Question

Whenever I want to read or write into a binary file using C, I do use the fread() and fwrite() functions. They need as a parameter the bytes of the datum that is being read or written so I use the sizeof() function. Now the question is:

The books says that I should declare a function like this:

fread(&variable,sizeof(TYPE_OF_VAR),quantity,file);

I’ve use the following statement which works most of the time but not always:

fread(&variable,sizeof(VARIABLE),quantity,file);  

Why does it works sometimes but sometimes it doesn’t?
Does it depends on the type of the variable (int, char, etc)?
Does it depends on the quantity of the datum that I use?

Answer 1

The thing to keep in mind is that sizeof is based on what the compiler knows about the type at compile time (ignoring VLA’s for now). If you give it a variable, it will use the type of that variable.

So, the only time I can think of where it wouldn’t work as you expect is with pointers.

basically what it boils down to is this:

int x[5];
int *y = &x[0];
int *p = malloc(sizeof(int) * 5);
sizeof(x); // == sizeof(int) * 5
sizeof(y); // == sizeof(int *)
sizeof(p); // == sizeof(int *)

This gets tricky when dealing with functions because arrays decay to pointers when passed to a function. Also note that all 3 of these are exactly equivalent:

int func(int *p);
int func(int p[5]);
int func(int p[]);

in all 3, p is a pointer, not an array.