2

This is my file1 called main.c 

#include <stdio.h>
#include <stdlib.h>

#define MONTHS 12
void ChangeDay(void);
int* days;

int main(void)
{
    days = (int*) malloc(MONTHS * sizeof(int));
    if(days != NULL)    
        ChangeDay();    
    else    
        return 1;   
    printf("%2d.\n", days[0]);
    return 0;
}

The global variable days is declared as a pointer to type int, and malloc is used to allocate space for 12 integers.

This is my file 2 called day.c

int days[];
void ChangeDay(void)
{
    days[0] = 31;
}

When the function ChangeDay is called, decimal value 31 is assigned to the first element of the array days .

This is code output :

root@where:~gcc -m32 -Wall -o day main.c day.c
day.c:1: warning: array ‘days’ assumed to have one element
root@where:~./day Segmentation fault

I will be grateful if you explain me this results .

My questions :

  • What is the right way to declare the variables( including arrays ) across multiple source files ?
  • How to access an element of a array using pointer, when they are declared in different files ?
Improve this question
3
  • 1
    in day.c you should add extern Commented Jul 24, 2013 at 19:34
  • The crash you are seeing is a consequence of violating of C's one definition rule, which leads to undefined behavior. Commented Jul 24, 2013 at 19:41
  • Don't cast the result of malloc. Commented Jul 24, 2013 at 19:50

3 Answers 3

Reset to default
5

Every declaration of an identifier for an object must have compatible type with the other declarations. int *days and int days[] are different types. The former is a pointer to an int. The latter is an array of int.

In the first file, use:

int *days;

In the second file, use:

extern int *days;

Additionally: int *days; is a tentative definition of days. When the compiler reaches the end of the translation unit (the source file being compiled), it changes the tentative definition into a definition of the object (with a zero initializer). extern int *days; is a declaration of days that is not a definition. It tells the compiler that days is a name for an object that exists somewhere else.

You should have only one definition for each object. Other files that refer to the object should only declare the name and not define the object.

Sometimes there is confusion about a declaration such as int days[] because using this in a function parameter declares the parameter to be of type int *. This is a special adjustment that happens only in function parameters and not in other declarations.

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

7 Comments

Eric but this is not segmentation - fault reason I think!
@GrijeshChauhan: I tested the original code and my code. My change fixes the problem.
because its correct fix even I was about to answer this, but check my answer, I given a link of codepade view that too.
I just wanted to share with you, Is it not surprising to see that code executing whereas if you use proper flags as I suggested you get a compilation time error!! Similarly I wounder why OP code could compile -because probably its a bug in compiler
@GrijeshChauhan: I compiled, as I habitually do, with GCC flags “-Wmost -pedantic -std=c99”. There is no compilation error here. Can you cite the section of the C standard that this purportedly violates?
|
1

Reason for segmentation fault: 

int days[]; // is errror 
void ChangeDay(void)
{
    days[0] = 31;
}

declaration this is error int days[];, you don't give it size and initialized it. So you can't use day[0] = 31; -- this is illegal and causes segmentation fault.

To get an error instead of warning compile your code like:

gcc -pedantic -error then only it will give you an error.

else this code will not give you an error, Its confusing but check this code its working Codepade. and this code will give you an error if you compile it proper flags as I suggested.

Additionally, as you wants to share same day[] array declare it as suggested by @Eric Postpischil

I am not deleting my answer presently as I want other users to view the linked code @ codepade and tell me whether it should be compile with new compilers? or its a bug in compiler?

Improve this answer

12 Comments

A declaration of int days[]; is not a problem by itself. It declares days with an incomplete type. You may define days in one file with int days[3];, for example, and declare it in another file with extern int days[];. However, since the question shows days being assigned memory with malloc, either it should be a pointer and not an array or it should be an array and the malloc should be deleted.
@EricPostpischil No int days[]; correct as function argument but as formal declaration its error according to C standard.
I am sorry, I do not understand that sentence. Are you saying that a declaration such as void foo(int days[]); is not legal? It is legal. It is adjusted to void foo(int *days); per C 2011 (N1570) 6.7.6.3 7. If you are saying it is not legal at file scope, that is also incorrect; int days[]; is a legal declaration.
@EricPostpischil ok Which compiler you have try the code I linked or OP code with flag I suggested or without that.
@GrijeshChauhan Thanks for help, I tried with GCC version 4.4.3 (Ubuntu 4.4.3-4ubuntu5.1), and code compiles fine with flag -pedantic -error.
|
0

You have to declare the array in one c file, and declare the variable as extern in the other c file.

Improve this answer

1 Comment

Yes, the array must be defined (not just declared) in one file and declared (not defined) with extern in another. However, the types must also be compatible. int * and int [] are incompatible. This answer will not solve the problem.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.