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; /* Last change time */
38 unsigned long sem_nsems; /* No. of semaphores in set */
39 };
40 The ipc_perm structure is defined as follows (the highlighted fields
42 are settable using IPC_SET):
43 struct ipc_perm {
45 key_t __key; /* Key supplied to semget(2) */
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 arg.buf.
58 The argument semnum is ignored. The calling process must
59 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 associated
63 with this semaphore set, updating also its sem_ctime member.
64 The following members of the structure are updated:
65 sem_perm.uid, sem_perm.gid, and (the least significant 9 bits
66 of) sem_perm.mode. The effective UID of the calling process
67 must match the owner (sem_perm.uid) or creator
68 (sem_perm.cuid) of the semaphore set, or the caller must be
69 privileged. The argument semnum is ignored.
70 IPC_RMID Immediately remove the semaphore set, awakening all processes
72 blocked in semop(2) calls on the set (with an error return
73 and errno set to EIDRM). The effective user ID of the call‐
74 ing process must match the creator or owner of the semaphore
75 set, or the caller must be privileged. The argument semnum
76 is ignored.
77 IPC_INFO (Linux-specific)
79 Return 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; /* Number of entries in semaphore
86 map; unused within kernel */
87 int semmni; /* Maximum number of semaphore sets */
88 int semmns; /* Maximum number of semaphores in all
89 semaphore sets */
90 int semmnu; /* System-wide maximum number of undo
91 structures; unused within kernel */
92 int semmsl; /* Maximum number of semaphores in a
93 set */
94 int semopm; /* Maximum number of operations for
95 semop(2) */
96 int semume; /* Maximum number of undo entries per
97 process; unused within kernel */
98 int semusz; /* Size of struct sem_undo */
99 int semvmx; /* Maximum semaphore value */
100 int semaem; /* Max. value that can be recorded for
101 semaphore adjustment (SEM_UNDO) */
102 };
103 The semmsl, semmns, semopm, and semmni settings can be
105 changed via /proc/sys/kernel/sem; see proc(5) for details.
106 SEM_INFO (Linux-specific)
108 Return a seminfo structure containing the same information as
109 for IPC_INFO, except that the following fields are returned
110 with information about system resources consumed by sema‐
111 phores: the semusz field returns the number of semaphore sets
112 that currently exist on the system; and the semaem field
113 returns the total number of semaphores in all semaphore sets
114 on the system.
115 SEM_STAT (Linux-specific)
117 Return a semid_ds structure as for IPC_STAT. However, the
118 semid argument is not a semaphore identifier, but instead an
119 index into the kernel's internal array that maintains infor‐
120 mation about all semaphore sets on the system.
121 SEM_STAT_ANY (Linux-specific, since Linux 4.17)
123 Return a seminfo structure containing the same information as
124 for SEM_STAT. However, sem_perm.mode is not checked for read
125 access for semid meaning that any user can employ this opera‐
126 tion (just as any user may read /proc/sysvipc/sem to obtain
127 the same information).
128 GETALL Return semval (i.e., the current value) for all semaphores of
130 the set into arg.array. The argument semnum is ignored. The
131 calling process must have read permission on the semaphore
132 set.
133 GETNCNT Return the value of semncnt for the semnum-th semaphore of
135 the set (i.e., the number of processes waiting for an
136 increase of semval for the semnum-th semaphore of the set).
137 The calling process must have read permission on the sema‐
138 phore set.
139 GETPID Return the value of sempid for the semnum-th semaphore of the
141 set. This is the PID of the process that last performed an
142 operation on that semaphore (but see NOTES). The calling
143 process must have read permission on the semaphore set.
144 GETVAL Return the value of semval for the semnum-th semaphore of the
146 set. The calling process must have read permission on the
147 semaphore set.
148 GETZCNT Return the value of semzcnt for the semnum-th semaphore of
150 the set (i.e., the number of processes waiting for semval of
151 the semnum-th semaphore of the set to become 0). The calling
152 process must have read permission on the semaphore set.
153 SETALL Set semval for all semaphores of the set using arg.array,
155 updating also the sem_ctime member of the semid_ds structure
156 associated with the set. Undo entries (see semop(2)) are
157 cleared for altered semaphores in all processes. If the
158 changes to semaphore values would permit blocked semop(2)
159 calls in other processes to proceed, then those processes are
160 woken up. The argument semnum is ignored. The calling
161 process must have alter (write) permission on the semaphore
162 set.
163 SETVAL Set the value of semval to arg.val for the semnum-th sema‐
165 phore of the set, updating also the sem_ctime member of the
166 semid_ds structure associated with the set. Undo entries are
167 cleared for altered semaphores in all processes. If the
168 changes to semaphore values would permit blocked semop(2)
169 calls in other processes to proceed, then those processes are
170 woken up. The calling process must have alter permission on
171 the semaphore set.
