150 pid_t shm_cpid; /* PID of creator */
151 pid_t shm_lpid; /* PID, last operation */
152 shmatt_t shm_nattch; /* no. of current attaches */
153 time_t shm_atime; /* time of last attach */
154 time_t shm_dtime; /* time of last detach */
155 time_t shm_ctime; /* time of last change */
156 };
157 shm_perm ipc_perm structure that specifies the access permissions on
159 the shared memory segment.
160 shm_segsz Size in bytes of the shared memory segment.
162 shm_cpid ID of the process that created the shared memory segment.
164 shm_lpid ID of the last process that executed a shmat(2) or shmdt(2)
166 system call.
167 shm_nattch Number of current alive attaches for this shared memory seg‐
169 ment.
170 shm_atime Time of the last shmat(2) system call.
172 shm_dtime Time of the last shmdt(2) system call.
174 shm_ctime Time of the last shmctl(2) system call that changed
176 shmid_ds.
177 IPC namespaces
179 For a discussion of the interaction of System V IPC objects and IPC
180 namespaces, see ipc_namespaces(7).
181
183 ipcmk(1), ipcrm(1), ipcs(1), lsipc(1), ipc(2), msgctl(2), msgget(2),
184 msgrcv(2), msgsnd(2), semctl(2), semget(2), semop(2), shmat(2), shm‐
185 ctl(2), shmdt(2), shmget(2), ftok(3), ipc_namespaces(7)
186
188 This page is part of release 5.04 of the Linux man-pages project. A
189 description of the project, information about reporting bugs, and the
190 latest version of this page, can be found at
191 https://www.kernel.org/doc/man-pages/.
192Linux 2019-10-10 SVIPC(7)