Why are "long *" and "int *" not compatible in 32-bit code?

Question

I was wondering why the following code doesn't compile:

void foo_int(int *a) { }
void foo_long(long *a) { }

int main()
{
    int i;
    long l;

    foo_long(&i);
    foo_int(&l);
}

I am using GCC, and neither calls work either in C or C++. Since it is a 32-bit system, both int and long are signed 32-bit integers (which can be verified with sizeof at compile time).

The reason I am asking is that I have two separate header files, neither are under my control, and one does something like: typedef unsigned long u32; and the other: typedef unsigned int uint32_t;. The declarations are basically compatible, except when I use them as pointers as in the above code snippet, I have to explicitly cast.

Any idea why this is?

Answer 1

Just because long and int both happen to be 32-bit on your particular compiler and hardware, does not mean that they will always both be 32-bit on every kind of hardware and every compiler.

C (and C++) were designed to be source-portable between different compilers and different hardware.