Concatenate string in C #include filename

Question

Is it possible to concatenate string from another macro when #including a file name (in C). For example,

I have,

#define AA 10 
#define BB 20

these are parameters that change with program runs

And the file include:

#include "file_10_20" // this changes correspondingly i.e. file_AA_BB

Is it possible to have something like #include "file_AA_BB" somehow? I googled to find that double pound operator can concat strings but couldn't find a way of doing it.

Any help would be appreciated.

try #include #(file_##AA##_##BB)

asaelr
– asaelr

2012-02-01 13:01:07 +00:00
Commented Feb 1, 2012 at 13:01 — asaelr
– asaelr, Commented Feb 1, 2012 at 13:01

Per Johansson · Accepted Answer · 2012-02-01 14:14:53Z

23

First I thought "that's easy", but it did take a few tries to figure out:

#define AA 10 
#define BB 20

#define stringify(x) #x
#define FILE2(a, b) stringify(file_ ## a ## _ ## b)
#define FILE(a, b) FILE2(a, b)

#include FILE(AA, BB)

As requested I'll try to explain. FILE(AA, BB) expands to FILE2(AA, BB) but then AA and BB is expanded before FILE2, so the next expansion is FILE2(10, 20) which expands to stringify(file_10_20) which becomes the string.

If you skip FILE2 you'll end up with stringify(file_AA_BB) which won't work. The C standard actually spends several pages defining how macro expansion is done. In my experience the best way to think is "if there wasn't enough expansion, add another layer of define"

Only stringily will not work because the # is applied before AA is replaced by 10. That's how you usually want it actually, e.g.:

#define debugint(x) warnx(#x " = %d", x)
debugint(AA);

will print

AA = 10

edited Feb 1, 2012 at 14:14

answered Feb 1, 2012 at 13:17

Per Johansson

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9 Comments

Artur Over a year ago

If you are able to explain me why FILE macro is necessary I'll upvote ;-). I've seen many such constructs already and never understood why just 2 macros do not work or even why asael's answer does not work. I am highly interested in knowing how preprocessor works in your example. Can someone explain?

wildplasser Over a year ago

Don't forget to #undef FILE, because it will collide with something in <stdio.h>

Per Johansson Over a year ago

Right, perhaps not the best choice of macro name.

anasimtiaz Over a year ago

First of all, thanks for the details. I am about to try this but before that I am trying to understand rather than just simply using it. I don't understand why there is a need to define FILE and FILE2. You do say that stringify alone will not work - I don't understand why that wont.

Artur Over a year ago

@Per: Thanks for explanation. I think everyone interested in this question would actually like to read delorie.com/gnu/docs/gcc/cpp_32.html

|

Community · Accepted Answer · 2017-05-23 12:17:39Z

6

It is usually used like this:

#define stringify(x)  #x
#define expand_and_stringify(x) stringify(x)

#define AA 10
#define BB 20

#define TEXT1 "AA = " stringify(AA) " BB = " stringify(BB)
#define TEXT2 "AA = " expand_and_stringify(AA) " BB = " expand_and_stringify(BB)

TEXT1 = "AA = AA BB = BB"
TEXT2 = "AA = 10 BB = 20"

It's called stringification. You should check this answer.

edited May 23, 2017 at 12:17

CommunityBot

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answered Feb 1, 2012 at 13:22

LihO

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1 Comment

Antonio Over a year ago

Isn't this not supposed to work for an include statement? See stackoverflow.com/questions/20524491/… and stackoverflow.com/questions/39038867/…

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