1SEMCTL(2) Linux Programmer's Manual SEMCTL(2)
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6 semctl - System V semaphore control operations
7
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 Sys‐
17 tem V semaphore set identified by semid, or on the semnum-th semaphore
18 of that 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; /* Creation time/time of last
38 modification via semctl() */
39 unsigned long sem_nsems; /* No. of semaphores in set */
40 };
41 The fields of the semid_ds structure are as follows:
42 sem_perm This is an ipc_perm structure (see below) that specifies the
44 access permissions on the semaphore set.
45 sem_otime Time of last semop(2) system call.
47 sem_ctime Time of creation of semaphore set or time of last semctl()
49 IPCSET, SETVAL, or SETVALL operation.
50 sem_nsems Number of semaphores in the set. Each semaphore of the set
52 is referenced by a nonnegative integer ranging from 0 to
53 sem_nsems-1.
54 The ipc_perm structure is defined as follows (the highlighted fields
56 are settable using IPC_SET):
57 struct ipc_perm {
59 key_t __key; /* Key supplied to semget(2) */
60 uid_t uid; /* Effective UID of owner */
61 gid_t gid; /* Effective GID of owner */
62 uid_t cuid; /* Effective UID of creator */
63 gid_t cgid; /* Effective GID of creator */
64 unsigned short mode; /* Permissions */
65 unsigned short __seq; /* Sequence number */
66 };
67 The least significant 9 bits of the mode field of the ipc_perm struc‐
69 ture define the access permissions for the shared memory segment. The
70 permission bits are as follows:
71 0400 Read by user
73 0200 Write by user
74 0040 Read by group
75 0020 Write by group
76 0004 Read by others
77 0002 Write by others
78 In effect, "write" means "alter" for a semaphore set. Bits 0100, 0010,
80 and 0001 (the execute bits) are unused by the system.
81 Valid values for cmd are:
83 IPC_STAT
85 Copy information from the kernel data structure associated with
86 semid into the semid_ds structure pointed to by arg.buf. The
87 argument semnum is ignored. The calling process must have read
88 permission on the semaphore set.
89 IPC_SET
91 Write the values of some members of the semid_ds structure
92 pointed to by arg.buf to the kernel data structure associated
93 with this semaphore set, updating also its sem_ctime member.
94 The following members of the structure are updated:
95 sem_perm.uid, sem_perm.gid, and (the least significant 9 bits
96 of) sem_perm.mode. The effective UID of the calling process
97 must match the owner (sem_perm.uid) or creator (sem_perm.cuid)
98 of the semaphore set, or the caller must be privileged. The
99 argument semnum is ignored.
100 IPC_RMID
102 Immediately remove the semaphore set, awakening all processes
103 blocked in semop(2) calls on the set (with an error return and
104 errno set to EIDRM). The effective user ID of the calling
105 process must match the creator or owner of the semaphore set, or
106 the caller must be privileged. The argument semnum is ignored.
107 IPC_INFO (Linux-specific)
109 Return information about system-wide semaphore limits and param‐
110 eters in the structure pointed to by arg.__buf. This structure
111 is of type seminfo, defined in <sys/sem.h> if the _GNU_SOURCE
112 feature test macro is defined:
113 struct seminfo {
115 int semmap; /* Number of entries in semaphore
116 map; unused within kernel */
117 int semmni; /* Maximum number of semaphore sets */
118 int semmns; /* Maximum number of semaphores in all
119 semaphore sets */
120 int semmnu; /* System-wide maximum number of undo
121 structures; unused within kernel */
122 int semmsl; /* Maximum number of semaphores in a
123 set */
124 int semopm; /* Maximum number of operations for
125 semop(2) */
126 int semume; /* Maximum number of undo entries per
127 process; unused within kernel */
128 int semusz; /* Size of struct sem_undo */
129 int semvmx; /* Maximum semaphore value */
130 int semaem; /* Max. value that can be recorded for
131 semaphore adjustment (SEM_UNDO) */
132 };
133 The semmsl, semmns, semopm, and semmni settings can be changed
135 via /proc/sys/kernel/sem; see proc(5) for details.
136 SEM_INFO (Linux-specific)
138 Return a seminfo structure containing the same information as
139 for IPC_INFO, except that the following fields are returned with
140 information about system resources consumed by semaphores: the
141 semusz field returns the number of semaphore sets that currently
142 exist on the system; and the semaem field returns the total num‐
143 ber of semaphores in all semaphore sets on the system.
144 SEM_STAT (Linux-specific)
146 Return a semid_ds structure as for IPC_STAT. However, the semid
147 argument is not a semaphore identifier, but instead an index
148 into the kernel's internal array that maintains information
149 about all semaphore sets on the system.
150 SEM_STAT_ANY (Linux-specific, since Linux 4.17)
152 Return a seminfo structure containing the same information as
153 for SEM_STAT. However, sem_perm.mode is not checked for read
154 access for semid meaning that any user can employ this operation
155 (just as any user may read /proc/sysvipc/sem to obtain the same
156 information).
157 GETALL Return semval (i.e., the current value) for all semaphores of
159 the set into arg.array. The argument semnum is ignored. The
160 calling process must have read permission on the semaphore set.
161 GETNCNT
163 Return the semncnt value for the semnum-th semaphore of the set
164 (i.e., the number of processes waiting for the semaphore's value
165 to increase). The calling process must have read permission on
166 the semaphore set.
167 GETPID Return the sempid value for the semnum-th semaphore of the set.
169 This is the PID of the process that last performed an operation
170 on that semaphore (but see NOTES). The calling process must
171 have read permission on the semaphore set.
