1SEMOP(2) Linux Programmer's Manual SEMOP(2)
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6 semop, semtimedop - System V semaphore operations
7
9 #include <sys/types.h>
10 #include <sys/ipc.h>
11 #include <sys/sem.h>
12 int semop(int semid, struct sembuf *sops, unsigned nsops);
14 int semtimedop(int semid, struct sembuf *sops, unsigned nsops,
16 struct timespec *timeout);
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 semtimedop(): _GNU_SOURCE
21
23 Each semaphore in a System V semaphore set has the following associated
24 values:
25 unsigned short semval; /* semaphore value */
27 unsigned short semzcnt; /* # waiting for zero */
28 unsigned short semncnt; /* # waiting for increase */
29 pid_t sempid; /* ID of process that did last op */
30 semop() performs operations on selected semaphores in the set indicated
32 by semid. Each of the nsops elements in the array pointed to by sops
33 specifies an operation to be performed on a single semaphore. The ele‐
34 ments of this structure are of type struct sembuf, containing the fol‐
35 lowing members:
36 unsigned short sem_num; /* semaphore number */
38 short sem_op; /* semaphore operation */
39 short sem_flg; /* operation flags */
40 Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an opera‐
42 tion specifies SEM_UNDO, it will be automatically undone when the
43 process terminates.
44 The set of operations contained in sops is performed in array order,
46 and atomically, that is, the operations are performed either as a com‐
47 plete unit, or not at all. The behavior of the system call if not all
48 operations can be performed immediately depends on the presence of the
49 IPC_NOWAIT flag in the individual sem_flg fields, as noted below.
50 Each operation is performed on the sem_num-th semaphore of the sema‐
52 phore set, where the first semaphore of the set is numbered 0. There
53 are three types of operation, distinguished by the value of sem_op.
54 If sem_op is a positive integer, the operation adds this value to the
56 semaphore value (semval). Furthermore, if SEM_UNDO is specified for
57 this operation, the system subtracts the value sem_op from the sema‐
58 phore adjustment (semadj) value for this semaphore. This operation can
59 always proceed—it never forces a thread to wait. The calling process
60 must have alter permission on the semaphore set.
61 If sem_op is zero, the process must have read permission on the sema‐
63 phore set. This is a "wait-for-zero" operation: if semval is zero, the
64 operation can immediately proceed. Otherwise, if IPC_NOWAIT is speci‐
65 fied in sem_flg, semop() fails with errno set to EAGAIN (and none of
66 the operations in sops is performed). Otherwise semzcnt (the count of
67 threads waiting until this semaphore's value becomes zero) is incre‐
68 mented by one and the thread sleeps until one of the following occurs:
69 · semval becomes 0, at which time the value of semzcnt is decremented.
71 · The semaphore set is removed: semop() fails, with errno set to
73 EIDRM.
74 · The calling thread catches a signal: the value of semzcnt is decre‐
76 mented and semop() fails, with errno set to EINTR.
77 · The time limit specified by timeout in a semtimedop() call expires:
79 semop() fails, with errno set to EAGAIN.
80 If sem_op is less than zero, the process must have alter permission on
82 the semaphore set. If semval is greater than or equal to the absolute
83 value of sem_op, the operation can proceed immediately: the absolute
84 value of sem_op is subtracted from semval, and, if SEM_UNDO is speci‐
85 fied for this operation, the system adds the absolute value of sem_op
86 to the semaphore adjustment (semadj) value for this semaphore. If the
87 absolute value of sem_op is greater than semval, and IPC_NOWAIT is
88 specified in sem_flg, semop() fails, with errno set to EAGAIN (and none
89 of the operations in sops is performed). Otherwise semncnt (the
90 counter of threads waiting for this semaphore's value to increase) is
91 incremented by one and the thread sleeps until one of the following
92 occurs:
93 · semval becomes greater than or equal to the absolute value of
95 sem_op: the operation now proceeds, as described above.
96 · The semaphore set is removed from the system: semop() fails, with
98 errno set to EIDRM.
99 · The calling thread catches a signal: the value of semncnt is decre‐
101 mented and semop() fails, with errno set to EINTR.
102 · The time limit specified by timeout in a semtimedop() call expires:
104 the system call fails, with errno set to EAGAIN.
105 On successful completion, the sempid value for each semaphore specified
107 in the array pointed to by sops is set to the caller's process ID. In
108 addition, the sem_otime is set to the current time.
109 semtimedop() behaves identically to semop() except that in those cases
111 where the calling thread would sleep, the duration of that sleep is
112 limited by the amount of elapsed time specified by the timespec struc‐
113 ture whose address is passed in the timeout argument. (This sleep
114 interval will be rounded up to the system clock granularity, and kernel
115 scheduling delays mean that the interval may overrun by a small
116 amount.) If the specified time limit has been reached, semtimedop()
117 fails with errno set to EAGAIN (and none of the operations in sops is
118 performed). If the timeout argument is NULL, then semtimedop() behaves
119 exactly like semop().
120
122 If successful semop() and semtimedop() return 0; otherwise they return
123 -1 with errno indicating the error.
