gprof gives no output

Question

I am trying to use gprof to profile some numerical code I am developing, but gprof seems to fail to collect data from my program. Here is my command line:

g++ -Wall -O3 -g -pg -o fftw_test fftw_test.cpp -lfftw3 -lfftw3_threads -lm && ./fftw_test

The gmon.out file is created, but it seems to have no data. When i run

gprof -b fftw_test gmon.out > gprof.out

All I get is

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total           
 time   seconds   seconds    calls  Ts/call  Ts/call  name    


                        Call graph


granularity: each sample hit covers 2 byte(s) no time propagated

index % time    self  children    called     name


Index by function name

Any insights?

The code does a lot of stuff, it does not simply call FFTW routines. It has functions that compute certain complex coefficients, functions that multiply input data by these coefficients, and so on.

Edit.: Including example code and results.

#include <cstdlib>
#include <ctime>

int main()
{
   std::srand( std::time( 0 ) );

   double sum = 0.0;

   for ( int i = 0; i < RAND_MAX; ++i )
      sum += std::rand() / ( double ) RAND_MAX;

   std::cout << sum << '\n';

   return 0;
}

Command lines:

$ g++ -Wall -O3 -g -pg -o gprof_test gprof_test.cpp && ./gprof_test
1.07374e+09
$ gprof -b gprof_test gmon.out > gprof.out
$ cat gprof.out

Result:

Flat profile:

Each sample counts as 0.01 seconds.
 no time accumulated

  %   cumulative   self              self     total           
 time   seconds   seconds    calls  Ts/call  Ts/call  name    


                        Call graph


granularity: each sample hit covers 2 byte(s) no time propagated

index % time    self  children    called     name


Index by function name

And that's it.

Answer 1

If you're using gcc 6, you're most likely running into this bug (note that the bug is not specific to Debian but depends on how gcc was built). A workaround is simply to compile using the `-no-pie' option which disables position-independent code generation.

This is a good start if you want to know more about PIE.