Strace: what a process does

strace is a command to show what a command or process is doing.

Analyzing strace's output

To analyze strace's output you must know that the first keyword in a line of output from strace is always a syscall like open or read. Then, in parantheses, the arguments follow, then the result. The meaning of the syscall and its parameters and results can be found with the command

man 2 syscall

output

Here is an example output from strace that we are going to analyze:

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

analysis

Every line follows the syntax

syscall(arguments) = return value

The meaning of the syscall, its expected arguments and its return value can be determined from its man page. For example to understand the first line call man 2 clock_gettime. It 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. What's this? Let's assume the process ID we are tracing is 7179. Then we can find out its file descriptors like this:

/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

1. 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.

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.

See also

• man strace
• http://try-linux.blogspot.de/2013/12/how-to-strace-process.html
• http://scn.sap.com/community/linux/blog/2014/04/11/dispwork-running-but-not-connected-to-message-server#
• TCPDump