imported>ThorstenStaerk |
imported>ThorstenStaerk |
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| − | <metadesc>How to strace a process. How to understand strace's output. How to follow sub-processes, how to format the string length so you get a sensible meaning, how to find the file descriptors and how to see where the process spends most of its time.</metadesc>
| + | Barcode Scanners are well supported under [[Linux]]. They typically have a [[USB]] connector and register as [[keyboard]] [[device]]s, sending the number they scan as if they were keyboard input. They do not require an additional driver, they just work as a second keyboard. |
| − | strace is a [[command]] that shows all requests from a running process to the kernel (aka syscalls). This helps you to do excellent troubleshooting and gain good performance tuning insights.
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| − | <pic border=1 src="http://www.linuxintro.org/strace-mindmap.jpg" width=50% align=right caption="strace mindmap" link="http://www.linuxintro.org/strace-mindmap.jpg" />
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| − | = starting strace =
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| − | You can start a new process with strace or you can attach strace to an already running process. To start a new process just put strace in front of its command:
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| − | strace <abbr title="print the first 99 characters of every output string">-s 99</abbr> <abbr title="follow child processes">-ff</abbr> <abbr title="command that you want to execute and trace">ls</abbr>
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| − | To attach strace to an already running process, in this example firefox, find out the process' id:
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| − | # ps -A | grep firefox
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| − | 2728 pts/1 00:00:02 firefox
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| − | Then call strace with the id as parameter:
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| − | strace <abbr title="print the first 99 characters of every output string">-s 99</abbr> <abbr title="follow child processes">-ff</abbr><abbr title="process id">p</abbr> 2728
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| − | | |
| − | = Understanding strace's output =
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| − | The bad news is that strace's output looks tedious like this:
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| − | <abbr title="Syscall. Find out more with the command man 2 clock_gettime.">clock_gettime</abbr>(CLOCK_MONOTONIC, {4433, 764347636}) = <abbr title="return code. 0 means there was no error.">0</abbr>
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| − | <abbr title="Syscall. Find out more with the command man 2 poll.">poll</abbr>([{fd=12, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 1 ([{fd=12, revents=POLLIN|POLLRDNORM}])
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| − | <abbr title="Syscall. Find out more with the command man 2 recv.">recv</abbr>(<abbr title="As we can tell from man 2 recv, the following is a file descriptor. Find out more with the command cat /proc/PID/fd/12">12</abbr>, "\37\36&\364T<\264\6Og\20\2\30%-\31C\3757\350{\260\0203\351\23&O\365(y\212"..., 16384, 0) = 1448
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| − | The good news is that every line follows the same scheme:
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| − | syscall(parameters) = return value
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| − | Whatever you see for syscall you can get help on using the man(2) pages. So for the above example
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| − | man 2 clock_gettime
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| − | man 2 poll
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| − | man 2 recv
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| − | Will give you the information what is going on. For example, clock_gettime retrieves the current time.
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| − | Wow, that was easy. Now to the next line, poll. According to [http://linux.die.net/man/2/poll man 2 poll] it waits for an event on a file descriptor, like "data to be read". The file descriptor is the first argument, 12. Fortunately you can look up the file descriptor's meaning if you know the process id. In this case the process id was 7179:
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| − | /proc/7179/fd # ll
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| − | total 0
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| − | lr-x------ 1 root root 64 Apr 19 08:02 0 -> /dev/null
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| − | lrwx------ 1 root root 64 Apr 19 08:02 1 -> socket:[156171]
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| − | l-wx------ 1 root root 64 Apr 19 08:02 2 -> /var/log/apache2/error_log
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| − | lr-x------ 1 root root 64 Apr 19 08:02 4 -> pipe:[156208]
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| − | l-wx------ 1 root root 64 Apr 19 08:02 5 -> pipe:[156208]
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| − | l-wx------ 1 root root 64 Apr 19 08:02 6 -> /var/log/apache2/access_log
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| − | lrwx------ 1 root root 64 Apr 19 08:02 7 -> anon_inode:[eventpoll]
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| − | lrwx------ 1 root root 64 Apr 19 08:02 12 -> socket:[155097]
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| − | Ok, to find out what socket:[155097] is we use the command
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| − | # lsof | grep 155097
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| − | httpd2-pr 7158 root 3u IPv4 155097 0t0 TCP *:http (LISTEN)
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| − | httpd2-pr 7179 wwwrun 3u IPv4 155097 0t0 TCP *:http (LISTEN)
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| − | So this is a socket listening to the http port.
