1semctl(2) System Calls Manual semctl(2)
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6 semctl - System V semaphore control operations
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9 Standard C library (libc, -lc)
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12 #include <sys/sem.h>
13 int semctl(int semid, int semnum, int cmd, ...);
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17 semctl() performs the control operation specified by cmd on the Sys‐
18 tem V semaphore set identified by semid, or on the semnum-th semaphore
19 of that set. (The semaphores in a set are numbered starting at 0.)
20 This function has three or four arguments, depending on cmd. When
22 there are four, the fourth has the type union semun. The calling pro‐
23 gram must define this union as follows:
24 union semun {
26 int val; /* Value for SETVAL */
27 struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
28 unsigned short *array; /* Array for GETALL, SETALL */
29 struct seminfo *__buf; /* Buffer for IPC_INFO
30 (Linux-specific) */
31 };
32 The semid_ds data structure is defined in <sys/sem.h> as follows:
34 struct semid_ds {
36 struct ipc_perm sem_perm; /* Ownership and permissions */
37 time_t sem_otime; /* Last semop time */
38 time_t sem_ctime; /* Creation time/time of last
39 modification via semctl() */
40 unsigned long sem_nsems; /* No. of semaphores in set */
41 };
42 The fields of the semid_ds structure are as follows:
44 sem_perm This is an ipc_perm structure (see below) that specifies the
46 access permissions on the semaphore set.
47 sem_otime Time of last semop(2) system call.
49 sem_ctime Time of creation of semaphore set or time of last semctl()
51 IPCSET, SETVAL, or SETALL operation.
52 sem_nsems Number of semaphores in the set. Each semaphore of the set
54 is referenced by a nonnegative integer ranging from 0 to
55 sem_nsems-1.
56 The ipc_perm structure is defined as follows (the highlighted fields
58 are settable using IPC_SET):
59 struct ipc_perm {
61 key_t __key; /* Key supplied to semget(2) */
62 uid_t uid; /* Effective UID of owner */
63 gid_t gid; /* Effective GID of owner */
64 uid_t cuid; /* Effective UID of creator */
65 gid_t cgid; /* Effective GID of creator */
66 unsigned short mode; /* Permissions */
67 unsigned short __seq; /* Sequence number */
68 };
69 The least significant 9 bits of the mode field of the ipc_perm struc‐
71 ture define the access permissions for the shared memory segment. The
72 permission bits are as follows:
73 0400 Read by user
75 0200 Write by user
76 0040 Read by group
77 0020 Write by group
78 0004 Read by others
79 0002 Write by others
80 In effect, "write" means "alter" for a semaphore set. Bits 0100, 0010,
82 and 0001 (the execute bits) are unused by the system.
83 Valid values for cmd are:
85 IPC_STAT
87 Copy information from the kernel data structure associated with
88 semid into the semid_ds structure pointed to by arg.buf. The
89 argument semnum is ignored. The calling process must have read
90 permission on the semaphore set.
91 IPC_SET
93 Write the values of some members of the semid_ds structure
94 pointed to by arg.buf to the kernel data structure associated
95 with this semaphore set, updating also its sem_ctime member.
96 The following members of the structure are updated:
98 sem_perm.uid, sem_perm.gid, and (the least significant 9 bits
99 of) sem_perm.mode.
100 The effective UID of the calling process must match the owner
102 (sem_perm.uid) or creator (sem_perm.cuid) of the semaphore set,
103 or the caller must be privileged. The argument semnum is ig‐
104 nored.
105 IPC_RMID
107 Immediately remove the semaphore set, awakening all processes
108 blocked in semop(2) calls on the set (with an error return and
109 errno set to EIDRM). The effective user ID of the calling
110 process must match the creator or owner of the semaphore set, or
111 the caller must be privileged. The argument semnum is ignored.
