The problem occurs because you access a char-array through a
char data; ... return *(double*)data;
But gcc assumes that your program will never access variables though pointers of different type. This assumption is called strict-aliasing and allows the compiler to make some optimizations:
If the compiler knows that your
*(double*) can in no way overlap with
data, it’s allowed to all sorts of things like reordering your code into:
return *(double*)data; for(int i=7;i>=0;i--) data[i] = fgetc(stream);
The loop is most likely optimized away and you end up with just:
Which leaves your data uninitialized. In this particular case the compiler might be able to see that your pointers overlap, but if you had declared it
char* data, it could have given bugs.
But, the strict-aliasing rule says that a char* and void* can point at any type. So you can rewrite it into:
double data; ... *(((char*)&data) + i) = fgetc(stream); ... return data;
Strict aliasing warnings are really important to understand or fix. They cause the kinds of bugs that are impossible to reproduce in-house because they occur only on one particular compiler on one particular operating system on one particular machine and only on full-moon and once a year, etc.