172
174 On failure, semctl() returns -1 with errno indicating the error.
175 Otherwise, the system call returns a nonnegative value depending on cmd
177 as follows:
178 GETNCNT the value of semncnt.
180 GETPID the value of sempid.
182 GETVAL the value of semval.
184 GETZCNT the value of semzcnt.
186 IPC_INFO the index of the highest used entry in the kernel's internal
188 array recording information about all semaphore sets. (This
189 information can be used with repeated SEM_STAT or
190 SEM_STAT_ANY operations to obtain information about all sema‐
191 phore sets on the system.)
192 SEM_INFO as for IPC_INFO.
194 SEM_STAT the identifier of the semaphore set whose index was given in
196 semid.
197 SEM_STAT_ANY
199 as for SEM_STAT.
200 All other cmd values return 0 on success.
202
204 On failure, errno will be set to one of the following:
205 EACCES The argument cmd has one of the values GETALL, GETPID, GETVAL,
207 GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SEM_STAT_ANY, SETALL, or
208 SETVAL and the calling process does not have the required per‐
209 missions on the semaphore set and does not have the
210 CAP_IPC_OWNER capability in the user namespace that governs its
211 IPC namespace.
212 EFAULT The address pointed to by arg.buf or arg.array isn't accessible.
214 EIDRM The semaphore set was removed.
216 EINVAL Invalid value for cmd or semid. Or: for a SEM_STAT operation,
218 the index value specified in semid referred to an array slot
219 that is currently unused.
220 EPERM The argument cmd has the value IPC_SET or IPC_RMID but the
222 effective user ID of the calling process is not the creator (as
223 found in sem_perm.cuid) or the owner (as found in sem_perm.uid)
224 of the semaphore set, and the process does not have the
225 CAP_SYS_ADMIN capability.
226 ERANGE The argument cmd has the value SETALL or SETVAL and the value to
228 which semval is to be set (for some semaphore of the set) is
229 less than 0 or greater than the implementation limit SEMVMX.
230
232 POSIX.1-2001, POSIX.1-2008, SVr4.
233 POSIX.1 specifies the sem_nsems field of the semid_ds structure as hav‐
235 ing the type unsigned short, and the field is so defined on most other
236 systems. It was also so defined on Linux 2.2 and earlier, but, since
237 Linux 2.4, the field has the type unsigned long.
238
240 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
241 or by any version of POSIX. However, some old implementations required
242 the inclusion of these header files, and the SVID also documented their
243 inclusion. Applications intended to be portable to such old systems
244 may need to include these header files.
245 The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(1)
247 program to provide information on allocated resources. In the future
248 these may modified or moved to a /proc filesystem interface.
249 Various fields in a struct semid_ds were typed as short under Linux 2.2
251 and have become long under Linux 2.4. To take advantage of this, a
252 recompilation under glibc-2.1.91 or later should suffice. (The kernel
253 distinguishes old and new calls by an IPC_64 flag in cmd.)
254 In some earlier versions of glibc, the semun union was defined in
256 <sys/sem.h>, but POSIX.1 requires that the caller define this union.
257 On versions of glibc where this union is not defined, the macro
258 _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
259 The following system limit on semaphore sets affects a semctl() call:
261 SEMVMX Maximum value for semval: implementation dependent (32767).
263 For greater portability, it is best to always call semctl() with four
265 arguments.
266 The sempid value
268 POSIX.1 defines sempid as the "process ID of [the] last operation" on a
269 semaphore, and explicitly notes that this value is set by a successful
270 semop(2) call, with the implication that no other interface affects the
271 sempid value.
272 While some implementations conform to the behavior specified in
274 POSIX.1, others do not. (The fault here probably lies with POSIX.1
275 inasmuch as it likely failed to capture the full range of existing
276 implementation behaviors.) Various other implementations also update
277 sempid for the other operations that update the value of a semaphore:
278 the SETVAL and SETALL operations, as well as the semaphore adjustments
279 performed on process termination as a consequence of the use of the
280 SEM_UNDO flag (see semop(2)).
281 Linux also updates sempid for SETVAL operations and semaphore adjust‐
283 ments. However, somewhat inconsistently, up to and including 4.5,
284 Linux did not update sempid for SETALL operations. This was rectified
285 in Linux 4.6.
286
288 ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7),
289 sysvipc(7)
290
292 This page is part of release 5.04 of the Linux man-pages project. A
293 description of the project, information about reporting bugs, and the
294 latest version of this page, can be found at
295 https://www.kernel.org/doc/man-pages/.
296Linux 2019-08-02 SEMCTL(2)