Invalid characters in C preprocessors/macros?

Question

What are the different invalid characters that I am not allowed use in a macro ?
It seems that #define TE$T 8 is working, so $ is valid.
Does somebody have a list of the invalid characters ? (or on the contrary the list of the valid ones).

Answer 1

It's your compiler that allows usage of $ as an identifier. It's not standard and you shouldn't expect other compilers to provide it or your compiler to allow it, if you compile with -pedantic or similar.

In the C11 draft's common extension appendix:

J.5.2 Specialized identifiers

1 Characters other than the underscore _, letters, and digits, that are not part of the basic source character set (such as the dollar sign $, or characters in national character sets) may appear in an identifier (6.4.2).

Section 6.4.2 shows what characters every conforming compiler has to support:

6.4.2 Identifiers
6.4.2.1 General
Syntax 1         identifier:
                 identifier-nondigit
                 identifier identifier-nondigit
                 identifier digit
         identifier-nondigit:
                 nondigit
                 universal-character-name
                 other implementation-defined characters
         nondigit: one of
                _ a b            c    d    e    f     g    h    i    j     k    l    m
                    n o          p    q    r    s     t    u    v    w     x    y    z
                    A B          C    D    E    F     G    H    I    J     K    L    M
                    N O          P    Q    R    S     T    U    V    W     X    Y    Z
         digit: one of
                0 1        2     3    4    5    6     7    8    9

You should restrict yourself to those.

Answer 2

Macro names should only consist of alphanumeric characters and underscores, i.e. 'a-z', 'A-Z', '0-9', and '_', and the first character should not be a digit. Some preprocessors also permit the dollar sign character '$', but you shouldn't use it.

Also have look on this... What are the valid characters for macro names?

Answer 3

It seems that #define TE$T 8 is working, so $ is valid.

That's not true. $ is NOT a valid character for identifiers in standard C. Some compilers, e.g, GCC , allows $ in identifiers as an extension. (See Dollar Signs)

So you are asking the wrong question, there's nothing special for names in macros, all the preprocessor does is text replacement.

Answer 4

Consider somefille.c

#include<stdio.h>
#define NAM$ "SomeName"
int main(void)
{
printf("Name - %s\n",NAM$);

return 0;
}

Compiling the above with

gcc -pedantic somefille.c -o somefille

gives you

somefille.c:2:9: warning: '$' in identifier or number [enabled by default]
 #define NAM$ "SomeName"

This [ page ] says.

-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

As per the strict standard the macro name must have no spaces in it, and it must conform to the same naming rules that C variables follow: Only letters, digits, and the underscore ( _ ) character can be used, and the first character cannot be a digit.

The problem is that various compilers do not comply with this. An example is gcc which I mentioned above.

Having said that, below rules are still obeyed:

1. A macro name must not begin with a number, if you violate this you may get an error like :

   error: macro names must be identifiers
   

2. A macro name must not contain spaces. For instance #define FULL NAME "Your name" gives you :

   error: ‘NAME’ undeclared (first use in this function)