Is this #define equivalent to this variable definition in C?

Question

Consider this #define:

#define msd_buffer              ((volatile char *) MSD_BUFFER_ADDRESS)

and this variable definition:

volatile char *msd_buffer = MSD_BUFFER_ADDRESS;

Are the two definitions of msd_buffer — as variable and as macro — equivalent?

Answer 1

A #define is a macro. It acts as a direct copy-pasta of what you pass into it. So, msd_buffer is not a variable, it is a cast to a volatile char array.

Assuming example usage like this:

msd_buffer[160] = 0xFF;

It would be the same as using it as a variable. However, it doesn't have all the features of a variable, such as assignment:

msd_buffer = NEW_BUFFER_ADDRESS; // invalid!

Or address taking:

my_func(&msd_buffer); // invalid!

If you understand these drawbacks (and perhaps there are more), then you are on your way to using a macro like this.

Answer 2

A define statement is just a textual substitution. Nothing more. So in your case, not really.