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| − | = limitations =
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| − | strace will show you all syscalls from a process, and that's it. This also mean that if your process hangs in an endless loop that does not do any syscall, strace will not show anything.
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| − | | |
| − | = interprocess communication =
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| − | Processes will typically communicate:
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| − | * via network
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| − | * via sockets
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| − | * via pipes
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| − | * via shared memory
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| − | * by writing into files
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| − | For example when using strace to [[troubleshoot sound]] you may see that speaker-test receives messages from file descriptor 7:
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| − | # strace -s 99 -ffp 32681
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| − | [pid 32682] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\0}U\0\0\0\0\0\22\10\277U\0\0\0\0\0\0\0\0001TU b|\0\17(\347TU b|\0\0175\342r\0\0\0\0\0\20\0\0r\0\0\0\0\0\2\260\220R\0\0\0\0\0\0\0\0R\0\0\0\0\0\2\260\220", 83}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 83
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| − | Now let's find out what file descriptor 7 of process 32681 is:
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| − | # ll /proc/32681/fd/7
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| − | lrwx------ 1 root root 64 Apr 4 18:17 /proc/32681/fd/7 -> socket:[198064]
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| − | So it is unix socket 198064. Let's find out what process it is:
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| − | # ss -p | grep 198064
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| − | u_str ESTAB 0 0 * 198064 * 198065 users:(("speaker-test",32681,7))
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| − | u_str ESTAB 0 0 /run/user/0/pulse/native 198065 * 198064 users:(("pulseaudio",2289,28))
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| − | = Search for a syscall =
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| − | With strace, you can search for a special syscall. For example,
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| − | strace -e open ''command''
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| − | will show you all files that have been touched (opened) by ''command''. This can be used e.g. to [[find out where configuration changes are stored]].
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| − | = Performance analysis =
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| − | With strace, you can find out the biggest time-consuming syscalls during a program run:
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| − | strace -c ls -R
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| − | Entries Repository Root
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| − | % time seconds usecs/call calls errors syscall
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| − | ------ ----------- ----------- --------- --------- ----------------
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| − | 67.09 0.412153 14 29664 getdents64
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| − | 27.70 0.170168 11 14849 14 open
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| − | 4.24 0.026043 0 123740 write
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| − | 0.72 0.004443 0 14837 close
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| − | 0.20 0.001204 0 14836 fstat
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| − | 0.05 0.000285 285 1 execve
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| − | 0.00 0.000000 0 12 read
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| − | 0.00 0.000000 0 4 3 stat
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| − | 0.00 0.000000 0 33 mmap
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| − | 0.00 0.000000 0 18 mprotect
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| − | 0.00 0.000000 0 4 munmap
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| − | 0.00 0.000000 0 12 brk
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| − | 0.00 0.000000 0 2 rt_sigaction
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| − | 0.00 0.000000 0 1 rt_sigprocmask
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| − | 0.00 0.000000 0 2 ioctl
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| − | 0.00 0.000000 0 1 1 access
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| − | 0.00 0.000000 0 3 mremap
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| − | 0.00 0.000000 0 1 fcntl
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| − | 0.00 0.000000 0 1 getrlimit
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| − | 0.00 0.000000 0 1 statfs
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| − | 0.00 0.000000 0 1 arch_prctl
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| − | 0.00 0.000000 0 3 1 futex
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| − | 0.00 0.000000 0 1 set_tid_address
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| − | 0.00 0.000000 0 8 fadvise64
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| − | 0.00 0.000000 0 1 set_robust_list
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| − | ------ ----------- ----------- --------- --------- ----------------
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| − | 100.00 0.614296 198036 19 total
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| − | Now you go
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| − | [http://linux.die.net/man/2/getdents64 man 2 getdents64]
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| − | to find out what that syscall is about.
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| | = See also = | | = See also = |
| − | * [[gdb|gdb, allowing you to monitor every action of a process]] | + | * [[Hardware]] |
| − | * [http://linux.die.net/man/1/strace man strace] | + | * [[Scanners]] |
| − | * http://try-linux.blogspot.de/2013/12/how-to-strace-process.html | + | * [http://en.wikipedia.org/wiki/Barcode_scanner Wikipedia's article] |
| − | * http://scn.sap.com/community/linux/blog/2014/04/11/dispwork-running-but-not-connected-to-message-server# | + | * [http://userbase.kde.org/Kbarcode KDE program to print barcodes] |
| − | * [[TCPDump]]
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| − | [[Category:low-level]] | + | [[Category:Hardware]] |
| − | [[Category:command]]
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| − | [[Category:mindmap]]
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