Question

Test the following code:

#include <stdio.h> #include <stdlib.h> main() {     const char *yytext='0';     const float f=(float)atof(yytext);     size_t t = *((size_t*)&f);     printf('t should be 0 but is %d\n', t); } 

Compile it with:

gcc -O3 test.c 

The GOOD output should be:

't should be 0 but is 0' 

But with my gcc 4.1.3, I have:

't should be 0 but is -1209357172' 

Answer 1

Use the compiler flag -fno-strict-aliasing.

With strict aliasing enabled, as it is by default for at least -O3, in the line:

size_t t = *((size_t*)&f); 

the compiler assumes that the size_t* does NOT point to the same memory area as the float*. As far as I know, this is standards-compliant behaviour (adherence with strict aliasing rules in the ANSI standard start around gcc-4, as Thomas Kammeyer pointed out).

If I recall correctly, you can use an intermediate cast to char* to get around this. (compiler assumes char* can alias anything)

In other words, try this (can’t test it myself right now but I think it will work):

size_t t = *((size_t*)(char*)&f);