1SEMCTL(2) Linux Programmer's Manual SEMCTL(2)
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6 semctl - semaphore control operations
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9 #include <sys/types.h>
10 #include <sys/ipc.h>
11 #include <sys/sem.h>
12 int semctl(int semid, int semnum, int cmd, ...);
14
16 semctl() performs the control operation specified by cmd on the sema‐
17 phore set identified by semid, or on the semnum-th semaphore of that
18 set. (The semaphores in a set are numbered starting at 0.)
19 This function has three or four arguments, depending on cmd. When
21 there are four, the fourth has the type union semun. The calling pro‐
22 gram must define this union as follows:
23 union semun {
25 int val; /* Value for SETVAL */
26 struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
27 unsigned short *array; /* Array for GETALL, SETALL */
28 struct seminfo *__buf; /* Buffer for IPC_INFO
29 (Linux specific) */
30 };
31 The semid_ds data structure is defined in <sys/sem.h> as follows:
33 struct semid_ds {
35 struct ipc_perm sem_perm; /* Ownership and permissions
36 time_t sem_otime; /* Last semop time */
37 time_t sem_ctime; /* Last change time */
38 unsigned short sem_nsems; /* No. of semaphores in set */
39 };
40 The ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
42 lighted fields are settable using IPC_SET):
43 struct ipc_perm {
45 key_t key; /* Key supplied to semget() */
46 uid_t uid; /* Effective UID of owner */
47 gid_t gid; /* Effective GID of owner */
48 uid_t cuid; /* Effective UID of creator */
49 gid_t cgid; /* Effective GID of creator */
50 unsigned short mode; /* Permissions */
51 unsigned short seq; /* Sequence number */
52 };
53 Valid values for cmd are:
55 IPC_STAT Copy information from the kernel data structure associated
57 with semid into the semid_ds structure pointed to by
58 arg.buf. The argument semnum is ignored. The calling
59 process must have read permission on the semaphore set.
60 IPC_SET Write the values of some members of the semid_ds structure
62 pointed to by arg.buf to the kernel data structure associ‐
63 ated with this semaphore set, updating also its sem_ctime
64 member. The following members of the structure are
65 updated: sem_perm.uid, sem_perm.gid, and (the least signif‐
66 icant 9 bits of) sem_perm.mode. The effective UID of the
67 calling process must match the owner (sem_perm.uid) or cre‐
68 ator (sem_perm.cuid) of the semaphore set, or the caller
69 must be privileged. The argument semnum is ignored.
70 IPC_RMID Immediately remove the semaphore set, awakening all pro‐
72 cesses blocked in semop() calls on the set (with an error
73 return and errno set to EIDRM). The effective user ID of
74 the calling process must match the creator or owner of the
75 semaphore set, or the caller must be privileged. The argu‐
76 ment semnum is ignored.
77 IPC_INFO (Linux specific)
79 Returns information about system-wide semaphore limits and
80 parameters in the structure pointed to by arg.__buf. This
81 structure is of type seminfo, defined in <sys/sem.h> if the
82 _GNU_SOURCE feature test macro is defined:
83 struct seminfo {
85 int semmap; /* # of entries in semaphore map;
86 unused */
87 int semmni; /* Max. # of semaphore sets */
88 int semmns; /* Max. # of semaphores in all
89 semaphore sets */
90 int semmnu; /* System-wide max. # of undo
91 structures; unused */
92 int semmsl; /* Max. # of semaphores in a set */
93 int semopm; /* Max. # of operations for semop() */
94 int semume; /* Max. # of undo entries per
95 process; unused */
96 int semusz; /* size of struct sem_undo */
97 int semvmx; /* Maximum semaphore value */
98 int semaem; /* Max. value that can be recorded for
99 semaphore adjustment (SEM_UNDO) */
100 };
101 The semmsl, semmns, semopm, and semmni settings can be
103 changed via /proc/sys/kernel/sem; see proc(5) for details.
104 SEM_INFO (Linux specific)
106 Returns a seminfo structure containing the same information
107 as for IPC_INFO, except that the following fields are
108 returned with information about system resources consumed
109 by semaphores: the semusz field returns the number of sema‐
110 phore sets that currently exist on the system; and the
111 semaem field returns the total number of semaphores in all
112 semaphore sets on the system.
113 SEM_STAT (Linux specific)
115 Returns a semid_ds structure as for IPC_STAT. However, the
116 semid argument is not a semaphore identifier, but instead
117 an index into the kernel's internal array that maintains
118 information about all semaphore sets on the system.
119 GETALL Return semval (i.e., the current value) for all semaphores
121 of the set into arg.array. The argument semnum is ignored.
122 The calling process must have read permission on the sema‐
123 phore set.
