In C is there a technical reason to not use lowercase macro definitions?

Question

I'm starting to learn C coming from a background in Java & Javascript and I'm wondering if there is a technical reason for the following being considered bad practice or if it's just the way the language evolved?

#define bool char
#define false 0
#define true 1
#define null NULL

Everywhere I look I always see defined variables (if that's the correct word) uppercased and I'm wondering if there is a good technical reason for this convention? I'm sure this must have been asked many times but I can't seem to find an answer.

If there is no reason for the standard then it would be but I'm asking if it's just opinion based or if there is a technical reason for the convention. Also I was under the impression bool was not in fact a standard C type? — LiamRyan, Nov 19 '16 at 16:30
`_Bool` is not really `int` because its size is probably different — phuclv, Nov 19 '16 at 16:41
@iharob [Is bool a native C type?](http://stackoverflow.com/q/1608318/995714), [Difference between _Bool and bool types in C?](http://stackoverflow.com/q/8724349/995714) — phuclv, Nov 19 '16 at 16:43
@iharob This question is about C. (There is no need to use a bold font, I can read normal fonts just fine) — 2501, Nov 19 '16 at 16:43
Some C standard macros use mixed case like `PRIu32`, `SCNx8` — chux - Reinstate Monica, Nov 19 '16 at 16:51

melpomene · Accepted Answer · 2016-11-19T16:34:20.513

7

They're called macros. And yes, by convention they're ALL_UPPERCASE to avoid clashes with normal identifiers, because they're just text replacement and don't follow the normal scoping rules of the language.

Consider the following:

#define foo bar
...
int foo, bar;  // error: preprocessing turns this into int bar, bar;

To avoid surprises like this (code that isn't what it seems like), we try to keep macros distinct.

edited Nov 19 '16 at 16:34

answered Nov 19 '16 at 16:31

melpomene

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Thank you for the answer but does that mean NULL is the only uppercase language-specific sequence in the entire C language? – LiamRyan Nov 19 '16 at 16:32
1

@LinuxN00b It is usually also a macro. – Iharob Al Asimi Nov 19 '16 at 16:34
@LinuxN00b I don't understand the question. The standard library provides several macros; `NULL` is one of them. – melpomene Nov 19 '16 at 16:35
Ah thanks, as I said I'm just starting into C so I didn't realize that the standard library was what defined NULL, I thought it was baked into the core language – LiamRyan Nov 19 '16 at 16:36
1

@LinuxN00b almost all macros in C are UPPERCASE. And there is [`FILE`](http://stackoverflow.com/q/5672746/995714) which is not a macro – phuclv Nov 19 '16 at 16:37
According to the standard (ISO/IEC 9899:1999 7.17/3) the `NULL` macro is defined in the `stddef.h` header file. – ad absurdum Nov 19 '16 at 16:38
@DavidBowling `NULL` is also defined by several other headers (e.g. `stdlib.h`). – melpomene Nov 19 '16 at 16:38
@melpomene-- just providing a citation from the standard :) – ad absurdum Nov 19 '16 at 16:41

score 1 · Answer 2 · answered Nov 19 '16 at 16:52

It's not the lowercase definitions are bad (even if there are against common convention), rather than reinventing the language is considered as such:

#define null NULL

This way you are going to confuse maintenance programmer, who expects NULL as idiomatic way to specify null pointer constant.

In C is there a technical reason to not use lowercase macro definitions?

2 Answers2