48 lines
2.5 KiB
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48 lines
2.5 KiB
HTML
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<TITLE>6.2.4 Atomic Operations with Pipes</TITLE>
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<A NAME="tex2html490" HREF="node14.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html488" HREF="node9.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html482" HREF="node12.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html492" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME="tex2html491" HREF="node14.html">6.2.5 Notes on half-duplex </A>
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<B>Up:</B> <A NAME="tex2html489" HREF="node9.html">6.2 Half-duplex UNIX Pipes</A>
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<B> Previous:</B> <A NAME="tex2html483" HREF="node12.html">6.2.3 Pipes the Easy </A>
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<BR> <P>
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<H2><A NAME="SECTION00724000000000000000">6.2.4 Atomic Operations with Pipes</A></H2>
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<P>
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In order for an operation to be considered ``atomic'', it must not be interrupted
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for any reason at all. The entire operation occurs at once. The POSIX
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standard dictates in /usr/include/posix1_lim.h that the maximum buffer size
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for an atomic operation on a pipe is:
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<P>
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<PRE> #define _POSIX_PIPE_BUF 512</PRE>
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<P>
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Up to 512 bytes can be written or retrieved from a pipe atomically. Anything
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that crosses this threshold will be split, and not atomic. Under Linux,
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however, the atomic operational limit is defined in ``linux/limits.h'' as:
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<P>
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<PRE> #define PIPE_BUF 4096</PRE>
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<P>
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As you can see, Linux accommodates the minimum number of bytes required by
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POSIX, quite considerably I might add. The atomicity of a pipe operation
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becomes important when more than one process is involved (FIFOS). For
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example, if the number of bytes written to a pipe exceeds the atomic limit
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for a single operation, and multiple processes are writing to the pipe,
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the data will be ``interleaved'' or ``chunked''. In other words, one process
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may insert data into the pipeline between the writes of another.
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<P>
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<P><ADDRESS>
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<I>Converted on: <BR>
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Fri Mar 29 14:43:04 EST 1996</I>
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