Thread: globals & multiple .c files

The keyword 'extern' is your friend.

FileA:

int myglobal;

FileB:

extern int myglobal;

This basicly just creates a reference point to the original variable.

Quzah.

Originally posted by FCF
Dear quzah,
Sorry to interrupt, i think i have to ask something.
Can u describe a bit the usage of "extern" in practical? I read my reference book and i can't understand clearly.
Thank u first for your help.

Sure. Let's assume that I have a table of items in a file:

Code:

struct vehicle {
    char *name;
    int weight;
    int color;
    float mpg;
} vehiclelist[] =
{
    { "Ferarri", 2000, COLOR_RED, 20.1  },
    { "Limo", 5000, COLOR_BLACK, 15.3 },
    { "Econo-car", 1200, COLOR_BLUE, 55.7 },
};

Ok, good enough. (If you're wondering, I declared a structure "struct vehicle", defined it, and followed that by creating a table (array) of these structures, called "vehiclelist")

Let's assume this list has a ton of entries.
Let's assume I'm using them in a progject that spans multiple .c files.

Now, I have one .c file called "output.c" that handles all of my "output" functions. This may be to disk, or perhaps to the screen, whatever. Anything considered output is all in this file.

Now, assuming I actually want to display any information on my vehicles, (which are contained in a file called "vehicle.c"--that has all of my vehicle related functions--getting speed, breaking, calculating impact damage, whatever), my 'output.c' file needs to know about this table.

Thus, in 'vehicle.c', the table is actually created, it exists there.

In 'output.c', I need a reference to that variable/table:

extern struct vehicle vehicletable[];

Now it can see the table.

Quzah.

Can also do

FileA:

#include "def.h"

FileB:
FileC:
FileD:
extern int myglobal;

(def.h not included in these files but a variables declared in def.h can be declared here as an extern)

quzah,

a bit of question, is extern on the

>> extern void function (int *)
and
>> void function (int *)

same?? or it might be have some adv and disadv on each statements

The extern keword is really a tag which states "Whatever follows is defined in some other file."

Thus:

extern int x;

Doesn't actually create a variable called 'x', it creates a sort of reference to another variable which is defined elsewhere. It's really a reference for the compiler so it knows what you're doing. You use it like you would that actual variable, because in effect you are actually using that variable.

The same thing goes for prototyping functions. You're saying "Hey, this function isn't defined here, but it's in some other file and it takes this format, so it's valid for me to use it this way."

In effect it makes a global reference (global in the true sense of the word, not limited to a single file, but the entire program) to whatever follows it, so you can use that item.

Quzah.