Why does #define not require a semicolon?

by 
#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.

More Related Contents:

  1. How do I check OS with a preprocessor directive?
  2. What are the applications of the ## preprocessor operator and gotchas to consider?
  3. Macro definition to determine big endian or little endian machine?
  4. What is the meaning of lines starting with a hash sign and number like ‘# 1 “a.c”‘ in the gcc preprocessor output?
  5. Are typedef and #define the same in c?
  6. Standard alternative to GCC’s ##__VA_ARGS__ trick?
  7. What are the valid characters for macro names?
  8. Convert a preprocessor token to a string
  9. How to make a char string from a C macro’s value?
  10. Does the C preprocessor strip comments or expand macros first? [duplicate]
  11. What does ## mean for the C(C++) preprocessor?
  12. # and ## in macros
  13. Are empty macro definitions allowed in C? How do they behave?
  14. Variadic macro trick
  15. How to use #if inside #define in the C preprocessor?
  16. Concatenate int to string using C Preprocessor
  17. #ifdef inside #define
  18. Spaces inserted by the C preprocessor
  19. Is there a good reason for always enclosing a define in parentheses in C?
  20. Detecting Endianness
  21. Why do most C developers use define instead of const? [duplicate]
  22. C Macros to create strings
  23. Macro and function with same name
  24. Is the sizeof(enum) == sizeof(int), always?
  25. What is a good reference documenting patterns of use of X-Macros in C (or possibly C++)?
  26. Should variable definition be in header files?
  27. How can I implement a dynamic dispatch table in C
  28. Why can’t I use sizeof in a preprocessor condition?
  29. How can I make a char string from a C macro’s value?
  30. Macros to create strings in C

Leave a Comment