1SEMCTL(P) POSIX Programmer's Manual SEMCTL(P)
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6 semctl - XSI semaphore control operations
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9 #include <sys/sem.h>
10 int semctl(int semid, int semnum, int cmd, ...);
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15 The semctl() 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 semctl() function provides a variety of semaphore control opera‐
21 tions as specified by cmd. The fourth argument is optional and depends
22 upon the operation requested. If required, it is of type union semun,
23 which the application shall explicitly declare:
24 union semun {
27 int val;
28 struct semid_ds *buf;
29 unsigned short *array;
30 } arg;
31 The following semaphore control operations as specified by cmd are exe‐
33 cuted with respect to the semaphore specified by semid and semnum. The
34 level of permission required for each operation is shown with each com‐
35 mand; see XSI Interprocess Communication . The symbolic names for the
36 values of cmd are defined in the <sys/sem.h> header:
37 GETVAL Return the value of semval; see <sys/sem.h>. Requires read per‐
39 mission.
40 SETVAL Set the value of semval to arg.val, where arg is the value of
42 the fourth argument to semctl(). When this command is success‐
43 fully executed, the semadj value corresponding to the specified
44 semaphore in all processes is cleared. Requires alter permis‐
45 sion; see XSI Interprocess Communication .
46 GETPID Return the value of sempid. Requires read permission.
48 GETNCNT
50 Return the value of semncnt. Requires read permission.
51 GETZCNT
53 Return the value of semzcnt. Requires read permission.
54 The following values of cmd operate on each semval in the set of sema‐
57 phores:
58 GETALL Return the value of semval for each semaphore in the semaphore
60 set and place into the array pointed to by arg.array, where arg
61 is the fourth argument to semctl(). Requires read permission.
62 SETALL Set the value of semval for each semaphore in the semaphore set
64 according to the array pointed to by arg.array, where arg is the
65 fourth argument to semctl(). When this command is successfully
66 executed, the semadj values corresponding to each specified sem‐
67 aphore in all processes are cleared. Requires alter permission.
68 The following values of cmd are also available:
71 IPC_STAT
73 Place the current value of each member of the semid_ds data
74 structure associated with semid into the structure pointed to by
75 arg.buf, where arg is the fourth argument to semctl(). The con‐
76 tents of this structure are defined in <sys/sem.h>. Requires
77 read permission.
78 IPC_SET
80 Set the value of the following members of the semid_ds data
81 structure associated with semid to the corresponding value found
82 in the structure pointed to by arg.buf, where arg is the fourth
83 argument to semctl():
84 sem_perm.uid
87 sem_perm.gid
88 sem_perm.mode
89 The mode bits specified in IPC General Description are copied into the
91 corresponding bits of the sem_perm.mode associated with semid. The
92 stored values of any other bits are unspecified.
93 This command can only be executed by a process that has an effective
95 user ID equal to either that of a process with appropriate privileges
96 or to the value of sem_perm.cuid or sem_perm.uid in the semid_ds data
97 structure associated with semid.
98 IPC_RMID
100 Remove the semaphore identifier specified by semid from the sys‐
101 tem and destroy the set of semaphores and semid_ds data struc‐
102 ture associated with it. This command can only be executed by a
103 process that has an effective user ID equal to either that of a
104 process with appropriate privileges or to the value of
105 sem_perm.cuid or sem_perm.uid in the semid_ds data structure
106 associated with semid.
107
110 If successful, the value returned by semctl() depends on cmd as fol‐
111 lows:
112 GETVAL The value of semval.
114 GETPID The value of sempid.
116 GETNCNT
118 The value of semncnt.
119 GETZCNT
121 The value of semzcnt.
122 All others
124 0.
125 Otherwise, semctl() shall return -1 and set errno to indicate the
128 error.
129
131 The semctl() function shall fail if:
132 EACCES Operation permission is denied to the calling process; see XSI
134 Interprocess Communication .
135 EINVAL The value of semid is not a valid semaphore identifier, or the
137 value of semnum is less than 0 or greater than or equal to
138 sem_nsems, or the value of cmd is not a valid command.
139 EPERM The argument cmd is equal to IPC_RMID or IPC_SET and the effec‐
141 tive user ID of the calling process is not equal to that of a
142 process with appropriate privileges and it is not equal to the
143 value of sem_perm.cuid or sem_perm.uid in the data structure
144 associated with semid.
145 ERANGE The argument cmd is equal to SETVAL or SETALL and the value to
147 which semval is to be set is greater than the system-imposed
148 maximum.
149 The following sections are informative.
152
154 None.
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157 The fourth parameter in the SYNOPSIS section is now specified as "..."
158 in order to avoid a clash with the ISO C standard when referring to the
159 union semun (as defined in Issue 3) and for backwards-compatibility.
160 The POSIX Realtime Extension defines alternative interfaces for inter‐
162 process communication. Application developers who need to use IPC
163 should design their applications so that modules using the IPC routines
164 described in XSI Interprocess Communication can be easily modified to
165 use the alternative interfaces.
166
168 None.
169
171 None.
172
174 XSI Interprocess Communication , Realtime , semget() , semop() ,
175 sem_close() , sem_destroy() , sem_getvalue() , sem_init() , sem_open()
176 , sem_post() , sem_unlink() , sem_wait() , the Base Definitions volume
177 of IEEE Std 1003.1-2001, <sys/sem.h>
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180 Portions of this text are reprinted and reproduced in electronic form
181 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
182 -- Portable Operating System Interface (POSIX), The Open Group Base
183 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
184 Electrical and Electronics Engineers, Inc and The Open Group. In the
185 event of any discrepancy between this version and the original IEEE and
186 The Open Group Standard, the original IEEE and The Open Group Standard
187 is the referee document. The original Standard can be obtained online
188 at http://www.opengroup.org/unix/online.html .
189IEEE/The Open Group 2003 SEMCTL(P)