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89 lines
2.2 KiB
Groff
89 lines
2.2 KiB
Groff
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.\"
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.\" $Id: semaphore.2,v 1.1 1997/02/27 07:32:15 gdr Exp $
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.\"
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.TH SEMAPHORE 2 "16 January 1997" GNO "System Calls"
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.SH NAME
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.BR scount ,
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.BR screate ,
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.BR sdelete ,
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.BR ssignal ,
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.BR swait
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\- semaphore operations
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.SH SYNOPSIS
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#include <gno/gno.h>
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.sp 1
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int \fBscount\fR (int \fIsem\fR);
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.br
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int \fBscreate\fR (int \fIcount\fR);
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.br
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int \fBsdelete\fR (int \fIsem\fR);
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.br
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int \fBssignal\fR (int \fIsem\fR);
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.br
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int \fBswait\fR (int \fIsem\fR);
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.SH DESCRIPTION
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.BR screate
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is used to allocate a semaphore from the kernel semapore manager.
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Semaphores are the most basic form of interprocess communication (IPC),
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and these routines provide the power necessary to solve a large number
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of synchronization and communication problems. (See an Operating Systems
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text).
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.LP
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The initial
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.IR count
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determines how many times
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.BR swait
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can be called before processes are blocked.
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.IR count
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must non-negative, and is usually set to 1.
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.BR screate
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returns a semaphore ID number as an integer. This ID must be used in
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all the other semaphore calls.
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.LP
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.BR sdelete
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releases the specified semaphore, and returns to a ready state all
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processes that were blocked on that semaphore.
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.LP
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.BR swait
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decrements the value of the semaphore (initially specified by
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.IR count )
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by one. If the semaphore count is less than zero, the process is blocked
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and queued for release by
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.BR ssignal .
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This is what is traditionally referred to as a semaphore-down operation.
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.LP
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.BR ssignal
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increments the semaphore count by one. If the semaphore count is less
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than zero,
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.BR ssignal
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unblocks a process blocked on the semaphore.
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The selection of the process to be unblocked is arbitrary; FIFO operation
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is not guaranteed.
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.LP
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.BR scount
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retuns the current value of the semaphore referred to by
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.IR sem .
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Note that depending on this value for synchronization can lead to race
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conditions.
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.SH "RETURN VALUE"
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On success,
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.BR screate
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returns the semaphore identifier,
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.BR scount
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returns the semaphore value, and
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.BR sdelete ,
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.BR ssignal ,
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and
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.BR swait
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return zero.
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All functions return -1 and set
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.BR errno
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on failure.
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.SH BUGS
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There is currently no mechanism for deallocating semaphores that are
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orphaned by abnormal process termination.
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.SH HISTORY
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These semaphore routines were designed for XINU, written by Douglas Comer.
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