112 IPC_INFO (Linux-specific)
114 Return information about system-wide semaphore limits and param‐
115 eters in the structure pointed to by arg.__buf. This structure
116 is of type seminfo, defined in <sys/sem.h> if the _GNU_SOURCE
117 feature test macro is defined:
118 struct seminfo {
120 int semmap; /* Number of entries in semaphore
121 map; unused within kernel */
122 int semmni; /* Maximum number of semaphore sets */
123 int semmns; /* Maximum number of semaphores in all
124 semaphore sets */
125 int semmnu; /* System-wide maximum number of undo
126 structures; unused within kernel */
127 int semmsl; /* Maximum number of semaphores in a
128 set */
129 int semopm; /* Maximum number of operations for
130 semop(2) */
131 int semume; /* Maximum number of undo entries per
132 process; unused within kernel */
133 int semusz; /* Size of struct sem_undo */
134 int semvmx; /* Maximum semaphore value */
135 int semaem; /* Max. value that can be recorded for
136 semaphore adjustment (SEM_UNDO) */
137 };
138 The semmsl, semmns, semopm, and semmni settings can be changed
140 via /proc/sys/kernel/sem; see proc(5) for details.
141 SEM_INFO (Linux-specific)
143 Return a seminfo structure containing the same information as
144 for IPC_INFO, except that the following fields are returned with
145 information about system resources consumed by semaphores: the
146 semusz field returns the number of semaphore sets that currently
147 exist on the system; and the semaem field returns the total num‐
148 ber of semaphores in all semaphore sets on the system.
149 SEM_STAT (Linux-specific)
151 Return a semid_ds structure as for IPC_STAT. However, the semid
152 argument is not a semaphore identifier, but instead an index
153 into the kernel's internal array that maintains information
154 about all semaphore sets on the system.
155 SEM_STAT_ANY (Linux-specific, since Linux 4.17)
157 Return a semid_ds structure as for SEM_STAT. However,
158 sem_perm.mode is not checked for read access for semid meaning
159 that any user can employ this operation (just as any user may
160 read /proc/sysvipc/sem to obtain the same information).
161 GETALL Return semval (i.e., the current value) for all semaphores of
163 the set into arg.array. The argument semnum is ignored. The
164 calling process must have read permission on the semaphore set.
165 GETNCNT
167 Return the semncnt value for the semnum-th semaphore of the set
168 (i.e., the number of processes waiting for the semaphore's value
169 to increase). The calling process must have read permission on
170 the semaphore set.
171 GETPID Return the sempid value for the semnum-th semaphore of the set.
173 This is the PID of the process that last performed an operation
174 on that semaphore (but see NOTES). The calling process must
175 have read permission on the semaphore set.
176 GETVAL Return semval (i.e., the semaphore value) for the semnum-th sem‐
178 aphore of the set. The calling process must have read permis‐
179 sion on the semaphore set.
180 GETZCNT
182 Return the semzcnt value for the semnum-th semaphore of the set
183 (i.e., the number of processes waiting for the semaphore value
184 to become 0). The calling process must have read permission on
185 the semaphore set.
186 SETALL Set the semval values for all semaphores of the set using
188 arg.array, updating also the sem_ctime member of the semid_ds
189 structure associated with the set. Undo entries (see semop(2))
190 are cleared for altered semaphores in all processes. If the
191 changes to semaphore values would permit blocked semop(2) calls
192 in other processes to proceed, then those processes are woken
193 up. The argument semnum is ignored. The calling process must
194 have alter (write) permission on the semaphore set.
195 SETVAL Set the semaphore value (semval) to arg.val for the semnum-th
197 semaphore of the set, updating also the sem_ctime member of the
198 semid_ds structure associated with the set. Undo entries are
199 cleared for altered semaphores in all processes. If the changes
200 to semaphore values would permit blocked semop(2) calls in other
201 processes to proceed, then those processes are woken up. The
202 calling process must have alter permission on the semaphore set.
203
205 On success, semctl() returns a nonnegative value depending on cmd as
206 follows:
207 GETNCNT
209 the value of semncnt.
210 GETPID the value of sempid.
212 GETVAL the value of semval.
214 GETZCNT
216 the value of semzcnt.
