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Saturday, October 03, 2009

Call Trace without modifying the source


While investigating about gcc flag "-fprofile-arcs", I came to know about a new(to me) gcc flag and this blog entry is about it. For any large C project it is hard to learn/find call graph through code walk. From C prospective unless otherwise you put a printf in each function entry/exit it is hard to find the call trace. 

GCC and ICC has a wonderful option "-finstrument-functions" to solve this. This option instructs the compiler to emit instructions to call a external function on each function's entry/exit. Defining these two functions like the following and adding the above option -finstrument-function to your makefile will do the magic.

void noinstrument __cyg_profile_func_enter(void *this_fn, void *call_site)
{
printf("%p called from %p\n", this_fn, call_site);
}

void noinstrument __cyg_profile_func_exit(void *this_fn, void *call_site)
{
printf("%p returns\n", this_fn);
}

Of course, you can do anything in these functions, for simplicity sake I just defined them as printfs.

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