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Top Forums UNIX for Beginners Questions & Answers UNIX Pipe -Exit when there are no bytes to read Post 303015515 by mr_manii on Friday 6th of April 2018 06:40:45 AM
Old 04-06-2018
Display UNIX Pipe -Exit when there are no bytes to read

Hi All,

I'm creating a program which reads millions of bytes from the PIPE and do some processing. As the data is more, the idea is to read the pipe parallely.

Sun Solaris 8
See the code below:
Code:
#!/bin/sh
MAXTHREAD=30
awk '{print $1}' metadata.csv > nvpipe &
while [ $c -le $MAXTHREAD ]
do
   ${BIN}/parallel_wot.sh &
   PID=$!
   sleep 3
   c=`expr $c + 1`
STRING=$STRING","$PID
done

parallel_wot.sh
Code:
cd $PIPEDIR/
while IFS=',' read DIR1 DIR2
do
 echo starting at `date`
  ${COMPUSETBIN}/prg1.sh prg2.sh $DIR1 $DIR2
  echo ending at `date`
  echo ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
done < nvpipe

Now the problem is, as the pipe is read in parallel, once the pipe is emptied all other processes are waiting to read from the pipe except 1. The questions I now have is , how to exit from reading the pipe if there are no records.

Last edited by Scrutinizer; 04-06-2018 at 08:11 AM.. Reason: Extra code tags
 

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PIPE(2) 							System Calls Manual							   PIPE(2)

NAME
pipe - create an interprocess channel SYNOPSIS
#include <u.h> #include <libc.h> int pipe(int fd[2]) DESCRIPTION
Pipe creates a buffered channel for interprocess I/O communication. Two file descriptors are returned in fd. Data written to fd[1] is available for reading from fd[0] and data written to fd[0] is available for reading from fd[1]. After the pipe has been established, cooperating processes created by subsequent fork(2) calls may pass data through the pipe with read and write calls. The bytes placed on a pipe by one write are contiguous even if many processes are writing. Write boundaries are preserved: each read terminates when the read buffer is full or after reading the last byte of a write, whichever comes first. The number of bytes available to a read(2) is reported in the Length field returned by fstat or dirfstat on a pipe (see stat(2)). When all the data has been read from a pipe and the writer has closed the pipe or exited, read(2) will return 0 bytes. Writes to a pipe with no reader will generate a note sys: write on closed pipe. SOURCE
/sys/src/libc/9syscall SEE ALSO
intro(2), read(2), pipe(3) DIAGNOSTICS
Sets errstr. BUGS
If a read or a write of a pipe is interrupted, some unknown number of bytes may have been transferred. When a read from a pipe returns 0 bytes, it usually means end of file but is indistinguishable from reading the result of an explicit write of zero bytes. PIPE(2)
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