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.
10
12 semctl - XSI semaphore control operations
13
15 #include <sys/sem.h>
16 int semctl(int semid, int semnum, int cmd, ...);
18
21 The semctl() 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 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 {
33 int val;
34 struct semid_ds *buf;
35 unsigned short *array;
36 } arg;
37 The following semaphore control operations as specified by cmd are exe‐
39 cuted with respect to the semaphore specified by semid and semnum. The
40 level of permission required for each operation is shown with each com‐
41 mand; see XSI Interprocess Communication . The symbolic names for the
42 values of cmd are defined in the <sys/sem.h> header:
43 GETVAL Return the value of semval; see <sys/sem.h>. Requires read per‐
45 mission.
46 SETVAL Set the value of semval to arg.val, where arg is the value of
48 the fourth argument to semctl(). When this command is success‐
49 fully executed, the semadj value corresponding to the specified
50 semaphore in all processes is cleared. Requires alter permis‐
51 sion; see XSI Interprocess Communication .
52 GETPID Return the value of sempid. Requires read permission.
54 GETNCNT
56 Return the value of semncnt. Requires read permission.
57 GETZCNT
59 Return the value of semzcnt. Requires read permission.
60 The following values of cmd operate on each semval in the set of sema‐
63 phores:
64 GETALL Return the value of semval for each semaphore in the semaphore
66 set and place into the array pointed to by arg.array, where arg
67 is the fourth argument to semctl(). Requires read permission.
68 SETALL Set the value of semval for each semaphore in the semaphore set
70 according to the array pointed to by arg.array, where arg is the
71 fourth argument to semctl(). When this command is successfully
72 executed, the semadj values corresponding to each specified sem‐
73 aphore in all processes are cleared. Requires alter permission.
74 The following values of cmd are also available:
77 IPC_STAT
79 Place the current value of each member of the semid_ds data
80 structure associated with semid into the structure pointed to by
81 arg.buf, where arg is the fourth argument to semctl(). The con‐
82 tents of this structure are defined in <sys/sem.h>. Requires
83 read permission.
84 IPC_SET
86 Set the value of the following members of the semid_ds data
87 structure associated with semid to the corresponding value found
88 in the structure pointed to by arg.buf, where arg is the fourth
89 argument to semctl():
90 sem_perm.uid
93 sem_perm.gid
94 sem_perm.mode
95 The mode bits specified in IPC General Description are copied into the
97 corresponding bits of the sem_perm.mode associated with semid. The
98 stored values of any other bits are unspecified.
99 This command can only be executed by a process that has an effective
101 user ID equal to either that of a process with appropriate privileges
102 or to the value of sem_perm.cuid or sem_perm.uid in the semid_ds data
103 structure associated with semid.
104 IPC_RMID
106 Remove the semaphore identifier specified by semid from the sys‐
107 tem and destroy the set of semaphores and semid_ds data struc‐
108 ture associated with it. This command can only be executed by a
109 process that has an effective user ID equal to either that of a
110 process with appropriate privileges or to the value of
111 sem_perm.cuid or sem_perm.uid in the semid_ds data structure
112 associated with semid.
113
116 If successful, the value returned by semctl() depends on cmd as fol‐
117 lows:
118 GETVAL The value of semval.
120 GETPID The value of sempid.
122 GETNCNT
124 The value of semncnt.
125 GETZCNT
127 The value of semzcnt.
128 All others
130 0.
131 Otherwise, semctl() shall return -1 and set errno to indicate the
134 error.
135
137 The semctl() function shall fail if:
138 EACCES Operation permission is denied to the calling process; see XSI
140 Interprocess Communication .
141 EINVAL The value of semid is not a valid semaphore identifier, or the
143 value of semnum is less than 0 or greater than or equal to
144 sem_nsems, or the value of cmd is not a valid command.
145 EPERM The argument cmd is equal to IPC_RMID or IPC_SET and the effec‐
147 tive user ID of the calling process is not equal to that of a
148 process with appropriate privileges and it is not equal to the
149 value of sem_perm.cuid or sem_perm.uid in the data structure
150 associated with semid.
151 ERANGE The argument cmd is equal to SETVAL or SETALL and the value to
153 which semval is to be set is greater than the system-imposed
154 maximum.
155 The following sections are informative.
158
160 None.
161
163 The fourth parameter in the SYNOPSIS section is now specified as "..."
164 in order to avoid a clash with the ISO C standard when referring to the
165 union semun (as defined in Issue 3) and for backwards-compatibility.
166 The POSIX Realtime Extension defines alternative interfaces for inter‐
168 process communication. Application developers who need to use IPC
169 should design their applications so that modules using the IPC routines
170 described in XSI Interprocess Communication can be easily modified to
171 use the alternative interfaces.
172
174 None.
175
177 None.
178
180 XSI Interprocess Communication, Realtime, semget(), semop(),
181 sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(),
182 sem_post(), sem_unlink(), sem_wait(), the Base Definitions volume of
183 IEEE Std 1003.1-2001, <sys/sem.h>
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186 Portions of this text are reprinted and reproduced in electronic form
187 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
188 -- Portable Operating System Interface (POSIX), The Open Group Base
189 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
190 Electrical and Electronics Engineers, Inc and The Open Group. In the
191 event of any discrepancy between this version and the original IEEE and
192 The Open Group Standard, the original IEEE and The Open Group Standard
193 is the referee document. The original Standard can be obtained online
194 at http://www.opengroup.org/unix/online.html .
195IEEE/The Open Group 2003 SEMCTL(3P)