How to use a global array in multiple modules

Question

I'm trying to access the programs array in my main file. It is declared in the header file and initialized in a separate module called fileReader. The error message I receive is

Undefined symbols for architecture x86_64: "_programs", referenced from: _main in test-0bf1e8.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "header.h"
#include "fileReader.c"

int main() {

    readPrograms();
    for (int i=0; i<4; i++) {
        printf("%s", programs[i]);
    } 

    return 0;
}

fileReader.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "header.h"

int readPrograms() {
    int i=0;
    int numProgs=0;
    char* programs[50];
    char line[50];

    FILE *file;
    file = fopen("files.txt", "r");

    while(fgets(line, sizeof(line), file)!=NULL) {
        //add each filename into array of programs
        programs[i]=strdup(line); 
        i++;
    }

    fclose(file);

    return 0;
}

header.h

extern char* programs[];

Thanks in advance

You don't have a global array. You need to create one if you want to use it in one module, let alone multiple modules. — Crowman
– Crowman, Commented Oct 23, 2014 at 0:53

Sergey Kalinichenko · Accepted Answer · 2014-10-23 02:12:11Z

0

You are not supposed to include C files from other C files, only the header files.

Here is what you need to fix:

Add a prototype of readPrograms function to the header.h
Remove #include "fileReader.c" from the main.c file
Add a definition of programs array to one of your C files (say, main.c).
Remove declaration of the local programs from readPrograms

The definition of programs that you put in main.c should look like this:

char* programs[50];

You can put it before or after your main() function.

edited Oct 23, 2014 at 2:12

answered Oct 23, 2014 at 0:55

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

user3192682 Over a year ago

Thanks for your help, I'm still getting the error message after making all of those changes. is the prototype just a line in the header file like so? int readPrograms();

Sergey Kalinichenko Over a year ago

@user3192682 Yes, that's the prototype. What errors do you get after making the changes?

user3192682 Over a year ago

the same error that I was receiving originally, it is posted above

Sergey Kalinichenko Over a year ago

@user3192682 I just tried compiling this, it compiles perfectly. I made exactly the four changes that I described in the answer, and compiled the code like this: gcc main.c fileReader.c. I got a.out which reads from files.txt, and prints the first four strings.

Sergey Kalinichenko Over a year ago

@user3192682 Here is a pastebin with your code. Comments show the names of files. I think you missed step #3, adding a definition. It's on line 23 of the pastebin.

|

Collectives™ on Stack Overflow

How to use a global array in multiple modules

1 Answer 1

7 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Related