<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --> <HTML> <HEAD> <TITLE>6.2.4 Atomic Operations with Pipes</TITLE> <META NAME="description" CONTENT="6.2.4 Atomic Operations with Pipes"> <META NAME="keywords" CONTENT="lpg"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <LINK REL=STYLESHEET HREF="lpg.css"> </HEAD> <BODY LANG="EN"> <A NAME="tex2html490" HREF="node14.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="/icons//next_motif.gif"></A> <A NAME="tex2html488" HREF="node9.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="/icons//up_motif.gif"></A> <A NAME="tex2html482" HREF="node12.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="/icons//previous_motif.gif"></A> <A NAME="tex2html492" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="/icons//contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html491" HREF="node14.html">6.2.5 Notes on half-duplex </A> <B>Up:</B> <A NAME="tex2html489" HREF="node9.html">6.2 Half-duplex UNIX Pipes</A> <B> Previous:</B> <A NAME="tex2html483" HREF="node12.html">6.2.3 Pipes the Easy </A> <BR> <P> <H2><A NAME="SECTION00724000000000000000">6.2.4 Atomic Operations with Pipes</A></H2> <P> In order for an operation to be considered ``atomic'', it must not be interrupted for any reason at all. The entire operation occurs at once. The POSIX standard dictates in /usr/include/posix1_lim.h that the maximum buffer size for an atomic operation on a pipe is: <P> <PRE> #define _POSIX_PIPE_BUF 512</PRE> <P> Up to 512 bytes can be written or retrieved from a pipe atomically. Anything that crosses this threshold will be split, and not atomic. Under Linux, however, the atomic operational limit is defined in ``linux/limits.h'' as: <P> <PRE> #define PIPE_BUF 4096</PRE> <P> As you can see, Linux accommodates the minimum number of bytes required by POSIX, quite considerably I might add. The atomicity of a pipe operation becomes important when more than one process is involved (FIFOS). For example, if the number of bytes written to a pipe exceeds the atomic limit for a single operation, and multiple processes are writing to the pipe, the data will be ``interleaved'' or ``chunked''. In other words, one process may insert data into the pipeline between the writes of another. <P> <BR> <HR> <P><ADDRESS> <I>Converted on: <BR> Fri Mar 29 14:43:04 EST 1996</I> </ADDRESS> </BODY> </HTML>
