1SEMOP(2) Linux Programmer's Manual SEMOP(2)
2
6 semop, semtimedop - 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 semaphore set has the following associated values:
24 unsigned short semval; /* semaphore value */
26 unsigned short semzcnt; /* # waiting for zero */
27 unsigned short semncnt; /* # waiting for increase */
28 pid_t sempid; /* process that did last op */
29 semop() performs operations on selected semaphores in the set indicated
31 by semid. Each of the nsops elements in the array pointed to by sops
32 specifies an operation to be performed on a single semaphore. The ele‐
33 ments of this structure are of type struct sembuf, containing the fol‐
34 lowing members:
35 unsigned short sem_num; /* semaphore number */
37 short sem_op; /* semaphore operation */
38 short sem_flg; /* operation flags */
39 Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an opera‐
41 tion specifies SEM_UNDO, it will be automatically undone when the
42 process terminates.
43 The set of operations contained in sops is performed in array order,
45 and atomically, that is, the operations are performed either as a com‐
46 plete unit, or not at all. The behavior of the system call if not all
47 operations can be performed immediately depends on the presence of the
48 IPC_NOWAIT flag in the individual sem_flg fields, as noted below.
49 Each operation is performed on the sem_num-th semaphore of the sema‐
51 phore set, where the first semaphore of the set is numbered 0. There
52 are three types of operation, distinguished by the value of sem_op.
53 If sem_op is a positive integer, the operation adds this value to the
55 semaphore value (semval). Furthermore, if SEM_UNDO is specified for
56 this operation, the system updates the process undo count (semadj) for
57 this semaphore. This operation can always proceed — it never forces a
58 process to wait. The calling process must have alter permission on the
59 semaphore set.
60 If sem_op is zero, the process must have read permission on the sema‐
62 phore set. This is a "wait-for-zero" operation: if semval is zero, the
63 operation can immediately proceed. Otherwise, if IPC_NOWAIT is speci‐
64 fied in sem_flg, semop() fails with errno set to EAGAIN (and none of
65 the operations in sops is performed). Otherwise semzcnt (the count of
66 processes waiting until this semaphore's value becomes zero) is incre‐
67 mented by one and the process sleeps until one of the following occurs:
68 · semval becomes 0, at which time the value of semzcnt is decremented.
70 · The semaphore set is removed: semop() fails, with errno set to
72 EIDRM.
73 · The calling process catches a signal: the value of semzcnt is decre‐
75 mented and semop() fails, with errno set to EINTR.
76 · The time limit specified by timeout in a semtimedop() call expires:
78 semop() fails, with errno set to EAGAIN.
79 If sem_op is less than zero, the process must have alter permission on
81 the semaphore set. If semval is greater than or equal to the absolute
82 value of sem_op, the operation can proceed immediately: the absolute
83 value of sem_op is subtracted from semval, and, if SEM_UNDO is speci‐
84 fied for this operation, the system updates the process undo count
85 (semadj) for this semaphore. If the absolute value of sem_op is
86 greater than semval, and IPC_NOWAIT is specified in sem_flg, semop()
87 fails, with errno set to EAGAIN (and none of the operations in sops is
88 performed). Otherwise semncnt (the counter of processes waiting for
89 this semaphore's value to increase) is incremented by one and the
90 process sleeps until one of the following occurs:
91 · semval becomes greater than or equal to the absolute value of
93 sem_op, at which time the value of semncnt is decremented, the abso‐
94 lute value of sem_op is subtracted from semval and, if SEM_UNDO is
95 specified for this operation, the system updates the process undo
96 count (semadj) for this semaphore.
97 · The semaphore set is removed from the system: semop() fails, with
99 errno set to EIDRM.
100 · The calling process catches a signal: the value of semncnt is decre‐
102 mented and semop() fails, with errno set to EINTR.
103 · The time limit specified by timeout in a semtimedop() call expires:
105 the system call fails, with errno set to EAGAIN.
106 On successful completion, the sempid value for each semaphore specified
108 in the array pointed to by sops is set to the process ID of the calling
109 process. In addition, the sem_otime is set to the current time.
110 semtimedop() behaves identically to semop() except that in those cases
112 were the calling process would sleep, the duration of that sleep is
113 limited by the amount of elapsed time specified by the timespec struc‐
114 ture whose address is passed in the timeout argument. If the specified
115 time limit has been reached, semtimedop() fails with errno set to
116 EAGAIN (and none of the operations in sops is performed). If the time‐
117 out argument is NULL, then semtimedop() behaves exactly like semop().
118
120 If successful semop() and semtimedop() return 0; otherwise they return
121 -1 with errno indicating the error.