124
126 On failure, errno is set to one of the following:
127 E2BIG The argument nsops is greater than SEMOPM, the maximum number of
129 operations allowed per system call.
130 EACCES The calling process does not have the permissions required to
132 perform the specified semaphore operations, and does not have
133 the CAP_IPC_OWNER capability.
134 EAGAIN An operation could not proceed immediately and either IPC_NOWAIT
136 was specified in sem_flg or the time limit specified in timeout
137 expired.
138 EFAULT An address specified in either the sops or the timeout argument
140 isn't accessible.
141 EFBIG For some operation the value of sem_num is less than 0 or
143 greater than or equal to the number of semaphores in the set.
144 EIDRM The semaphore set was removed.
146 EINTR While blocked in this system call, the thread caught a signal;
148 see signal(7).
149 EINVAL The semaphore set doesn't exist, or semid is less than zero, or
151 nsops has a nonpositive value.
152 ENOMEM The sem_flg of some operation specified SEM_UNDO and the system
154 does not have enough memory to allocate the undo structure.
155 ERANGE For some operation sem_op+semval is greater than SEMVMX, the
157 implementation dependent maximum value for semval.
158
160 semtimedop() first appeared in Linux 2.5.52, and was subsequently back‐
161 ported into kernel 2.4.22. Glibc support for semtimedop() first
162 appeared in version 2.3.3.
163
165 SVr4, POSIX.1-2001.
166
168 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
169 or by any version of POSIX. However, some old implementations required
170 the inclusion of these header files, and the SVID also documented their
171 inclusion. Applications intended to be portable to such old systems
172 may need to include these header files.
173 The sem_undo structures of a process aren't inherited by the child pro‐
175 duced by fork(2), but they are inherited across an execve(2) system
176 call.
177 semop() is never automatically restarted after being interrupted by a
179 signal handler, regardless of the setting of the SA_RESTART flag when
180 establishing a signal handler.
181 A semaphore adjustment (semadj) value is a per-process, per-semaphore
183 integer that is the negated sum of all operations performed on a sema‐
184 phore specifying the SEM_UNDO flag. Each process has a list of semadj
185 values—one value for each semaphore on which it has operated using
186 SEM_UNDO. When a process terminates, each of its per-semaphore semadj
187 values is added to the corresponding semaphore, thus undoing the effect
188 of that process's operations on the semaphore (but see BUGS below).
189 When a semaphore's value is directly set using the SETVAL or SETALL
190 request to semctl(2), the corresponding semadj values in all processes
191 are cleared.
192 The semval, sempid, semzcnt, and semnct values for a semaphore can all
194 be retrieved using appropriate semctl(2) calls.
195 The following limits on semaphore set resources affect the semop()
197 call:
198 SEMOPM Maximum number of operations allowed for one semop() call (32)
200 (on Linux, this limit can be read and modified via the third
201 field of /proc/sys/kernel/sem).
202 SEMVMX Maximum allowable value for semval: implementation dependent
204 (32767).
205 The implementation has no intrinsic limits for the adjust on exit maxi‐
207 mum value (SEMAEM), the system wide maximum number of undo structures
208 (SEMMNU) and the per-process maximum number of undo entries system
209 parameters.
210
212 When a process terminates, its set of associated semadj structures is
213 used to undo the effect of all of the semaphore operations it performed
214 with the SEM_UNDO flag. This raises a difficulty: if one (or more) of
215 these semaphore adjustments would result in an attempt to decrease a
216 semaphore's value below zero, what should an implementation do? One
217 possible approach would be to block until all the semaphore adjustments
218 could be performed. This is however undesirable since it could force
219 process termination to block for arbitrarily long periods. Another
220 possibility is that such semaphore adjustments could be ignored alto‐
221 gether (somewhat analogously to failing when IPC_NOWAIT is specified
222 for a semaphore operation). Linux adopts a third approach: decreasing
223 the semaphore value as far as possible (i.e., to zero) and allowing
224 process termination to proceed immediately.
225 In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
227 prevents a thread that is waiting for a semaphore value to become zero
228 from being woken up when the value does actually become zero. This bug
229 is fixed in kernel 2.6.11.
230
232 The following code segment uses semop() to atomically wait for the
233 value of semaphore 0 to become zero, and then increment the semaphore
234 value by one.
235 struct sembuf sops[2];
237 int semid;
238 /* Code to set semid omitted */
240 sops[0].sem_num = 0; /* Operate on semaphore 0 */
242 sops[0].sem_op = 0; /* Wait for value to equal 0 */
243 sops[0].sem_flg = 0;
244 sops[1].sem_num = 0; /* Operate on semaphore 0 */
246 sops[1].sem_op = 1; /* Increment value by one */
247 sops[1].sem_flg = 0;
248 if (semop(semid, sops, 2) == -1) {
250 perror("semop");
251 exit(EXIT_FAILURE);
252 }
253
255 clone(2), semctl(2), semget(2), sigaction(2), capabilities(7),
256 sem_overview(7), svipc(7), time(7)
257
259 This page is part of release 3.53 of the Linux man-pages project. A
260 description of the project, and information about reporting bugs, can
261 be found at http://www.kernel.org/doc/man-pages/.
262Linux 2013-04-17 SEMOP(2)