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POData in linux FIFO seems lost
 

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    <p>I have a bash script which wants to do some work in parallel, I did this by putting each job in an subshell which is run in the background. While the number of jobs running simultaneously should under some limit, I achieve this by first put some lines in a FIFO, then just before forking the subshell, the parent script is required to read a line from this FIFO. Only after it gets a line can it fork the subshell. Up to now, everything works fine. But when I tried to read a line from the FIFO in the subshell, it seems that only one subshell can get a line, even if there are apparently more lines in the FIFO. So I wonder why other subshell(s) cannot read a line even when there are more lines in the FIFO. <br /> My testing code looks something like this: <br /></p> <pre><code> #!/bin/sh fifo_path="/tmp/fy_u_test2.fifo" mkfifo $fifo_path #open fifo for r/w at fd 6 exec 6&lt;&gt; $fifo_path process_num=5 #put $process_num lines in the FIFO for ((i=0; i&lt;${process_num}; i++)); do echo "$i" done >&6 delay_some(){ local index="$1" echo "This is what u can see. $index \n" sleep 20; } #In each iteration, try to read 2 lines from FIFO, one from this shell, #the other from the subshell for i in 1 2 do date >>/tmp/fy_date #If a line can be read from FIFO, run a subshell in bk, otherwise, block. read -u6 echo " $$ Read --- $REPLY --- from 6 \n" >> /tmp/fy_date { delay_some $i #Try to read a line from FIFO, __ only one subshell succeeds the following line. __ read -u6 echo " $$ This is in child # $i, read --- $REPLY --- from 6 \n" >> /tmp/fy_date } & done </code></pre> <p><br /> And the output file /tmp/fy_date has content of: <br /></p> <pre><code> Mon Apr 26 16:02:18 CST 2010 32561 Read --- 0 --- from 6 \n Mon Apr 26 16:02:18 CST 2010 32561 Read --- 1 --- from 6 \n 32561 This is in child # 1, read --- 2 --- from 6 \n </code></pre> <p>There, I expect a line like this:</p> <pre><code> 32561 This is in child # 2, read --- 3 --- from 6 \n </code></pre> <p>But it never appears, and the child #2 process is blocked there until I issue:<br /> echo something > /tmp/fy_u_test2.fifo</p>
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