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Two pass C preprocessing?

Is there a way to get the C preprocessor (GCC) to run two passes, to fully expand macros?

I'm trying to define macros that support port I/O on a microcontroller using abstract names for pins:

#define LED_RED E, (1<<6)

#define SetPin(port, mask)    PORT ## port ## SET = (mask)
#define ClearPin(port, mask)  PORT ## port ## CLR = (mask)
#define ReadPin(port, mask)   (PORT ## port) & (mask)

Then in my code:

SetPin(LED_RED);
ClearPin(LED_RED);
result = ReadPin(LED_RED);

This is meant to expand to:

PORTESET = (1<<6);
PORTECLR = (1<<6);
result = (PORTE) & (1<<6);

This doesn't compile - I get:

error: macro "SetPin" requires 2 arguments, but only 1 given.

Although this does compile and work OK:

SetPin(E, (1<<6));

So...how to get the C compiler to fully expand these macros?

Or, if not that, how to make this kind of thing work?

over 4 years ago · Santiago Trujillo

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Accepted Answer · 2022-05-10T20:03:44.450Z

You have to pass the arguments through an additional macro.

#define LED_RED E, (1<<6)
#define SetPin2(port, mask)    PORT ## port ## SET = (mask)
#define SetPin(x) SetPin2(x)

SetPin(LED_RED);

This is due to the order of macro replacement:

First, the arguments of a function-like macro are identified. This already fails if the number of arguments is wrong (as in your code).
Then, the argument tokens are put into the replacement list. Unless they are next to ## or #, they get macro-expanded before.
Finally, the resulting replacement list is scanned for further macro replacements.

With the additional macro "in between", the 2nd steps gets the chance to replace LED_RED.

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Two pass C preprocessing?

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