How do define global variable visible across several .c files

Question

assume there is a header file： a.h and source file : a1.c a2.c ... an.c; n>=1， which means it may only have one source file, or have several source files.

my question is that how to define a global variable g in a.h; which should be visible by a1.c ... an.c.

there are restrictions:

in a1.c a2.c ... an.c; "a.h" can only be included in the first line, which means there should be no code in the source file before the line of #include "a.h"
g must be defined in the a.h.

can only compile the code by the following way:

gcc -c a1.c  -o  a1.o
...       
gcc -c an.c  -o  an.o
gcc  -o a.exe    a1.o ... an.o

there is an answer that define g in a.h like this: extern int g; however, according to the c's specification J.5.11; it's undefined behavior.

is there any other solution?

You need to learn how to capital first letter of the sentence. — Krypton
– Krypton, Commented Sep 2, 2014 at 1:44
possible duplicate of How do I share a variable between source files in C? With `extern`, but how? — n0p
– n0p, Commented Sep 2, 2014 at 7:52

Junyoung LEE · Accepted Answer · 2014-09-02 01:57:34Z

6

As you said, variable must be defined in the c file and declaration should be located in header file.
You have to define a global variable in any c file and declare 'extern' in header file.

Example)

1) define global variable in a1.c
    int g;

2) declare global variable in a.h
    extern int g;

3) include header file in other c files
    #include "a.h"
    // to do something

answered Sep 2, 2014 at 1:57

Junyoung LEE

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