1SEMGET(3P) POSIX Programmer's Manual SEMGET(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 semget - get set of XSI semaphores
13
15 #include <sys/sem.h>
16 int semget(key_t key, int nsems, int semflg);
18
21 The semget() function operates on XSI semaphores (see the Base Defini‐
22 tions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore). It is
23 unspecified whether this function interoperates with the realtime
24 interprocess communication facilities defined in Realtime .
25 The semget() function shall return the semaphore identifier associated
27 with key.
28 A semaphore identifier with its associated semid_ds data structure and
30 its associated set of nsems semaphores (see <sys/sem.h>) is created for
31 key if one of the following is true:
32 * The argument key is equal to IPC_PRIVATE.
34 * The argument key does not already have a semaphore identifier asso‐
36 ciated with it and (semflg &IPC_CREAT) is non-zero.
37 Upon creation, the semid_ds data structure associated with the new sem‐
39 aphore identifier is initialized as follows:
40 * In the operation permissions structure sem_perm.cuid, sem_perm.uid,
42 sem_perm.cgid, and sem_perm.gid shall be set equal to the effective
43 user ID and effective group ID, respectively, of the calling
44 process.
45 * The low-order 9 bits of sem_perm.mode shall be set equal to the low-
47 order 9 bits of semflg.
48 * The variable sem_nsems shall be set equal to the value of nsems.
50 * The variable sem_otime shall be set equal to 0 and sem_ctime shall
52 be set equal to the current time.
53 * The data structure associated with each semaphore in the set shall
55 not be initialized. The semctl() function with the command SETVAL or
56 SETALL can be used to initialize each semaphore.
57
59 Upon successful completion, semget() shall return a non-negative inte‐
60 ger, namely a semaphore identifier; otherwise, it shall return -1 and
61 set errno to indicate the error.
62
64 The semget() function shall fail if:
65 EACCES A semaphore identifier exists for key, but operation permission
67 as specified by the low-order 9 bits of semflg would not be
68 granted; see XSI Interprocess Communication .
69 EEXIST A semaphore identifier exists for the argument key but ((semflg
71 &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
72 EINVAL The value of nsems is either less than or equal to 0 or greater
74 than the system-imposed limit, or a semaphore identifier exists
75 for the argument key, but the number of semaphores in the set
76 associated with it is less than nsems and nsems is not equal to
77 0.
78 ENOENT A semaphore identifier does not exist for the argument key and
80 (semflg &IPC_CREAT) is equal to 0.
81 ENOSPC A semaphore identifier is to be created but the system-imposed
83 limit on the maximum number of allowed semaphores system-wide
84 would be exceeded.
85 The following sections are informative.
88
90 Creating a Semaphore Identifier
91 The following example gets a unique semaphore key using the ftok()
92 function, then gets a semaphore ID associated with that key using the
93 semget() function (the first call also tests to make sure the semaphore
94 exists). If the semaphore does not exist, the program creates it, as
95 shown by the second call to semget(). In creating the semaphore for the
96 queuing process, the program attempts to create one semaphore with
97 read/write permission for all. It also uses the IPC_EXCL flag, which
98 forces semget() to fail if the semaphore already exists.
99 After creating the semaphore, the program uses a call to semop() to
101 initialize it to the values in the sbuf array. The number of processes
102 that can execute concurrently without queuing is initially set to 2.
103 The final call to semget() creates a semaphore identifier that can be
104 used later in the program.
105 #include <sys/types.h>
108 #include <stdio.h>
109 #include <sys/ipc.h>
110 #include <sys/sem.h>
111 #include <sys/stat.h>
112 #include <errno.h>
113 #include <unistd.h>
114 #include <stdlib.h>
115 #include <pwd.h>
116 #include <fcntl.h>
117 #include <limits.h>
118 ...
119 key_t semkey;
120 int semid, pfd, fv;
121 struct sembuf sbuf;
122 char *lgn;
123 char filename[PATH_MAX+1];
124 struct stat outstat;
125 struct passwd *pw;
126 ...
127 /* Get unique key for semaphore. */
128 if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
129 perror("IPC error: ftok"); exit(1);
130 }
131 /* Get semaphore ID associated with this key. */
134 if ((semid = semget(semkey, 0, 0)) == -1) {
135 /* Semaphore does not exist - Create. */
138 if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
139 S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
140 {
141 /* Initialize the semaphore. */
142 sbuf.sem_num = 0;
143 sbuf.sem_op = 2; /* This is the number of runs
144 without queuing. */
145 sbuf.sem_flg = 0;
146 if (semop(semid, &sbuf, 1) == -1) {
147 perror("IPC error: semop"); exit(1);
148 }
149 }
150 else if (errno == EEXIST) {
151 if ((semid = semget(semkey, 0, 0)) == -1) {
152 perror("IPC error 1: semget"); exit(1);
153 }
154 }
155 else {
156 perror("IPC error 2: semget"); exit(1);
157 }
158 }
159 ...
160
162 The POSIX Realtime Extension defines alternative interfaces for inter‐
163 process communication. Application developers who need to use IPC
164 should design their applications so that modules using the IPC routines
165 described in XSI Interprocess Communication can be easily modified to
166 use the alternative interfaces.
167
169 None.
170
172 None.
173
175 XSI Interprocess Communication, Realtime, semctl(), semop(),
176 sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(),
177 sem_post(), sem_unlink(), sem_wait(), the Base Definitions volume of
178 IEEE Std 1003.1-2001, <sys/sem.h>
179
181 Portions of this text are reprinted and reproduced in electronic form
182 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
183 -- Portable Operating System Interface (POSIX), The Open Group Base
184 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
185 Electrical and Electronics Engineers, Inc and The Open Group. In the
186 event of any discrepancy between this version and the original IEEE and
187 The Open Group Standard, the original IEEE and The Open Group Standard
188 is the referee document. The original Standard can be obtained online
189 at http://www.opengroup.org/unix/online.html .
190IEEE/The Open Group 2003 SEMGET(3P)