Strace: what a process does
So you have a process that is crashing or misbehaving and you found the tool strace to tell you what the process does. However you don't understand its output? Then this article is for you.
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Contents
starting strace
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:
strace -s 99 -ff ls
To attach strace to an already running process, in this example firefox, find out the process' id:
# ps -A | grep firefox 2728 pts/1 00:00:02 firefox
Then call strace with the id as parameter:
strace -s 99 -ffp 2728
Understanding strace's output
The bad news is that strace's output looks tedious like this:
clock_gettime(CLOCK_MONOTONIC, {4433, 764347636}) = 0
poll([{fd=12, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 1 ([{fd=12, revents=POLLIN|POLLRDNORM}])
recv(12, "\37\36&\364T<\264\6Og\20\2\30%-\31C\3757\350{\260\0203\351\23&O\365(y\212"..., 16384, 0) = 1448
The good news is that every line follows the same scheme:
syscall(parameters) = return value
Whatever you see for syscall you can get help on using the man(2) pages. So for the above example
man 2 clock_gettime man 2 poll man 2 recv
Will give you the information what is going on. For example, clock_gettime retrieves the current time.
Wow, that was easy. Now to the next line, poll. According to 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:
/proc/7179/fd # ll total 0 lr-x------ 1 root root 64 Apr 19 08:02 0 -> /dev/null lrwx------ 1 root root 64 Apr 19 08:02 1 -> socket:[156171] l-wx------ 1 root root 64 Apr 19 08:02 2 -> /var/log/apache2/error_log lr-x------ 1 root root 64 Apr 19 08:02 4 -> pipe:[156208] l-wx------ 1 root root 64 Apr 19 08:02 5 -> pipe:[156208] l-wx------ 1 root root 64 Apr 19 08:02 6 -> /var/log/apache2/access_log lrwx------ 1 root root 64 Apr 19 08:02 7 -> anon_inode:[eventpoll] lrwx------ 1 root root 64 Apr 19 08:02 12 -> socket:[155097]
Ok, to find out what socket:[155097] is we use the command
# lsof | grep 155097 httpd2-pr 7158 root 3u IPv4 155097 0t0 TCP *:http (LISTEN) httpd2-pr 7179 wwwrun 3u IPv4 155097 0t0 TCP *:http (LISTEN)
So this is a socket listening to the http port.
limitations
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.
interprocess communication
Processes will typically communicate:
- via network
- via sockets
- via pipes
- via shared memory
- by writing into files
For example when using strace to troubleshoot sound you may see that speaker-test receives messages from file descriptor 7:
# strace -s 99 -ffp 32681
[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
Now let's find out what file descriptor 7 of process 32681 is:
# ll /proc/32681/fd/7 lrwx------ 1 root root 64 Apr 4 18:17 /proc/32681/fd/7 -> socket:[198064]
So it is unix socket 198064. Let's find out what process it is:
# ss -p | grep 198064
u_str ESTAB 0 0 * 198064 * 198065 users:(("speaker-test",32681,7))
u_str ESTAB 0 0 /run/user/0/pulse/native 198065 * 198064 users:(("pulseaudio",2289,28))
Search for a syscall
With strace, you can search for a special syscall. For example,
strace -e open command
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.
Performance analysis
With strace, you can find out the biggest time-consuming syscalls during a program run:
strace -c ls -R Entries Repository Root % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 67.09 0.412153 14 29664 getdents64 27.70 0.170168 11 14849 14 open 4.24 0.026043 0 123740 write 0.72 0.004443 0 14837 close 0.20 0.001204 0 14836 fstat 0.05 0.000285 285 1 execve 0.00 0.000000 0 12 read 0.00 0.000000 0 4 3 stat 0.00 0.000000 0 33 mmap 0.00 0.000000 0 18 mprotect 0.00 0.000000 0 4 munmap 0.00 0.000000 0 12 brk 0.00 0.000000 0 2 rt_sigaction 0.00 0.000000 0 1 rt_sigprocmask 0.00 0.000000 0 2 ioctl 0.00 0.000000 0 1 1 access 0.00 0.000000 0 3 mremap 0.00 0.000000 0 1 fcntl 0.00 0.000000 0 1 getrlimit 0.00 0.000000 0 1 statfs 0.00 0.000000 0 1 arch_prctl 0.00 0.000000 0 3 1 futex 0.00 0.000000 0 1 set_tid_address 0.00 0.000000 0 8 fadvise64 0.00 0.000000 0 1 set_robust_list ------ ----------- ----------- --------- --------- ---------------- 100.00 0.614296 198036 19 total
Now you go
man 2 getdents64
to find out what that syscall is about.
