1SEMCTL(3P) POSIX Programmer's Manual SEMCTL(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.
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13 semctl — XSI semaphore control operations
14
16 #include <sys/sem.h>
17 int semctl(int semid, int semnum, int cmd, ...);
19
21 The semctl() function operates on XSI semaphores (see the Base Defini‐
22 tions volume of POSIX.1‐2008, Section 4.16, Semaphore). It is unspeci‐
23 fied whether this function interoperates with the realtime interprocess
24 communication facilities defined in Section 2.8, Realtime.
25 The semctl() function provides a variety of semaphore control opera‐
27 tions as specified by cmd. The fourth argument is optional and depends
28 upon the operation requested. If required, it is of type union semun,
29 which the application shall explicitly declare:
30 union semun {
32 int val;
33 struct semid_ds *buf;
34 unsigned short *array;
35 } arg;
36 The following semaphore control operations as specified by cmd are exe‐
38 cuted with respect to the semaphore specified by semid and semnum. The
39 level of permission required for each operation is shown with each com‐
40 mand; see Section 2.7, XSI Interprocess Communication. The symbolic
41 names for the values of cmd are defined in the <sys/sem.h> header:
42 GETVAL Return the value of semval; see <sys/sem.h>. Requires read
44 permission.
45 SETVAL Set the value of semval to arg.val, where arg is the value
47 of the fourth argument to semctl(). When this command is
48 successfully executed, the semadj value corresponding to
49 the specified semaphore in all processes is cleared. Also,
50 the sem_ctime timestamp shall be set to the current time,
51 as described in Section 2.7.1, IPC General Description.
52 Requires alter permission; see Section 2.7, XSI Interpro‐
53 cess Communication.
54 GETPID Return the value of sempid. Requires read permission.
56 GETNCNT Return the value of semncnt. Requires read permission.
58 GETZCNT Return the value of semzcnt. Requires read permission.
60 The following values of cmd operate on each semval in the set of sema‐
62 phores:
63 GETALL Return the value of semval for each semaphore in the sema‐
65 phore set and place into the array pointed to by arg.array,
66 where arg is the fourth argument to semctl(). Requires
67 read permission.
68 SETALL Set the value of semval for each semaphore in the semaphore
70 set according to the array pointed to by arg.array, where
71 arg is the fourth argument to semctl(). When this command
72 is successfully executed, the semadj values corresponding
73 to each specified semaphore in all processes are cleared.
74 Also, the sem_ctime timestamp shall be set to the current
75 time, as described in Section 2.7.1, IPC General Descrip‐
76 tion. Requires alter permission.
77 The following values of cmd are also available:
79 IPC_STAT Place the current value of each member of the semid_ds data
81 structure associated with semid into the structure pointed
82 to by arg.buf, where arg is the fourth argument to sem‐
83 ctl(). The contents of this structure are defined in
84 <sys/sem.h>. Requires read permission.
85 IPC_SET Set the value of the following members of the semid_ds data
87 structure associated with semid to the corresponding value
88 found in the structure pointed to by arg.buf, where arg is
89 the fourth argument to semctl():
90 sem_perm.uid
92 sem_perm.gid
93 sem_perm.mode
94 The mode bits specified in Section 2.7.1, IPC General
96 Description are copied into the corresponding bits of the
97 sem_perm.mode associated with semid. The stored values of
98 any other bits are unspecified. The sem_ctime timestamp
99 shall be set to the current time, as described in Section
100 2.7.1, IPC General Description.
101 This command can only be executed by a process that has an
103 effective user ID equal to either that of a process with
104 appropriate privileges or to the value of sem_perm.cuid or
105 sem_perm.uid in the semid_ds data structure associated with
106 semid.
107 IPC_RMID Remove the semaphore identifier specified by semid from the
109 system and destroy the set of semaphores and semid_ds data
110 structure associated with it. This command can only be exe‐
111 cuted by a process that has an effective user ID equal to
112 either that of a process with appropriate privileges or to
113 the value of sem_perm.cuid or sem_perm.uid in the semid_ds
114 data structure associated with semid.
115
117 If successful, the value returned by semctl() depends on cmd as fol‐
118 lows:
119 GETVAL The value of semval.
121 GETPID The value of sempid.
123 GETNCNT The value of semncnt.
125 GETZCNT The value of semzcnt.
127 All others 0.
129 Otherwise, semctl() shall return −1 and set errno to indicate the
131 error.
132
134 The semctl() function shall fail if:
135 EACCES Operation permission is denied to the calling process; see Sec‐
137 tion 2.7, XSI Interprocess Communication.
138 EINVAL The value of semid is not a valid semaphore identifier, or the
140 value of semnum is less than 0 or greater than or equal to
141 sem_nsems, or the value of cmd is not a valid command.
142 EPERM The argument cmd is equal to IPC_RMID or IPC_SET and the effec‐
144 tive user ID of the calling process is not equal to that of a
145 process with appropriate privileges and it is not equal to the
146 value of sem_perm.cuid or sem_perm.uid in the data structure
147 associated with semid.
148 ERANGE The argument cmd is equal to SETVAL or SETALL and the value to
150 which semval is to be set is greater than the system-imposed
151 maximum.
152 The following sections are informative.
154
156 Refer to semop().
157
159 The fourth parameter in the SYNOPSIS section is now specified as "..."
160 in order to avoid a clash with the ISO C standard when referring to the
161 union semun (as defined in Issue 3) and for backwards-compatibility.
162 The POSIX Realtime Extension defines alternative interfaces for inter‐
164 process communication. Application developers who need to use IPC
165 should design their applications so that modules using the IPC routines
166 described in Section 2.7, XSI Interprocess Communication can be easily
167 modified to use the alternative interfaces.
168
170 None.
171
173 None.
174
176 Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
177 semget(), semop(), sem_close(), sem_destroy(), sem_getvalue(),
178 sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()
179 The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore,
181 <sys_sem.h>
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184 Portions of this text are reprinted and reproduced in electronic form
185 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
186 -- Portable Operating System Interface (POSIX), The Open Group Base
187 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
188 cal and Electronics Engineers, Inc and The Open Group. (This is
189 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
190 event of any discrepancy between this version and the original IEEE and
191 The Open Group Standard, the original IEEE and The Open Group Standard
192 is the referee document. The original Standard can be obtained online
193 at http://www.unix.org/online.html .
194 Any typographical or formatting errors that appear in this page are
196 most likely to have been introduced during the conversion of the source
197 files to man page format. To report such errors, see https://www.ker‐
198 nel.org/doc/man-pages/reporting_bugs.html .
199IEEE/The Open Group 2013 SEMCTL(3P)