2 .\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
4 .\" All rights reserved.
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28 .Dd September 22, 1995
33 .Nd atomic array of operations on a semaphore set
35 .Fd #include <sys/types.h>
36 .Fd #include <sys/ipc.h>
37 .Fd #include <sys/sem.h>
39 .Fn semop "int semid" "struct sembuf array[]" "unsigned nops"
42 atomically performs the array of operations indicated by
44 on the semaphore set indicated by
50 Each operation is encoded in a
52 which is defined as follows:
58 u_short sem_num; /* semaphore # */
59 short sem_op; /* semaphore operation */
60 short sem_flg; /* operation flags */
69 determine an operation to be performed on semaphore number
71 in the set. The values SEM_UNDO and IPC_NOWAIT may be
75 member in order to modify the behavior of the given operation.
77 The operation performed depends as follows on the value of
80 .\" This section is based on the description of semop() in
81 .\" Stevens, _Advanced Programming in the UNIX Environment_.
87 is positive, the semaphore's value is incremented by
89 value. If SEM_UNDO is specified, the semaphore's adjust on exit
90 value is decremented by
92 value. A positive value for
94 generally corresponds to a process releasing a resource
95 associated with the semaphore.
99 is negative depends on the current value of the semaphore:
102 If the current value of the semaphore is greater than or equal to
103 the absolute value of
105 then the value is decremented by the absolute value of
107 If SEM_UNDO is specified, the semaphore's adjust on exit
108 value is incremented by the absolute value of
111 If the current value of the semaphore is less than
113 value, one of the following happens:
114 .\" XXX a *second* sublist?
117 If IPC_NOWAIT was specified, then
119 returns immediately with a return value of EAGAIN.
121 If some other process has removed the semaphore with the IPC_RMID
126 returns immediately with a return value of EINVAL.
128 Otherwise, the calling process is put to sleep until the semaphore's
129 value is greater than or equal to the absolute value of
131 When this condition becomes true, the semaphore's value is decremented
132 by the absolute value of
134 and the semaphore's adjust on exit value is incremented by the
141 generally means that a process is waiting for a resource to become
148 is zero, the process waits for the semaphore's value to become zero.
149 If it is already zero, the call to
151 can return immediately. Otherwise, the calling process is put to
152 sleep until the semaphore's value becomes zero.
155 For each semaphore a process has in use, the kernel maintains an
156 `adjust on exit' value, as alluded to earlier. When a process
157 exits, either voluntarily or involuntarily, the adjust on exit value
158 for each semaphore is added to the semaphore's value. This can
159 be used to insure that a resource is released if a process terminates
164 returns 0; otherwise, -1 is returned and
166 is set to indicate the error.
172 No semaphore set corresponds to
175 Permission denied due to mismatch between operation and mode of
178 The semaphore's value was less than
180 and IPC_NOWAIT was specified.
182 Too many operations were specified.
185 .\" I'd have thought this would be EINVAL, but the source says
189 was not in the range of valid semaphores for the set.