__CONCAT(3)

💅 __CONCAT(3) — okay bestie, let's talk about the __CONCAT and __STRING macros

So this is a page from the Library Functions Manual (the 3 in __CONCAT(3) is giving "section 3, library functions" ✨). Sit down, grab a strawberry 🍓, and let's get into it.

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🌸 NAME 🌸

__CONCAT, __STRING — argument substitution, but make it preprocessor chic 🎀

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✨ SYNOPSIS ✨

#include <sys/cdefs.h>

xy
__CONCAT(x, y);

const char *
__STRING(x);

(Yes, xy up there is literally the return-type placeholder from the page — I'm not touching it, she stays exactly as written 💅)

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💖 DESCRIPTION 💖

Okay so first up, the __CONCAT macro. It's lowkey just vibing with the cpp(1) preprocessor to glue two tokens together. When the macro expands, x and y get combined into a single token — but only if the result actually forms a valid token. If the two tokens together don't make a valid token, then nope, they can't be concatenated, periodt. This whole thing is known as "token concatenation" or "token pasting". 🫶

Then there's __STRING(), the icon. It uses the conventional '#' preprocessing operator to swap the argument x out for a string literal. She's also known as "stringification". Slay. 🌈

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🦋 EXAMPLES 🦋

Not me showing you that these two printf(3) calls produce the same output 👀 :

#define Net     0x01
#define BSD     0x02

#define NetBSD  "NetBSD"

(void)printf("%s\n", __CONCAT(Net, BSD));
(void)printf("%s%s\n", __STRING(Net), __STRING(BSD));

Same output, two different vibes. Main character energy from both fr fr. 💜

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🎀 SEE ALSO 🎀

• cpp(1)
• cdefs(3)

Go read them, iykyk 📚

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👑 HISTORY 👑

The __CONCAT() and __STRING() macros first appeared in NetBSD 1.3. Little baby macros, all grown up now 😭💖

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🔥 CAVEATS 🔥

Bestie, listen to me carefully, because there are many small details that direct the proper use of these macros and I need you to focus 🙈:

• While all leading and trailing whitespace is ignored when __STRING() is used, it is undefined whether cpp(1) puts white space between the tokens when __CONCAT() is used. Undefined! Do not come crying to me about whitespace. 💀
• Also worth noting: the C preprocessor converts all comments to whitespace before any macros are even considered.
• The use of either macro is discouraged in complex constructs. Keep it simple, keep it cute. 🧚

Use of this macro is non-portable — she is part of the implementation namespace and should only be used in NetBSD code. No sneaking her into your portable project, ngl. 🚨

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⭐ 💅 ⭐

And that's the page! Source/version and date, for the record: this one's from Debian, dated October 17, 2013 — so yes, she's an elder icon but she still ate. Byeee 🫶💖🎀