xref: /dragonfly/lib/libc/sys/semop.2 (revision 956939d5)
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2.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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26.\" $FreeBSD: src/lib/libc/sys/semop.2,v 1.7.2.6 2001/12/14 18:34:01 ru Exp $
27.\" $DragonFly: src/lib/libc/sys/semop.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
28.\"
29.Dd September 22, 1995
30.Dt SEMOP 2
31.Os
32.Sh NAME
33.Nm semop
34.Nd atomic array of operations on a semaphore set
35.Sh LIBRARY
36.Lb libc
37.Sh SYNOPSIS
38.In sys/types.h
39.In sys/ipc.h
40.In sys/sem.h
41.Ft int
42.Fn semop "int semid" "struct sembuf array[]" "unsigned nops"
43.Sh DESCRIPTION
44.Fn Semop
45atomically performs the array of operations indicated by
46.Fa array
47on the semaphore set indicated by
48.Fa semid .
49The length of
50.Fa array
51is indicated by
52.Fa nops .
53Each operation is encoded in a
54.Fa "struct sembuf" ,
55which is defined as follows:
56.Bd -literal
57.\"
58.\" From <sys/sem.h>
59.\"
60struct sembuf {
61        u_short sem_num;        /* semaphore # */
62        short   sem_op;         /* semaphore operation */
63        short   sem_flg;        /* operation flags */
64};
65.Ed
66.Pp
67For each element in
68.Fa array ,
69.Fa sem_op
70and
71.Fa sem_flg
72determine an operation to be performed on semaphore number
73.Fa sem_num
74in the set.  The values SEM_UNDO and IPC_NOWAIT may be
75.Em OR Ns 'ed
76into the
77.Fa sem_flg
78member in order to modify the behavior of the given operation.
79.Pp
80The operation performed depends as follows on the value of
81.Fa sem_op :
82.\"
83.\" This section is based on the description of semop() in
84.\" Stevens, _Advanced Programming in the UNIX Environment_.
85.\"
86.Bl -bullet
87.It
88When
89.Fa sem_op
90is positive, the semaphore's value is incremented by
91.Fa sem_op Ns 's
92value.  If SEM_UNDO is specified, the semaphore's adjust on exit
93value is decremented by
94.Fa sem_op Ns 's
95value.  A positive value for
96.Fa sem_op
97generally corresponds to a process releasing a resource
98associated with the semaphore.
99.It
100The behavior when
101.Fa sem_op
102is negative depends on the current value of the semaphore:
103.Bl -bullet
104.It
105If the current value of the semaphore is greater than or equal to
106the absolute value of
107.Fa sem_op ,
108then the value is decremented by the absolute value of
109.Fa sem_op .
110If SEM_UNDO is specified, the semaphore's adjust on exit
111value is incremented by the absolute value of
112.Fa sem_op .
113.It
114If the current value of the semaphore is less than
115.Fa sem_op Ns 's
116value, one of the following happens:
117.\" XXX a *second* sublist?
118.Bl -bullet
119.It
120If IPC_NOWAIT was specified, then
121.Fn semop
122returns immediately with a return value of
123.Er EAGAIN .
124.It
125If some other process has removed the semaphore with the IPC_RMID
126option of
127.Fn semctl ,
128then
129.Fn semop
130returns immediately with a return value of
131.Er EINVAL .
132.It
133Otherwise, the calling process is put to sleep until the semaphore's
134value is greater than or equal to the absolute value of
135.Fa sem_op .
136When this condition becomes true, the semaphore's value is decremented
137by the absolute value of
138.Fa sem_op ,
139and the semaphore's adjust on exit value is incremented by the
140absolute value of
141.Fa sem_op .
142.El
143.Pp
144A negative value for
145.Fa sem_op
146generally means that a process is waiting for a resource to become
147available.
148.El
149.Pp
150.It
151When
152.Fa sem_op
153is zero, the process waits for the semaphore's value to become zero.
154If it is already zero, the call to
155.Fn semop
156can return immediately.  Otherwise, the calling process is put to
157sleep until the semaphore's value becomes zero.
158.El
159.Pp
160For each semaphore a process has in use, the kernel maintains an
161`adjust on exit' value, as alluded to earlier.  When a process
162exits, either voluntarily or involuntarily, the adjust on exit value
163for each semaphore is added to the semaphore's value.  This can
164be used to insure that a resource is released if a process terminates
165unexpectedly.
166.Sh RETURN VALUES
167.Rv -std semop
168.Sh ERRORS
169.Fn Semop
170will fail if:
171.Bl -tag -width Er
172.It Bq Er EINVAL
173No semaphore set corresponds to
174.Fa semid .
175.It Bq Er EACCES
176Permission denied due to mismatch between operation and mode of
177semaphore set.
178.It Bq Er EAGAIN
179The semaphore's value was less than
180.Fa sem_op ,
181and IPC_NOWAIT was specified.
182.It Bq Er E2BIG
183Too many operations were specified.
184.It Bq Er EFBIG
185.\"
186.\" I'd have thought this would be EINVAL, but the source says
187.\" EFBIG.
188.\"
189.Fa sem_num
190was not in the range of valid semaphores for the set.
191.El
192.Sh SEE ALSO
193.Xr semctl 2 ,
194.Xr semget 2
195