Thread: True Global Variables

Originally Posted by Bladactania

I want to be able to access a variable in one .c file that I declare in another .c file the first #includes the second). How do I declare it?

You declare it as extern in the various header files and/or source file and define it in exactly one source file.

You do know that it is usually a good idea to avoid global variables, right?

Originally Posted by Bladactania

One question though, is it ok to use a "global" variable within one source file?

Maybe, in the same sense that it may be okay to use a global variable across different source files. Why do you need to use the global variable in the first place? A temporary solution can become a permanent solution that might actually be problematic in the future.

Originally Posted by Bladactania

As far as using globals within one source file, I generally prefer to declar my constants at the very top of the source for easy editing.

Oh, but global constants are not global variables. They are far less problematic to trace than global variables since they do not add state to the program (you can be pretty sure that the constant has the particular value.)

Originally Posted by Bladactania

Can you pass by reference in C or must I pass pointers?

No passing by reference in the language itself; pointers are used to simulate pass by reference.

Originally Posted by Bladactania

Yes... It's not a permanent solution, just a temporary one.

Once your code grows and you start referencing this global variable all over the place, you'll find it becomes a lot less temporary

One question though, is it ok to use a "global" variable within one source file?

As usual, not preferred, but if you must use a file scope variable in a single module at least make it static.

The usual cure against having a lot of parameters is to keep related variables together in a struct.

Code:

typedef struct
{
    char* first_name;
    char* last_name;
    int id;
} Employee;

void do_something(Employee* emp);
void interact(Employee* first, Employee* second);

//vs.

void do_something(char* first_name, char* last_name, int id);
void interact(char* first_name_a, char* last_name_a, int id_a, char* first_name_b, char* last_name_b, int id_b);

However, it is good to know (at the place of call) that these function work with the data of this or that employee, rather than limiting the usage only to some globally visible ones. If you know that the function is correct and it doesn't rely on globals, nothing you do later with other code breaks this function. You know what goes in and what will come out since it is tested (and it is easier to ensure that each single function in isolation is OK than to ensure that for a program where every part depends on every other part). Once tested, you can always just step over it in the debugger.