How to use "extern struct" to share variables in c programming and compile with gcc?
c
Solution
The problem is with the scope. Your variables (`fo` & `ba`) have local scope as they are declared within `main`.So, their visibility is restricted to within `main` function. Please make them global variables and it should work.
Problem
I want some shared variables should be accessed among source files, main.c and second.c and my header file is all.h defined the shared data type, ``` #ifndef ALL_H #define ALL_H struct foo { double v; int i; }; struct bar { double x; double y; }; #endif ``` main.c is given below ``` /* TEST*/ #include "all.h" #include "second.h" int main(int argc, char* argv[]) { struct foo fo; // should be accessed in second.c fo.v= 1.1; fo.i = 12; struct bar ba; // should be accessed in second.c ba.x= 2.1; ba.y= 2.2; sec(); // function defined in second.c return 0; } ``` second.h is given below ``` #include <stdio.h> #include "all.h" int sec(); ``` second.c is given below ``` #include "second.h" extern struct foo fo; extern struct bar ba; int sec() { printf("OK is %f\n", fo.v+ba.x); return 0; } ``` I thought i have all the declaration and include the headers. But when i compile ``` gcc -o main main.c second.c or gcc -c second.c gcc -c main.c gcc -o main main.o second.o ``` It will give some error like ``` second.o: In function `sec': second.c:(.text+0x8): undefined reference to `fo' second.c:(.text+0xe): undefined reference to `ba' collect2: ld returned 1 exit status ``` I think somewhere of the use of `extern` was wrong or i use the `gcc` incorrectly?