As I guessed in my comment. The problem is simply that you don't declare the variable in the file load.c
.
For a quick solution, add these following lines to load.c
:
#define STRING__SIZE 501
extern char sys_mybuf[STRING__SIZE];
This tells the compiler that sys_mybuf
is a global variable declared in another translation unit (e.g. source file).
What you really should do, is put this in a header file, and include that header file by all files that needs to access the global data.