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Linux Processes: Structure, Hangs and Core Dumps

Efficient and effective resolution practices

2.  Acquire the current process ID.

$ echo $$ # note that the current PID = the shell

3.  From the same shell window, start the gzip process on the named pipe.

$ gzip < /tmp/named_pipe > /tmp/pipe.out.gz

4.  Find the process with a parent of 5032.

$ ps -emo pid,ppid,state,comm,time,pri,size,wchan |grep 5032
5236 5032 S bash 00:00:00 30 1040 pipe_wait
Notice that the command name is bash, and it is in the sleep state, sleeping on wait channel pipe_wait. Yet gzip
was the command executed.

5.  In another shell window, start a trace on the parent before executing the gzip command.

$ strace -o /tmp/pipe.strace -f -F -r -T -v -p 5032
Process 5032 attached - interrupt to quit ........Parent shell \
Process 5236 attached ....................................The gzip \
process being forked
Process 5032 suspended

As mentioned earlier, fork() essentially creates a process structure by copying the parent. Until execve() executes the binary, the new executable is not loaded into memory, so ps -ef |grep gzip does not show the process. In this case, the gzip process waits for something to be sent to the pipe before executing gzip.

6.  A review of the trace explains why the ps -ef | grep gzip command does not show the process.

PID Time call()
5032 0.000079 fork() = 5236 <0.000252>.........."GZIP was
executed at command line"
5032 0.000678 setpgid(5236, 5236) = 0 <0.000008>
5032 0.000130 rt_sigprocmask(SIG_SETMASK, [RTMIN], NULL, 8) = 0 <0.000007>
5032 0.000074 waitpid(-1, <unfinished ...>..........."man
waitpid: -1 means wait on child"
5236 0.000322 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
5236 0.000078 getpid() = 5236 <0.000006>
5236 0.000050 rt_sigprocmask(SIG_SETMASK, [RTMIN], NULL, 8) = 0 <0.000007>
5236 0.000067 rt_sigaction(SIGTSTP, {SIG_DFL}, {SIG_IGN}, 8) = 0 <0.000009>
5236 0.000060 rt_sigaction(SIGTTIN, {SIG_DFL}, {SIG_IGN}, 8) = 0 <0.000007>
5236 0.000057 rt_sigaction(SIGTTOU, {SIG_DFL}, {SIG_IGN}, 8) = 0 <0.000007>
5236 0.000055 setpgid(5236, 5236) = 0 <0.000008>
5236 0.000044 rt_sigprocmask(SIG_BLOCK, [CHLD TSTP TTIN TTOU], [RTMIN], 8) = 0 <0.000007>
5236 0.000071 ioctl(255, TIOCSPGRP, [5236]) = 0 <0.000058>
5236 0.000102 rt_sigprocmask(SIG_SETMASK, [RTMIN], NULL, 8) = 0 <0.000007>
5236 0.000060 rt_sigaction(SIGINT, {SIG_DFL}, {0x8087030, [], SA_RESTORER, 0x4005aca8}, 8) = 0 <0.000007>
5236 0.000075 rt_sigaction(SIGQUIT, {SIG_DFL}, {SIG_IGN}, 8) = 0 <0.000007>
5236 0.000057 rt_sigaction(SIGTERM, {SIG_DFL}, {SIG_IGN}, 8) = 0 <0.000007>
5236 0.000058 rt_sigaction(SIGCHLD, {SIG_DFL}, {0x80776a0, [], SA_RESTORER, 0x4005aca8}, 8) = 0 <0.000007>
5236 0.000262 open("/tmp/named_pipe", O_RDONLY|O_LARGEFILE) = 3 <141.798572>
5236 141.798719 dup2(3, 0) = 0 <0.000008>
5236 0.000051 close(3) = 0 <0.000008>
5236 0.000167 open("/tmp/pipe.out.gz", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3 <0.000329>
5236 0.000394 dup2(3, 1) = 1 <0.000007>
5236 0.000042 close(3) = 0 <0.000008>
5236 0.000127 execve("/usr//bin/gzip", ["gzip"]

So 141.79 seconds after opening, the named pipe data was received, evecve() executed gzip, and the data was compressed and redirected to the file /tmp/pipe.out.gz. Only at this point would the gzip process show up in the ps listing. So what was initially thought to be a hung process is simply a sleeping process waiting on data.

7.  Now ps -ef | grep gzip works.

$ ps -ef | grep 5236
chris 5236 5032 0 17:01 pts/4 00:00:00 gzip

Process Cores
Now that we have sufficiently covered structure and hangs as they pertain to Linux processes, let us move on to process core dumps. A core dump enables the user to visually inspect a process's last steps. This section details how cores are created and how to best use them.

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James Kirkland is the advocate for Red Hat's initiatives and solutions for the Internet of Things(IoT) and is the architect of its three-tier strategy for IoT deployments. For the past five years, James has been focused on IoT solutions for the transportation and energy sectors. A frequent public speaker and writer on a wide range of technical topics, James is also the co-author of Linux Troubleshooting for System Administrators and Power Users (ISBN: 0131855158) published by Prentice Hall PTR. He has been working with UNIX and Linux variants over the course of 20 years in his positions at Red Hat, and in previous roles at Racemi and Hewlett-Packard.

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Most Recent Comments
Linux News Desk 07/13/06 04:59:25 PM EDT

Troubleshooting a Linux process follows the same general methodology as that used with traditional UNIX systems. In both systems, for process hangs, we identify the system resources being used by the process and attempt to identify the cause for the process to stop responding. With application core dumps, we must identify the signal for which the process terminated and proceed with acquiring a stack trace to identify system calls made by the process at the time it died. There exists neither a 'golden' troubleshooting path nor a set of instructions that can be applied for all cases. Some conditions are much easier to solve than others, but with a good understanding of the fundamentals, a solution is not far from reach.

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