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// entropy.cpp
// Author: Wojtek Jamrozy (www.wojtekrj.net)
#include <cstdio>
#include <cstring>
#include <cmath>
unsigned char buffer[524290];
long long sum;
long long count[256];
 
int main(int argc, char* argv[]) 
{	
	if(argc!=2)
	{
		printf("Usage: entropy <file>");
		return 1;
	}
	FILE * pFile;
	pFile = fopen (argv[1] , "rb");
	if (pFile == NULL)
		perror ("Error opening file");
	else {
		int rnumber;
		while(rnumber = fread (buffer,1,524288,pFile))
		{
			sum += rnumber;
			for(int i=0;i<rnumber;++i)
				count[buffer[i]]++;
		}
		fclose (pFile);
		long double result=0.0, p;
		for(int i=0;i<256;++i)
		{
			if(!count[i]) continue;
			p = (long double)count[i]/(long double)sum;
			result -= p*log(p);
		}
		result /= log(256);
		printf("Numer of samples: %lld\n", sum);
		printf("Entropy %.10Lf bits per byte (%.2LF %%)\n", result*8, result*100.0);
	}
	return 0;
}

To get better performance, compile it with following syntax:

g++ -O2 -static entropy.cpp -o entropy

Usage:

./entropy file

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valgrind your_program

Example of usage:
We have following C++ program (tst.cpp):

int main() {
	int * tab = new int[1000000];
	return 0;
}

Obviously it generates memory leak. We compile it to tst file and execute:

valgrind tst

==20777== Memcheck, a memory error detector.
==20777== Copyright (C) 2002-2008, and GNU GPL'd, by Julian Seward et al.
==20777== Using LibVEX rev 1884, a library for dynamic binary translation.
==20777== Copyright (C) 2004-2008, and GNU GPL'd, by OpenWorks LLP.
==20777== Using valgrind-3.4.1-Debian, a dynamic binary instrumentation framework.
==20777== Copyright (C) 2000-2008, and GNU GPL'd, by Julian Seward et al.
==20777== For more details, rerun with: -v
==20777==
==20777==
==20777== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 17 from 1)
==20777== malloc/free: in use at exit: 4,000,000 bytes in 1 blocks.
==20777== malloc/free: 1 allocs, 0 frees, 4,000,000 bytes allocated.
==20777== For counts of detected errors, rerun with: -v
==20777== searching for pointers to 1 not-freed blocks.
==20777== checked 93,180 bytes.
==20777==
==20777== LEAK SUMMARY:
==20777==    definitely lost: 0 bytes in 0 blocks.
==20777==      possibly lost: 4,000,000 bytes in 1 blocks.
==20777==    still reachable: 0 bytes in 0 blocks.
==20777==         suppressed: 0 bytes in 0 blocks.
==20777== Rerun with --leak-check=full to see details of leaked memory.

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