Macro Definitions Syntax

Question

Why don't we put semicolons at the end of macro definitions?

I know that the preprocessor handles these parts of the code. Is there a relation between these two? If yes, what kind?

Is it related to a difference between how the preprocessor handles things differently when compared to the compiler? If yes, what are these differences?

AntonH · Accepted Answer · 2017-03-13T19:58:10.800

6

The macro definitions are just straight-up text replacement, including if you put a semi-colon ; there. So

#define MYVAR 4;

if used thusly:

for (i=0; i<MYVAR ; i++)

would be translated to:

for (i=0; i<4; ; i++)

Which is erroneous, it would cause a syntax error. Don't be fooled by moments where it could work, or appear to work, without side-effects. For example:

int j = MYVAR;

translates to:

int j = 4;;

Which works, but is not a good way to work.

edited Mar 13 '17 at 19:58

answered Mar 13 '17 at 19:49

AntonH

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Also `int j = MYVAR` without the semicolon (macro voodoo) is a bad thing. Don't do it. – Marco Mar 13 '17 at 19:52
I see. So it prevents flaws in the syntax. Thank you very much. – Mar 13 '17 at 19:52
@d3l I think my eyes would bug if I ever saw that :) – AntonH Mar 13 '17 at 19:53

score 1 · Answer 2 · edited Mar 13 '17 at 19:55

1

In addition to the example given by AntonH

#define MAX(a, b) (a > b ? a : b)
int x = MAX(4, 5);

If you defined this macro with a semicolon, you could write a statement without a semicolon, which would look quite strange:

#define MAX(a, b) (a > b ? a : b);
int x = MAX(4, 5)

edited Mar 13 '17 at 19:55

AntonH

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answered Mar 13 '17 at 19:51

Millie Smith

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AnT stands with Russia · Answer 3 · 2017-03-13T20:56:01.503

Even if we forget about preprocessor, there are lots of places in C language-proper where we don't put a ; at the end of things. In fact, one can argue that putting a ; at the end of something is more of an exception than a rule in C. Declaration ends in a ; and expression statement ends in a ; and jump statements end in a ; - that's about it. And almost everywhere when ; is needed by the language-proper, it is needed for some justifiable grammatical reasons.

What would be the reason to require ; at the end of macro definitions? I don't see any. That's probably why it isn't there.

score 0 · Answer 4 · answered Mar 13 '17 at 20:23

Preprocessing directives (#define, #include, #if, #ifdef, #ifndef, etc.) are not statements, so they are not terminated by semicolons; instead, they are terminated by newline characters. If you want a preprocessing directive to span several source lines, you need to escape the newline character like so:

#define FOREACH( n, arg, func )     \
  for( size_t i = 0; i < (n); i++ ) \
  {                                 \
    (func)((arg), i);               \
  }

Like the others have said, macro expansion is just dumb text substitution, so if you put a semicolon at the end of a macro definition it will be part of the expansion, which is almost never what you want.

Macro Definitions Syntax

4 Answers4