172 GETVAL Return semval (i.e., the semaphore value) for the semnum-th sem‐
174 aphore of the set. The calling process must have read permis‐
175 sion on the semaphore set.
176 GETZCNT
178 Return the semzcnt value for the semnum-th semaphore of the set
179 (i.e., the number of processes waiting for the semaphore value
180 to become 0). The calling process must have read permission on
181 the semaphore set.
182 SETALL Set the semval values for all semaphores of the set using
184 arg.array, updating also the sem_ctime member of the semid_ds
185 structure associated with the set. Undo entries (see semop(2))
186 are cleared for altered semaphores in all processes. If the
187 changes to semaphore values would permit blocked semop(2) calls
188 in other processes to proceed, then those processes are woken
189 up. The argument semnum is ignored. The calling process must
190 have alter (write) permission on the semaphore set.
191 SETVAL Set the semaphore value (semval) to arg.val for the semnum-th
193 semaphore of the set, updating also the sem_ctime member of the
194 semid_ds structure associated with the set. Undo entries are
195 cleared for altered semaphores in all processes. If the changes
196 to semaphore values would permit blocked semop(2) calls in other
197 processes to proceed, then those processes are woken up. The
198 calling process must have alter permission on the semaphore set.
199
201 On failure, semctl() returns -1 with errno indicating the error.
202 Otherwise, the system call returns a nonnegative value depending on cmd
204 as follows:
205 GETNCNT
207 the value of semncnt.
208 GETPID the value of sempid.
210 GETVAL the value of semval.
212 GETZCNT
214 the value of semzcnt.
215 IPC_INFO
217 the index of the highest used entry in the kernel's internal
218 array recording information about all semaphore sets. (This
219 information can be used with repeated SEM_STAT or SEM_STAT_ANY
220 operations to obtain information about all semaphore sets on the
221 system.)
222 SEM_INFO
224 as for IPC_INFO.
225 SEM_STAT
227 the identifier of the semaphore set whose index was given in
228 semid.
229 SEM_STAT_ANY
231 as for SEM_STAT.
232 All other cmd values return 0 on success.
234
236 On failure, errno will be set to one of the following:
237 EACCES The argument cmd has one of the values GETALL, GETPID, GETVAL,
239 GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SEM_STAT_ANY, SETALL, or
240 SETVAL and the calling process does not have the required per‐
241 missions on the semaphore set and does not have the
242 CAP_IPC_OWNER capability in the user namespace that governs its
243 IPC namespace.
244 EFAULT The address pointed to by arg.buf or arg.array isn't accessible.
246 EIDRM The semaphore set was removed.
248 EINVAL Invalid value for cmd or semid. Or: for a SEM_STAT operation,
250 the index value specified in semid referred to an array slot
251 that is currently unused.
252 EPERM The argument cmd has the value IPC_SET or IPC_RMID but the
254 effective user ID of the calling process is not the creator (as
255 found in sem_perm.cuid) or the owner (as found in sem_perm.uid)
256 of the semaphore set, and the process does not have the
257 CAP_SYS_ADMIN capability.
258 ERANGE The argument cmd has the value SETALL or SETVAL and the value to
260 which semval is to be set (for some semaphore of the set) is
261 less than 0 or greater than the implementation limit SEMVMX.
262
264 POSIX.1-2001, POSIX.1-2008, SVr4.
265 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
272 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
273 or by any version of POSIX. However, some old implementations required
274 the inclusion of these header files, and the SVID also documented their
275 inclusion. Applications intended to be portable to such old systems
276 may need to include these header files.
277 The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(1)
279 program to provide information on allocated resources. In the future
280 these may modified or moved to a /proc filesystem interface.
281 Various fields in a struct semid_ds were typed as short under Linux 2.2
283 and have become long under Linux 2.4. To take advantage of this, a
284 recompilation under glibc-2.1.91 or later should suffice. (The kernel
285 distinguishes old and new calls by an IPC_64 flag in cmd.)
286 In some earlier versions of glibc, the semun union was defined in
288 <sys/sem.h>, but POSIX.1 requires that the caller define this union.
289 On versions of glibc where this union is not defined, the macro
290 _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
291 The following system limit on semaphore sets affects a semctl() call:
293 SEMVMX Maximum value for semval: implementation dependent (32767).
295 For greater portability, it is best to always call semctl() with four
297 arguments.
298 The sempid value
300 POSIX.1 defines sempid as the "process ID of [the] last operation" on a
301 semaphore, and explicitly notes that this value is set by a successful
302 semop(2) call, with the implication that no other interface affects the
303 sempid value.
304 While some implementations conform to the behavior specified in
306 POSIX.1, others do not. (The fault here probably lies with POSIX.1
307 inasmuch as it likely failed to capture the full range of existing
308 implementation behaviors.) Various other implementations also update
309 sempid for the other operations that update the value of a semaphore:
310 the SETVAL and SETALL operations, as well as the semaphore adjustments
311 performed on process termination as a consequence of the use of the
312 SEM_UNDO flag (see semop(2)).
313 Linux also updates sempid for SETVAL operations and semaphore adjust‐
315 ments. However, somewhat inconsistently, up to and including Linux
316 4.5, the kernel did not update sempid for SETALL operations. This was
317 rectified in Linux 4.6.
318
320 See shmop(2).
321
323 ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7),
324 sysvipc(7)
325
327 This page is part of release 5.07 of the Linux man-pages project. A
328 description of the project, information about reporting bugs, and the
329 latest version of this page, can be found at
330 https://www.kernel.org/doc/man-pages/.
331Linux 2020-04-11 SEMCTL(2)