Obfuscation is a commonly used technique to protect software from the reverse engineering process. Advanced obfuscations usually rely on semantic properties of programs and thus may be performed on source programs. This raises the question of how to be sure that the binary code (that is effectively running) is still obfuscated. This paper presents a data obfuscation of C programs and a methodology to evaluate how the obfuscation resists to the GCC compiler. Information generated by the compiler (including effects of relevant optimizations that could deobfuscate programs) and a study of the disassembled binary code, as well as a dynamic analysis of the performances of binary code show that our obfuscation is worthwhile.