1SEMGET(P) POSIX Programmer's Manual SEMGET(P)
2
6 semget - get set of XSI semaphores
7
9 #include <sys/sem.h>
10 int semget(key_t key, int nsems, int semflg);
12
15 The semget() function operates on XSI semaphores (see the Base Defini‐
16 tions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore). It is
17 unspecified whether this function interoperates with the realtime
18 interprocess communication facilities defined in Realtime .
19 The semget() function shall return the semaphore identifier associated
21 with key.
22 A semaphore identifier with its associated semid_ds data structure and
24 its associated set of nsems semaphores (see <sys/sem.h>) is created for
25 key if one of the following is true:
26 * The argument key is equal to IPC_PRIVATE.
28 * The argument key does not already have a semaphore identifier asso‐
30 ciated with it and (semflg &IPC_CREAT) is non-zero.
31 Upon creation, the semid_ds data structure associated with the new sem‐
33 aphore identifier is initialized as follows:
34 * In the operation permissions structure sem_perm.cuid, sem_perm.uid,
36 sem_perm.cgid, and sem_perm.gid shall be set equal to the effective
37 user ID and effective group ID, respectively, of the calling
38 process.
39 * The low-order 9 bits of sem_perm.mode shall be set equal to the low-
41 order 9 bits of semflg.
42 * The variable sem_nsems shall be set equal to the value of nsems.
44 * The variable sem_otime shall be set equal to 0 and sem_ctime shall
46 be set equal to the current time.
47 * The data structure associated with each semaphore in the set shall
49 not be initialized. The semctl() function with the command SETVAL or
50 SETALL can be used to initialize each semaphore.
51
53 Upon successful completion, semget() shall return a non-negative inte‐
54 ger, namely a semaphore identifier; otherwise, it shall return -1 and
55 set errno to indicate the error.
56
58 The semget() function shall fail if:
59 EACCES A semaphore identifier exists for key, but operation permission
61 as specified by the low-order 9 bits of semflg would not be
62 granted; see XSI Interprocess Communication .
63 EEXIST A semaphore identifier exists for the argument key but ((semflg
65 &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
66 EINVAL The value of nsems is either less than or equal to 0 or greater
68 than the system-imposed limit, or a semaphore identifier exists
69 for the argument key, but the number of semaphores in the set
70 associated with it is less than nsems and nsems is not equal to
71 0.
72 ENOENT A semaphore identifier does not exist for the argument key and
74 (semflg &IPC_CREAT) is equal to 0.
75 ENOSPC A semaphore identifier is to be created but the system-imposed
77 limit on the maximum number of allowed semaphores system-wide
78 would be exceeded.
79 The following sections are informative.
82
84 Creating a Semaphore Identifier
85 The following example gets a unique semaphore key using the ftok()
86 function, then gets a semaphore ID associated with that key using the
87 semget() function (the first call also tests to make sure the semaphore
88 exists). If the semaphore does not exist, the program creates it, as
89 shown by the second call to semget(). In creating the semaphore for the
90 queuing process, the program attempts to create one semaphore with
91 read/write permission for all. It also uses the IPC_EXCL flag, which
92 forces semget() to fail if the semaphore already exists.
93 After creating the semaphore, the program uses a call to semop() to
95 initialize it to the values in the sbuf array. The number of processes
96 that can execute concurrently without queuing is initially set to 2.
97 The final call to semget() creates a semaphore identifier that can be
98 used later in the program.
99 #include <sys/types.h>
102 #include <stdio.h>
103 #include <sys/ipc.h>
104 #include <sys/sem.h>
105 #include <sys/stat.h>
106 #include <errno.h>
107 #include <unistd.h>
108 #include <stdlib.h>
109 #include <pwd.h>
110 #include <fcntl.h>
111 #include <limits.h>
112 ...
113 key_t semkey;
114 int semid, pfd, fv;
115 struct sembuf sbuf;
116 char *lgn;
117 char filename[PATH_MAX+1];
118 struct stat outstat;
119 struct passwd *pw;
120 ...
121 /* Get unique key for semaphore. */
122 if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
123 perror("IPC error: ftok"); exit(1);
124 }
125 /* Get semaphore ID associated with this key. */
128 if ((semid = semget(semkey, 0, 0)) == -1) {
129 /* Semaphore does not exist - Create. */
132 if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
133 S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
134 {
135 /* Initialize the semaphore. */
136 sbuf.sem_num = 0;
137 sbuf.sem_op = 2; /* This is the number of runs
138 without queuing. */
139 sbuf.sem_flg = 0;
140 if (semop(semid, &sbuf, 1) == -1) {
141 perror("IPC error: semop"); exit(1);
142 }
143 }
144 else if (errno == EEXIST) {
145 if ((semid = semget(semkey, 0, 0)) == -1) {
146 perror("IPC error 1: semget"); exit(1);
147 }
148 }
149 else {
150 perror("IPC error 2: semget"); exit(1);
151 }
152 }
153 ...
154
156 The POSIX Realtime Extension defines alternative interfaces for inter‐
157 process communication. Application developers who need to use IPC
158 should design their applications so that modules using the IPC routines
159 described in XSI Interprocess Communication can be easily modified to
160 use the alternative interfaces.
161
163 None.
164
166 None.
167
169 XSI Interprocess Communication , Realtime , semctl() , semop() ,
170 sem_close() , sem_destroy() , sem_getvalue() , sem_init() , sem_open()
171 , sem_post() , sem_unlink() , sem_wait() , the Base Definitions volume
172 of IEEE Std 1003.1-2001, <sys/sem.h>
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175 Portions of this text are reprinted and reproduced in electronic form
176 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
177 -- Portable Operating System Interface (POSIX), The Open Group Base
178 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
179 Electrical and Electronics Engineers, Inc and The Open Group. In the
180 event of any discrepancy between this version and the original IEEE and
181 The Open Group Standard, the original IEEE and The Open Group Standard
182 is the referee document. The original Standard can be obtained online
183 at http://www.opengroup.org/unix/online.html .
184IEEE/The Open Group 2003 SEMGET(P)