This is a follow up to my [pretty function tracing]{pretty-function-tracing} article. I base this work on the code presented there.

Some one asked me how to get the gcc -finstrument-functions feature working. If you don’t know this flag will modify the entry and exit to/from each function to make a call out to a special set of functions used for tracing.

While I’ve read about this feature, I never actually tried it. So here is what I learned…

When you compile a C file with -finstrument-function, each function entry and exit will receive the following respective callouts. You’re responsible to create these functions.

  • __cyg_profile_func_enter(void *this_fn, void *call_site)
  • __cyg_profile_func_exit(void *this_fn, void *call_site)

In the above two pointers are passed to the instruction that represents the function, this_fn, and the location from which the call was made, call_site.

This could be as easy as implementing:

void __cyg_profile_func_enter(void *this_fn, void *call_site)
{
        printf("entry to %p from %p\n", this_fn, call_site);
}
__cyg_profile_func_exit(void *this_fn, void *call_site)
{
        printf("exit from %p to %p\n", this_fn, call_site);
}

It just prints the hex numbers of functions, which then requires post processing.

The code I wrote uses the backtrace_symbols() glibc function that converts an offset in the running executable into a string that describes the symbol. It’s usually used along with the backtrace() function to generate stack dumps (or backtraces). See the manpage if you’re interested.

A few notes:

  • Don’t pass the -finstrument-function parameter when compiling the tracing function as that would lead to recursion.

    In my example I put that code into a separate file cyg.c.

  • To get symbol resolution to work use -g when building and -g -rdynamic when linking. See my Makefile.

  • My sample code is available in a git repo: git://git.jukie.net/function-tracer.git

I now get traces like this:

# make
# ./a
,-< main(./a) from __libc_start_main+0xe6(/lib/libc.so.6)
| ,-< f1(./a) from main+0x27(./a)
| | ,-< f1a(./a) from f1+0x1a(./a)
| | `-> f1a(./a) from f1+0x1a(./a)
| `-> f1(./a) from main+0x27(./a)
| ,-< f2(./a) from main+0x34(./a)
| | ,-< f2a(./a) from f2+0x1a(./a)
| | `-> f2a(./a) from f2+0x1a(./a)
| `-> f2(./a) from main+0x34(./a)
`-> main(./a) from __libc_start_main+0xe6(/lib/libc.so.6)

To get any more out of this trace (like values passed in function parameters) you’d have to use a disassembler.

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