124 GETNCNT The system call returns the value of semncnt (i.e., the
126 number of processes waiting for the value of this semaphore
127 to increase) for the semnum-th semaphore of the set (i.e.
128 the number of processes waiting for an increase of semval
129 for the semnum-th semaphore of the set). The calling
130 process must have read permission on the semaphore set.
131 GETPID The system call returns the value of sempid for the sem‐
133 num-th semaphore of the set (i.e. the PID of the process
134 that executed the last semop() call for the semnum-th sema‐
135 phore of the set). The calling process must have read per‐
136 mission on the semaphore set.
137 GETVAL The system call returns the value of semval for the sem‐
139 num-th semaphore of the set. The calling process must have
140 read permission on the semaphore set.
141 GETZCNT The system call returns the value of semzcnt (i.e., the
143 number of processes waiting for the value of this semaphore
144 to become zero) for the semnum-th semaphore of the set
145 (i.e. the number of processes waiting for semval of the
146 semnum-th semaphore of the set to become 0). The calling
147 process must have read permission on the semaphore set.
148 SETALL Set semval for all semaphores of the set using arg.array,
150 updating also the sem_ctime member of the semid_ds struc‐
151 ture associated with the set. Undo entries (see semop(2))
152 are cleared for altered semaphores in all processes. If
153 the changes to semaphore values would permit blocked
154 semop() calls in other processes to proceed, then those
155 processes are woken up. The argument semnum is ignored.
156 The calling process must have alter (write) permission on
157 the semaphore set.
158 SETVAL Set the value of semval to arg.val for the semnum-th sema‐
160 phore of the set, updating also the sem_ctime member of the
161 semid_ds structure associated with the set. Undo entries
162 are cleared for altered semaphores in all processes. If
163 the changes to semaphore values would permit blocked
164 semop() calls in other processes to proceed, then those
165 processes are woken up. The calling process must have
166 alter permission on the semaphore set.
167
169 On failure semctl() returns -1 with errno indicating the error.
170 Otherwise the system call returns a nonnegative value depending on cmd
172 as follows:
173 GETNCNT the value of semncnt.
175 GETPID the value of sempid.
177 GETVAL the value of semval.
179 GETZCNT the value of semzcnt.
181 IPC_INFO the index of the highest used entry in the kernel's internal
183 array recording information about all semaphore sets. (This
184 information can be used with repeated SEM_STAT operations to
185 obtain information about all semaphore sets on the system.)
186 SEM_INFO As for IPC_INFO.
188 SEM_STAT the identifier of the semaphore set whose index was given in
190 semid.
191 All other cmd values return 0 on success.
193
195 On failure, errno will be set to one of the following:
196 EACCES The argument cmd has one of the values GETALL, GETPID, GET‐
198 VAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL
199 and the calling process does not have the required permis‐
200 sions on the semaphore set and does not have the
201 CAP_IPC_OWNER capability.
202 EFAULT The address pointed to by arg.buf or arg.array isn't acces‐
204 sible.
205 EIDRM The semaphore set was removed.
207 EINVAL Invalid value for cmd or semid. Or: for a SEM_STAT opera‐
209 tion, the index value specified in semid referred to an
210 array slot that is currently unused.
211 EPERM The argument cmd has the value IPC_SET or IPC_RMID but the
213 effective user ID of the calling process is not the creator
214 (as found in sem_perm.cuid) or the owner (as found in
215 sem_perm.uid) of the semaphore set, and the process does not
216 have the CAP_SYS_ADMIN capability.
217 ERANGE The argument cmd has the value SETALL or SETVAL and the
219 value to which semval is to be set (for some semaphore of
220 the set) is less than 0 or greater than the implementation
221 limit SEMVMX.
222
224 The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(8)
225 program to provide information on allocated resources. In the future
226 these may modified or moved to a /proc file system interface.
227 Various fields in a struct semid_ds were shorts under Linux 2.2 and
229 have become longs under Linux 2.4. To take advantage of this, a recom‐
230 pilation under glibc-2.1.91 or later should suffice. (The kernel dis‐
231 tinguishes old and new calls by an IPC_64 flag in cmd.)
232 In some earlier versions of glibc, the semun union was defined in
234 <sys/sem.h>, but POSIX.1-2001 requires that the caller define this
235 union. On versions of glibc where this union is not defined, the macro
236 _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
237 The following system limit on semaphore sets affects a semctl() call:
239 SEMVMX Maximum value for semval: implementation dependent (32767).
241 For greater portability it is best to always call semctl() with four
243 arguments.
244 Under Linux, semctl() is not a system call, but is implemented via the
246 system call ipc(2).
247
249 SVr4, POSIX.1-2001.
250
252 ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), svipc(7)
253Linux 2.6.9 2004-11-10 SEMCTL(2)