strace(1) all the things!

Presented at the Marist College Computer Society meeting, 11/12/14

1. strace(1) all the things Dan Miller Software Engineer Etsy
2. etsy.com/careers
3. @jazzdan strace(1) all the things What is strace(1)? How to see what a program does without the source A quick aside about man(1) pages How does ls(1) work?
4. What is strace?
5. @jazzdan strace(1) is a unix utility for observing system calls
6. How do we see what a program does without reading the source?
7. @jazzdan $ls
8. @jazzdan $strace ls
10. @jazzdan strace ls • execve(2) • open(2) • close(2) • getdents(2) • write(2)
11. A quick aside on Unix man pages
12. @jazzdan $man man
16. How does ls work?
17. execve("/bin/ls", ["ls"], [/* 23 vars */]) = 0
19. execve() executes the program pointed to by filename. filename must be either a binary executable, or a script starting with a line of the form: ! #! interpreter [optional-arg]
20. int execve(const char *filename, char *const argv[], char *const envp[]);
21. open( “/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC ) = 3
23. Given a pathname for a file, open() returns a file descriptor, a small, nonnegative integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.).
24. int open(const char *pathname, int flags)
25. open( “/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC ) = 3
26. @jazzdan Wait… what is libc?
27. @jazzdan $man libc
29. open( “.", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC ) = 3
30. getdents( 3, /* 4 entries */, 32768 ) = 712
32. int getdents(unsigned int fd, struct linux_dirent *dirp, unsigned int count);
33. getdents( 3, /* 23 entries */, 32768 )= 712
34. open( “.", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC ) = 3
35. close(3)
37. int close(int fd);
38. close(3)
39. write( 1, “hello_world\n", hello_world ) = 23
41. exit_group(0) = ?
43. This system call is equivalent to exit(2) except that it terminates not only the calling thread, but all threads in the calling process's thread group.
44. void exit_group(int status);
45. @jazzdan When is strace(1) useful? • Anything involving files or sockets • Path issues • See what is being sent over a network interface
46. @jazzdan When is strace(1) not useful? • In production (performance is terrible) • When no syscalls are being executed
47. One Cool Trick for Performance Analysis (perf folks hate ‘em)
48. @jazzdan $strace -c ls
50. Questions?