Why does #define not require a semicolon? - c

Question

I was writing some test code in C. By mistake I had inserted a ; after a #define, which gave me errors. Why is a semicolon not required for #defines?
More specifically :
Method 1: works
const int MAX_STRING = 256;
int main(void) {
char buffer[MAX_STRING];
}
Method 2: Does not work - compilation error.
#define MAX_STRING 256;
int main(void) {
char buffer[MAX_STRING];
}
What is the reason of the different behavior of those codes? Are those both MAX_STRINGs not constants?

Answer 1

#define MAX_STRING 256;
means:
whenever you find MAX_STRING when preprocessing, replace it with 256;. In your case it'll make method 2:
#include <stdio.h>
#include <stdlib.h>
#define MAX_STRING 256;
int main(void) {
char buffer [256;];
}
which isn't valid syntax. Replace
#define MAX_STRING 256;
with
#define MAX_STRING 256
The difference between your two codes is that in first method you declare a constant equal to 256 but in the second code you define MAX_STRING to stand for 256; in your source file.
The #define directive is used to define values or macros that are used by the preprocessor to manipulate the program source code before it is compiled. Because preprocessor definitions are substituted before the compiler acts on the source code, any errors that are introduced by #define are difficult to trace.
The syntax is:
#define CONST_NAME VALUE
if there is a ; at the end, it's considered as a part of VALUE.
to understand how exactly #defines work, try defining:
#define FOREVER for(;;)
...
FOREVER {
/perform something forever.
}
Interesting remark by John Hascall:
Most compilers will give you a way to see the output after the preprocessor phase, this can aid with debugging issues like this.
In gcc it can be done with flag -E.

Answer 2

#define is a preprocessor directive, not a statement or declaration as defined by the C grammar (both of those are required to end with a semicolon). The rules for the syntax of each one are different.

Answer 3

define is a preprocessor directive, and is a simple replacement, it is not a declaration.
BTW, as a replacement it may contain some ; as part of it:
// Ugly as hell, but valid
#define END_STATEMENT ;
int a = 1 END_STATEMENT // preprocessed to -> int a = 1;

Answer 4

Both constants? No.
The first method does not produce a constant in C language. Const-qualified variables do not qualify as constants in C. Your first method works only because past-C99 C compilers support variable-length arrays (VLA). Your buffer is a VLA in the first case specifically because MAX_STRING is not a constant. Try declaring the same array in file scope and you'll get an error, since VLAs are not allowed in file scope.
The second method can be used to assign names to constant values in C, but you have to do it properly. The ; in macro definition should not be there. Macros work through textual substitution and you don't want to substitute that extra ; into your array declaration. The proper way to define that macro would be
#define MAX_STRING 256
In C language, when it comes to defining proper named constants, you are basically limited to macros and enums. Don't try to use const "constants", unless you really know that it will work for your purposes.

Answer 5

Because that is how the syntax was decided for the precompiler directives.
Only statements end with a ; in c/c++, #define is a pre-processor directive and not a statement.

Answer 6

The second version does not define a constant as far as the language is concerned, just a substitution rule for a block of text. Once the preprocessor has done it's job, the compiler sees
char buffer [256;];
which is not syntactically valid.
The moral of the story: prefer the const int MAX_STRING = 256; way as that helps you, the compiler, and the debugger.

Answer 7

This preprocessor directive:
#define MAX_STRING 256;
tells the preprocessor to replace all MAX_STRINGs with 256; - and with the semicolon. Preprocessor statements don't need a semicolon at the end. If you put one, the preprocessor actually thinks you mean it with a semicolon.
If you are confused with #defines for constants, const int would probably be easier to comprehend.
If you want to learn more on how to properly use these preprocessor directives, try looking at this website.