122
124 On failure, errno is set to one of the following:
125 E2BIG The argument nsops is greater than SEMOPM, the maximum number of
127 operations allowed per system call.
128 EACCES The calling process does not have the permissions required to
130 perform the specified semaphore operations, and does not have
131 the CAP_IPC_OWNER capability.
132 EAGAIN An operation could not proceed immediately and either IPC_NOWAIT
134 was specified in sem_flg or the time limit specified in timeout
135 expired.
136 EFAULT An address specified in either the sops or the timeout argument
138 isn't accessible.
139 EFBIG For some operation the value of sem_num is less than 0 or
141 greater than or equal to the number of semaphores in the set.
142 EIDRM The semaphore set was removed.
144 EINTR While blocked in this system call, the process caught a signal;
146 see signal(7).
147 EINVAL The semaphore set doesn't exist, or semid is less than zero, or
149 nsops has a nonpositive value.
150 ENOMEM The sem_flg of some operation specified SEM_UNDO and the system
152 does not have enough memory to allocate the undo structure.
153 ERANGE For some operation sem_op+semval is greater than SEMVMX, the
155 implementation dependent maximum value for semval.
156
158 semtimedop() first appeared in Linux 2.5.52, and was subsequently back‐
159 ported into kernel 2.4.22. Glibc support for semtimedop() first
160 appeared in version 2.3.3.
161
163 SVr4, POSIX.1-2001.
164
166 The sem_undo structures of a process aren't inherited by the child pro‐
167 duced by fork(2), but they are inherited across an execve(2) system
168 call.
169 semop() is never automatically restarted after being interrupted by a
171 signal handler, regardless of the setting of the SA_RESTART flag when
172 establishing a signal handler.
173 semadj is a per-process integer which is simply the (negative) count of
175 all semaphore operations performed specifying the SEM_UNDO flag. When
176 a semaphore's value is directly set using the SETVAL or SETALL request
177 to semctl(2), the corresponding semadj values in all processes are
178 cleared.
179 The semval, sempid, semzcnt, and semnct values for a semaphore can all
181 be retrieved using appropriate semctl(2) calls.
182 The following limits on semaphore set resources affect the semop()
184 call:
185 SEMOPM Maximum number of operations allowed for one semop() call (32)
187 (on Linux, this limit can be read and modified via the third
188 field of /proc/sys/kernel/sem).
189 SEMVMX Maximum allowable value for semval: implementation dependent
191 (32767).
192 The implementation has no intrinsic limits for the adjust on exit maxi‐
194 mum value (SEMAEM), the system wide maximum number of undo structures
195 (SEMMNU) and the per-process maximum number of undo entries system
196 parameters.
197
199 When a process terminates, its set of associated semadj structures is
200 used to undo the effect of all of the semaphore operations it performed
201 with the SEM_UNDO flag. This raises a difficulty: if one (or more) of
202 these semaphore adjustments would result in an attempt to decrease a
203 semaphore's value below zero, what should an implementation do? One
204 possible approach would be to block until all the semaphore adjustments
205 could be performed. This is however undesirable since it could force
206 process termination to block for arbitrarily long periods. Another
207 possibility is that such semaphore adjustments could be ignored alto‐
208 gether (somewhat analogously to failing when IPC_NOWAIT is specified
209 for a semaphore operation). Linux adopts a third approach: decreasing
210 the semaphore value as far as possible (i.e., to zero) and allowing
211 process termination to proceed immediately.
212 In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
214 prevents a process that is waiting for a semaphore value to become zero
215 from being woken up when the value does actually become zero. This bug
216 is fixed in kernel 2.6.11.
217
219 The following code segment uses semop() to atomically wait for the
220 value of semaphore 0 to become zero, and then increment the semaphore
221 value by one.
222 struct sembuf sops[2];
224 int semid;
225 /* Code to set semid omitted */
227 sops[0].sem_num = 0; /* Operate on semaphore 0 */
229 sops[0].sem_op = 0; /* Wait for value to equal 0 */
230 sops[0].sem_flg = 0;
231 sops[1].sem_num = 0; /* Operate on semaphore 0 */
233 sops[1].sem_op = 1; /* Increment value by one */
234 sops[1].sem_flg = 0;
235 if (semop(semid, sops, 2) == -1) {
237 perror("semop");
238 exit(EXIT_FAILURE);
239 }
240
242 semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7),
243 svipc(7), time(7)
244
246 This page is part of release 3.25 of the Linux man-pages project. A
247 description of the project, and information about reporting bugs, can
248 be found at http://www.kernel.org/doc/man-pages/.
249Linux 2008-10-04 SEMOP(2)