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
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15 #include <sys/sem.h>
16 int semget(key_t key, int nsems, int semflg);
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20 The semget() function operates on XSI semaphores (see the Base Defini‐
21 tions volume of POSIX.1‐2017, Section 4.17, Semaphore). It is unspeci‐
22 fied whether this function interoperates with the realtime interprocess
23 communication facilities defined in Section 2.8, Realtime.
24 The semget() function shall return the semaphore identifier associated
26 with key.
27 A semaphore identifier with its associated semid_ds data structure and
29 its associated set of nsems semaphores (see <sys/sem.h>) is created for
30 key if one of the following is true:
31 * The argument key is equal to IPC_PRIVATE.
33 * The argument key does not already have a semaphore identifier asso‐
35 ciated with it and (semflg &IPC_CREAT) is non-zero.
36 Upon creation, the semid_ds data structure associated with the new sem‐
38 aphore identifier is initialized as follows:
39 * In the operation permissions structure sem_perm.cuid, sem_perm.uid,
41 sem_perm.cgid, and sem_perm.gid shall be set to the effective user
42 ID and effective group ID, respectively, of the calling process.
43 * The low-order 9 bits of sem_perm.mode shall be set to the low-order
45 9 bits of semflg.
46 * The variable sem_nsems shall be set to the value of nsems.
48 * The variable sem_otime shall be set to 0 and sem_ctime shall be set
50 to the current time, as described in Section 2.7.1, IPC General
51 Description.
52 * The data structure associated with each semaphore in the set need
54 not be initialized. The semctl() function with the command SETVAL
55 or SETALL can be used to initialize each semaphore.
56
58 Upon successful completion, semget() shall return a non-negative inte‐
59 ger, namely a semaphore identifier; otherwise, it shall return -1 and
60 set errno to indicate the error.
61
63 The semget() function shall fail if:
64 EACCES A semaphore identifier exists for key, but operation permission
66 as specified by the low-order 9 bits of semflg would not be
67 granted; see Section 2.7, XSI Interprocess Communication.
68 EEXIST A semaphore identifier exists for the argument key but ((semflg
70 &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
71 EINVAL The value of nsems is either less than or equal to 0 or greater
73 than the system-imposed limit, or a semaphore identifier exists
74 for the argument key, but the number of semaphores in the set
75 associated with it is less than nsems and nsems is not equal to
76 0.
77 ENOENT A semaphore identifier does not exist for the argument key and
79 (semflg &IPC_CREAT) is equal to 0.
80 ENOSPC A semaphore identifier is to be created but the system-imposed
82 limit on the maximum number of allowed semaphores system-wide
83 would be exceeded.
84 The following sections are informative.
86
88 Refer to semop().
89
91 The POSIX Realtime Extension defines alternative interfaces for inter‐
92 process communication. Application developers who need to use IPC
93 should design their applications so that modules using the IPC routines
94 described in Section 2.7, XSI Interprocess Communication can be easily
95 modified to use the alternative interfaces.
96
98 None.
99
101 A future version may require that the value of the semval, sempid, sem‐
102 ncnt, and semzcnt members of all semaphores in a semaphore set be ini‐
103 tialized to zero when a call to semget() creates a semaphore set. Many
104 semaphore implementations already do this and it greatly simplifies
105 what an application must do to initialize a semaphore set.
106
108 Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
109 ftok(), semctl(), semop(), sem_close(), sem_destroy(), sem_getvalue(),
110 sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()
111 The Base Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore,
113 <sys_sem.h>
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116 Portions of this text are reprinted and reproduced in electronic form
117 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
118 table Operating System Interface (POSIX), The Open Group Base Specifi‐
119 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
120 Electrical and Electronics Engineers, Inc and The Open Group. In the
121 event of any discrepancy between this version and the original IEEE and
122 The Open Group Standard, the original IEEE and The Open Group Standard
123 is the referee document. The original Standard can be obtained online
124 at http://www.opengroup.org/unix/online.html .
125 Any typographical or formatting errors that appear in this page are
127 most likely to have been introduced during the conversion of the source
128 files to man page format. To report such errors, see https://www.ker‐
129 nel.org/doc/man-pages/reporting_bugs.html .
130IEEE/The Open Group 2017 SEMGET(3P)