gcc错误：未定义引用***

Question

In my main .c file, I have defined NUMBER as:

#define NUMBER '0'

In another .c file2, I have declared it as an "extern int" variable and used it. But while compiling gcc gives the following error message:

/tmp/ccsIkxdR.o: In function `file2':
file2.c:(.text+0xfd): undefined reference to `NUMBER'
collect2: error: ld returned 1 exit status

Please suggest me a way out. Thanks in advance.

Answer 1

When you use #define is defines a macro for the pre-processor . This macro will only be visible in the source file you defined it in. No other source file will see this macro definition, and the pre-processor will not be able to expand the macro for you in the other source file so the compiler sees the symbol NUMBER and it doesn't have a declaration for any such symbol.

To fix this you have two choices:

1. Put the macro in a header file that you include in both source files.
2. Define NUMBER as a proper variable instead of a macro, and then have an extern declaration in the other source file.

Answer 2

When you #define something (ie create a pre-processor macro) in a C file, it works as text replacement, it's not the declaration of a variable. So, when you write #define NUMBER '0' and write extern int NUMBER; later, the compiler converts it to extern int '0'; before compilation, which is quite meaningless and erroneous.

If you want to define a constant and access it from elsewhere, you can write:

const int NUMBER = '0';

and

extern int NUMBER;

Answer 3

Since your NUMBER is of type int , you could declare it as an enumeration constant:

enum { NUMBER = '0' };

You'd have to put that in a header file (.h) and include that header in your compilation unit (.c file).