Multiple definition of ... linker error

I defined a special file: config.h

My project also has files:

t.c, t.h
pp.c, pp.h
b.c b.h
l.cpp

and #includes:

in t.c:

    #include "t.h"
#include "b.h"
#include "pp.h"
#include "config.h"

in b.c: 

    #include "b.h"
#include "pp.h"

in pp.c:

    #include "pp.h"
#include "config.h"

in l.cpp:

    #include "pp.h"
#include "t.h"
#include "config.h"

there are no include directives in my *.h files, only in *.c files. I defined this in config.h:

const char *names[i] =
{
"brian", "stefan", "steve"
};

and need that array in l.cpp, t.c, pp.c but Im getting this error:

pp.o:(.data+0x0): multiple definition of `names'
l.o:(.data+0x0): first defined here
t.o:(.data+0x0): multiple definition of `names'
l.o:(.data+0x0): first defined here
collect2: ld returned 1 exit status
make: *** [link] Error 1

I have include guards in every *.h file I use in my project. Any help solving this?

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最佳答案

Don't define variables in headers. Put declarations in header and definitions in one of the .c files.

In config.h

extern const char *names[];

In some .c file:

const char *names[] = {
"brian", "stefan", "steve" };

If you put a definition of a global variable in a header file, then this definition will go to every .c file that includes this header, and you will get multiple definition error because a varible may be declared multiple times but can be defined only once.

Also, one more thing you can do if you have to define your variables inside of a header file you can use the static keyword.

static const char *names[] = {
"brian", "stefan", "steve" };

This way variable names will be defined only once in your entire program and can be accessed multiple number of times.

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Declarations of public functions go in header files, yes, but definitions are absolutely valid in headers as well! You may declare the definition as static (only 1 copy allowed for the entire program) if you are defining things in a header for utility functions that you don't want to have to define again in each c file. I.E. defining an enum and a static function to translate the enum to a string. Then you won't have to rewrite the enum to string translator for each .c file that includes the header. :)

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Make your definitions static in the header files.


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