c preprocessor macro concatenate arguments after expansion

Question

I am writing a driver for avr gpio, I have a function that takes an input which is an enum. I made a macro that calls this function after concatenating the port name with "__" so I can always use initPort(PORTA,1,...).

#define initPort(port,mask,dir,pullup) GPIO_Init(port ## __,mask,dir,pullup)

typedef enum {
PORTA__,
PORTB__,
PORTC__,
PORTD__
} PORT;

void GPIO_Init(PORT p, uint8_t pins, Direction dir,uint8_t pullup) {
switch (p) {
    case PORTA__:

now when I want to use that function I use: initPort(PORTA,1,...) and this works fine. The problem is when I want to use something like:

#define LED_PORT PORTA
initPort(LED_PORT,1,...) 

what happens now is that the argument of GPIO_Init is now LED_PORT__ and not PORTA__

is it possible to fix this or I have to use another way?

Answer 1

The usual technique for this is to add a level of indirection, like this:

#define PORTNAME(port)     port ## __
#define initPort(port, mask, dir, pullup) GPIO_Init(PORTNAME(port), mask, dir, pullup)

typedef enum {
    PORTA__,
    PORTB__,
    PORTC__,
    PORTD__
} PORT;

void GPIO_Init(PORT p, uint8_t pins, Direction dir, uint8_t pullup)
{
    switch (p)
    {
    case PORTA__:
        break;
    }
}

#define LED_PORT PORTA
initPort(LED_PORT, 1, 9, 43)

This can be preprocessed (I ran cpp port31.c to get this output):

# 1 "port31.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "port31.c"




typedef enum {
    PORTA__,
    PORTB__,
    PORTC__,
    PORTD__
} PORT;

void GPIO_Init(PORT p, uint8_t pins, Direction dir, uint8_t pullup)
{
    switch (p)
    {
    case PORTA__:
        break;
    }
}


GPIO_Init(PORTA__, 1, 9, 43)

This looks like the result you are after.

There are other questions on SO which describe the general technique.

Answer 2

You actually can do it by forcing the preprocessor to perform an extra pass before:

#define initPortS(port,mask,dir,pullup) GPIO_Init(port ## __,mask,dir,pullup)
#define initPort(...) initPortS(__VA_ARGS__)
#define LED_PORT PORTA

initPort(LED_PORT,1,2,3);

This will do:

1st pass:

initPort(LED_PORT,1,2,3); -> initPortS(PORTA,1,2,3);

2nd pass:

initPortS(PORTA,1,2,3); -> GPIO_Init(PORTA__,1,2,3);

Here is a demo

Possible pitfall:

If PORTA is a defined symbol, it will get expanded too on the second pass. So if you have a line such as

#define PORTA XXX

somewhere in the code, it will expand into

GPIO_Init(XXX__,1,2,3);

Answer 3

What you are asking is impossible. However, you can create a function called LED_PORTinitPort() and call that by concatentating the first argument of the macro to initPort(), which acheives the same goal.