Member Avatar for Layla_2401

Hi all,

I have a C project consisting of five files as follows:

main.c
file1.c
file1.h
file2.c
file2.h

I have defined an array in file file1.h as follows:

static struct prnc keys[MAXP];

the array was populated in file file1.c

Now I need to use the contents of this array again in file file2.c , I don't get a compilation error when i try to print the cotents of the array in file2.c, but at run time I get nothing as if the code is never encountered. So I tried the following two approaches:

1- I changed the definition of the array in file1.h to:

extern struct prnc keys[MAXP];

This time I got a compilation error generated by references to the array in file file1.c, the error message stated that the array is unknown. Why?
so I tried this:

2- I kept the definition in file1.h as:

static struct prnc keys[MAXP];

and added the following line in file2.c:

extern struct prnc keys[MAXP];

the project compiles but at run time the array seems to be empty.

My question is:

The array is populated in file1.c, and I want to reference its contents in

file2.c

, how can I do that? cause I really don't think that answer is to repopulate it in file2.c! or is it?!


Thanks in advance for your help.

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  • Latest Post Latest Post by Narue

Dani AI

Generated

Short answer: put a single definition (storage) in one .c file and put an extern declaration in the header that every .c includes. was right to recommend extern; , your earlier symptoms come from two different linkage issues (internal linkage when you used static in a header, and a missing definition when you used extern but didn’t provide the storage).

Why you saw those behaviors

  • A static at file scope in a header gives each translation unit its own private copy. Filling one copy in file1.c does nothing to the separate copy in file2.c.
  • An extern in the header is only a declaration; it requires exactly one definition (the actual array without extern) in a single .c file. If that defining .c isn’t compiled/linked, you get an undefined-reference (linker) error.
  • Placing a plain uninitialized definition in a header and including it in multiple .c files can sometimes “work” because some toolchains coalesce uninitialized symbols, but that is non‑portable and fragile (and will fail under C++ or stricter linkers).

Practical pattern (rename to avoid copy/paste confusion)

/* shared.h */
#ifndef SHARED_H
#define SHARED_H
#define NUM_KEYS 128
struct prnc;           /* forward if needed */
extern struct prnc global_prncs[NUM_KEYS];
void init_prncs(void);
#endif
/* file1.c */
#include "shared.h"
struct prnc global_prncs[NUM_KEYS];   /* single definition */
void init_prncs(void) { /* populate global_prncs here */ }
/* file2.c */
#include "shared.h"
/* use global_prncs (after init_prncs() has been called) */

Tips and good practice

  • Make sure file1.c is part of the build/link step (gcc main.c file1.c file2.c -o prog).
  • Prefer accessor functions (e.g., const struct prnc *get_prncs(void)) instead of exposing globals.
  • If mixing C and C++ compile units, watch name mangling (use extern "C" for C linkage).
  • Use include guards in headers to avoid double inclusion.

Recommended Answers

All 6 Replies

Member Avatar for Narue

Lose the static qualifier. extern should be placed on the array in the header file to ensure that it's a declaration. Then include file1.h in both file1.c and file2.c. Make sure that the code to fill the array is called first if you define it in file1.c.

Member Avatar for Layla_2401

That's exactly what I did the first time and which resulted in the compilation error:
"undefined reference to 'keys' "
where keys is the name of the array.
I'm using bloodshed Dev C++ compiler if it makes any difference.
Suggestions?

Member Avatar for Layla_2401

Ok check this out,

I defined the array in file1.h as:

struct prnc keys[MAXP];

then, I included the following in file2.c:

extern struct prnc keys[MAXP];

and it worked!
Please let me know if you think this is a poor scheme or that I should go about this in another way.
But either way I really appreciate your help, Thank you : )

Member Avatar for Narue

>Please let me know if you think this is a poor scheme
I think it's a poor scheme, but since it works for you and you can't seem to figure out how to set up your file dependencies properly, go ahead and use it.

Member Avatar for Layla_2401

But I want to know the proper way if you can kindly tell me,

file1.c has the following header files:

#include "file1.h"

file2.c has the following header files:

#include "file1.h"
#include "file2.h"

But applying the scheme you're suggesting -which is approach #1 in my first post- does not work.

Member Avatar for Narue

>But I want to know the proper way if you can kindly tell me
I did kindly tell you. Use extern declarations in your header files, provide a single source file for definitions, and the linker will do the rest. You only need a declaration for the code to compile.

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