217 IPC_INFO
219 the index of the highest used entry in the kernel's internal ar‐
220 ray recording information about all semaphore sets. (This in‐
221 formation can be used with repeated SEM_STAT or SEM_STAT_ANY op‐
222 erations to obtain information about all semaphore sets on the
223 system.)
224 SEM_INFO
226 as for IPC_INFO.
227 SEM_STAT
229 the identifier of the semaphore set whose index was given in
230 semid.
231 SEM_STAT_ANY
233 as for SEM_STAT.
234 All other cmd values return 0 on success.
236 On failure, semctl() returns -1 and sets errno to indicate the error.
238
240 EACCES The argument cmd has one of the values GETALL, GETPID, GETVAL,
241 GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SEM_STAT_ANY, SETALL, or
242 SETVAL and the calling process does not have the required per‐
243 missions on the semaphore set and does not have the
244 CAP_IPC_OWNER capability in the user namespace that governs its
245 IPC namespace.
246 EFAULT The address pointed to by arg.buf or arg.array isn't accessible.
248 EIDRM The semaphore set was removed.
250 EINVAL Invalid value for cmd or semid. Or: for a SEM_STAT operation,
252 the index value specified in semid referred to an array slot
253 that is currently unused.
254 EPERM The argument cmd has the value IPC_SET or IPC_RMID but the ef‐
256 fective user ID of the calling process is not the creator (as
257 found in sem_perm.cuid) or the owner (as found in sem_perm.uid)
258 of the semaphore set, and the process does not have the
259 CAP_SYS_ADMIN capability.
260 ERANGE The argument cmd has the value SETALL or SETVAL and the value to
262 which semval is to be set (for some semaphore of the set) is
263 less than 0 or greater than the implementation limit SEMVMX.
264
266 POSIX.1 specifies the sem_nsems field of the semid_ds structure as hav‐
267 ing the type unsigned short, and the field is so defined on most other
268 systems. It was also so defined on Linux 2.2 and earlier, but, since
269 Linux 2.4, the field has the type unsigned long.
270 The sempid value
272 POSIX.1 defines sempid as the "process ID of [the] last operation" on a
273 semaphore, and explicitly notes that this value is set by a successful
274 semop(2) call, with the implication that no other interface affects the
275 sempid value.
276 While some implementations conform to the behavior specified in
278 POSIX.1, others do not. (The fault here probably lies with POSIX.1
279 inasmuch as it likely failed to capture the full range of existing im‐
280 plementation behaviors.) Various other implementations also update
281 sempid for the other operations that update the value of a semaphore:
282 the SETVAL and SETALL operations, as well as the semaphore adjustments
283 performed on process termination as a consequence of the use of the
284 SEM_UNDO flag (see semop(2)).
285 Linux also updates sempid for SETVAL operations and semaphore adjust‐
287 ments. However, somewhat inconsistently, up to and including Linux
288 4.5, the kernel did not update sempid for SETALL operations. This was
289 rectified in Linux 4.6.
290
292 POSIX.1-2008.
293
295 POSIX.1-2001, SVr4.
296 Various fields in a struct semid_ds were typed as short under Linux 2.2
298 and have become long under Linux 2.4. To take advantage of this, a re‐
299 compilation under glibc-2.1.91 or later should suffice. (The kernel
300 distinguishes old and new calls by an IPC_64 flag in cmd.)
301 In some earlier versions of glibc, the semun union was defined in
303 <sys/sem.h>, but POSIX.1 requires that the caller define this union.
304 On versions of glibc where this union is not defined, the macro
305 _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
306
308 The IPC_INFO, SEM_STAT, and SEM_INFO operations are used by the ipcs(1)
309 program to provide information on allocated resources. In the future
310 these may modified or moved to a /proc filesystem interface.
311 The following system limit on semaphore sets affects a semctl() call:
313 SEMVMX Maximum value for semval: implementation dependent (32767).
315 For greater portability, it is best to always call semctl() with four
317 arguments.
318
320 See shmop(2).
321
323 ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7),
324 sysvipc(7)
325Linux man-pages 6.05 2023-03-30 semctl(2)