Stracing a program

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Stracing a program

Postby lik » Sun Aug 09, 2009 2:26 am

Suppose some program on your system refuses to work or it works, but much slower then you've expected. How can you investigate the reason of such undesirable behaviour? First of all, you can read and analyze logs that this program does (if it does). It's very useful to run unkindly program with --verbose argument or somehow else increase the log level, if the program allows that. But what can you do, if logs are poor and it's impossible to infer the reason of badness from them? One way is to use strace program to follow system calls performed by given process.
Use of strace

Commonly to use strace you should give the following command:
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strace -o strace.out -ff touch /tmp/file

* -o strace.out option means that strace program will output all information to the file named strace.out;
* -ff means to strace the forked children of the program. Child straces outputs will be placed to strace.out.PID files, where PID is a pid of the child. If you want all the output to a single file, use -f argument instead (i.e. single f not double).
* touch /tmp/file is the program with arguments which is to be straced.

Strace results
So this is what we have in strace.out:
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execve("/usr/bin/touch", ["touch", "/tmp/file"], [/* 51 vars */]) = 0
uname({sys="Linux", node="dhcp0-138", ...}) = 0
brk(0)                                  = 0x804f000
access("/etc/", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=47843, ...}) = 0
mmap2(NULL, 47843, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f1a000
close(3)                                = 0
open("/lib/", O_RDONLY)        = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\360V\1"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1227872, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f19000
mmap2(NULL, 1142148, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7e02000
mmap2(0xb7f13000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x110) = 0xb7f13000
mmap2(0xb7f17000, 7556, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7f17000
close(3)                                = 0
mprotect(0xb7f13000, 4096, PROT_READ)   = 0
munmap(0xb7f1a000, 47843)               = 0
open("/dev/urandom", O_RDONLY)          = 3
read(3, "v\0265\313", 4)                = 4
close(3)                                = 0
brk(0)                                  = 0x804f000
brk(0x8070000)                          = 0x8070000
open("/tmp/file", O_WRONLY|O_NONBLOCK|O_CREAT|O_NOCTTY|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)
utime("/tmp/file", NULL)                = -1 EACCES (Permission denied)
write(2, "touch: ", 7)                  = 7
write(2, "cannot touch `/tmp/file\'", 24) = 24
write(2, ": Permission denied", 19)     = 19
write(2, "\n", 1)                       = 1
exit_group(1)                           = ?

In this case we see, that the problem is in access to /tmp/file:
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open("/tmp/file", O_WRONLY|O_NONBLOCK|O_CREAT|O_NOCTTY|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)
utime("/tmp/file", NULL)                = -1 EACCES (Permission denied)

Analysis of strace.out, in conjunction with reading the source code, is realy very powerful thing. Even if you can not understand strace output yourself — send it to the developers, it'll be really helpful.

Additional notes

Sometimes (for example if a program works slow) you are also intersted in time that the program spends in syscalls. Than options -tt and -T are helpful.

As you've seen strace program shows arguments of syscalls. But if arguments are a pointers it shows us only 32 bytes from these pointers. To increase this number use -s option.
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