CODEWITHSUNDEEP
gccmacrosconcatenationc-preprocessor
togarha
togarha

Reputation: 175

Error when concatenate in macro gcc preprocessor

I'm getting an error when I try to use ## in macro this is what I try to make:

With this defines:

#define PORT 2
#define PIN 3

I want that preprocessor generates:

PM2.3=1

when I call a macro like this:

SetPort(PORT,PIN)

Then, I see that I can make the substitution PORT and PIN at the same time that concatenation, then I think I must to use 2 defines:

#define SetP2(PORT,PIN) PM##PORT.PIN = 1
#define SetPort(PORT,PIN) SetP2(PORT,PIN)

but I get an error on:

#define PIN 3  --> expected identifier before numeric constant

and a warning on:

SetPort(PORT,PIN) --> Syntax error

Any idea?

Upvotes: 1

Views: 538

Answers (1)

Digital Trauma
Digital Trauma

Reputation: 15996

This works for me:

$ cat portpin.c
#define PORT 2
#define PIN 3

#define SetP2(prefix,prt) prefix ## prt
#define SetPort(prt,pn) SetP2(PM,prt).pn = 1

SetPort(PORT,PIN)
$ gcc -E portpin.c 
# 1 "portpin.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "portpin.c"


PM2. 3 = 1
$

I don't know how important it is for there to be no space between the . and the 3, but the preprocessor seems to want to insert it.

UPDATE:

Actually I tried your original code, and it seems to produce the same result, so my answer above is probably not much use to you.

UPDATE 2:

It turns out the OP is expecting the pre-processor to generate PM2.no3=1 and not PM2.3=1. This can easily be done as follows:

$ cat portpin.c
#define PORT 2
#define PIN 3

#define SetP2(PORT,PIN) PM##PORT.no##PIN=1
#define SetPort(PORT,PIN) SetP2(PORT,PIN)

SetPort(PORT,PIN)
$ gcc -E portpin.c
# 1 "portpin.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "portpin.c"

PM2.no3=1
$

Upvotes: 1

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