Strace: what a process does
strace is a command that shows a process' calls to the kernel (aka syscalls). This helps you to do excellent troubleshooting and gain good performance tuning insights.
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Contents
Understand strace's output
Let's start with a real-world example: You want to troubleShoot sound on your computer. You know that speaker-test does some test noise. To find out what exactly happens, issue:
# strace -s 99 -ff speaker-test
You will find a lot of lines like
[pid 15287] write(6, "W", 1) = 1
[pid 15287] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\2+U\0\0\0\0\0\31vqU\0\0\0\0\0\0\0\0001TU \240\326\0\4\252\354TU \240\326\0\4\262%r\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
[pid 15287] write(6, "W", 1) = 1
Looks complicated, right? But fear not, every line follows the same pattern:
[pid process id] syscall(parameters) = return code
You will only find the first column with the process id if you use the -ff option (follow sub-processes). For the syscall (write or recvmsg in the above example) it is easy to obtain information - just type
man 2 syscall
in the above example
man 2 write
and
man 2 recvmsg
And you will find abundant information what the syscall does. If you understand 50% you are doing well :) The return code will also be documented in the syscall's man page.
So what happens here? Obviously speaker-test calls the kernel's recvmsg method. The interesting thing we get from man 2 recvmsg is that the first parameter is sockfd, the file descriptor of a socket. So it hands over a socket as a parameter. Let's look at process' file descriptors:
# cd /proc/15287/fd ls -l total 0 lrwx------ 1 root root 64 Apr 4 22:55 0 -> /dev/pts/4 lrwx------ 1 root root 64 Apr 4 22:55 1 -> /dev/pts/4 lrwx------ 1 root root 64 Apr 4 22:55 2 -> /dev/pts/4 lr-x------ 1 root root 64 Apr 4 22:55 3 -> pipe:[297487] l-wx------ 1 root root 64 Apr 4 22:55 4 -> pipe:[297487] lr-x------ 1 root root 64 Apr 4 22:55 5 -> pipe:[297488] l-wx------ 1 root root 64 Apr 4 22:55 6 -> pipe:[297488] lrwx------ 1 root root 64 Apr 4 22:55 7 -> socket:[297495]
aha! file descriptor 7 is the unix socket 297495. Let's look what this is connected to:
# ss -p | grep 297495 u_str ESTAB 0 0 /run/user/0/pulse/native 297496 * 297495 users:(("pulseaudio",2289,28)) u_str ESTAB 0 0 * 297495 * 297496 users:(("speaker-test",15286,7))
We see - the socket 297495 is connected to the socket 297496. What is written into 297495 will be readable from 297496 and the other way round. This couple of sockets connects the speaker-test process with the pulseaudio process. 2289 is pulseaudio's process id, and 15286 is the root process id of speaker-test. This system is using pulseaudio for sound which is an important information for troubleshooting sound. In my example I had been messing around with pulseaudio's configuration and I could fix the problem quickly once I understood I was using it.
strace start options
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
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.
Examples
- find out why SAP does not start
- find out why shellinabox shows forkpty failed
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.
