C Using #define to point to or alias a function

Question

I'm a little confused on if I can use #define to point to a function. I have a codec/DSP who's tool automatically generates pages of code like this:

SIGMA_WRITE_REGISTER(address, data, length);
SIGMA_WRITE_REGISTER(address, data, length);
SIGMA_WRITE_REGISTER(address, data, length);
....

Then in another .h file they do this:

    #define SIGMA_WRITE_REGISTER( address, data, length ) {
/*TODO: implement macro or define as function*/}

Which helpfully doesn't define anything about writing registers. That's fine though wrote some code for my micro to write registers over I2C and that seems to be working. Now I don't want to just paste that code into the above define and have it instantiate it 1000 times. I was hoping I could just use the is define as an alias to my function?

Something like:

#define SIGMA_WRITE_REGISTER(address, data, length) { my_i2_c_func(address, data, length)}

I tried something like this and it compiled, I'm not so sure it's working though. Is this a valid thing to do or am I barking up the wrong tree?

Answer 1

Yes, you can surely use #define to point to a full-fledged function or alias.

Consider a simpler example below, just for understanding

#define STRLEN(x) my_strlen(x)

and

int my_strlen(char *p)
{                      
     // check for NULL pointer argument?
     int x;

     for (x = 0; *p++; x++);

     return x;
}

now, in your code, you can use STRLEN as you wish.

Note: Regarding the presence of { }, you can either get rid of them, or use a do..while loop, or define the function as a part of macro itself. Choice is yours. However, as MACRO is expanded during the pre-processing stage [resemble a textual replacement], you need to be extra bit careful about the {} and the ; usage. The MACRO usage should not break the code.

Answer 2

It's (almost) valid, but you need a semicolon after the last close parenthesis and before the close brace.

The braces in the replacement are completely superfluous, so you'll remove them anyway, and then you don't need the semicolon you just added. The braces give you a null statement after each statement block (and if you keep the semicolon in the macro, you also get a null statement after each macro invocation.

The comments in the .h file indicate that you can replace the macro with a function (call). What you're doing is basically fine.