How to use variable in main function that is declare in other function ?

If I want to use variable in main function that is declare in another function so How to do that, for example x is declare in function foo but I want to use it in main function so I am making it extern but when I did it show the error

How to use variable in main function that is declare in other function ?

Code:

---

#include<stdio.h>

void foo ()
{
  extern int x = 2;
}
 
int main()
{
 
    x++;
   
    printf( "x = %d", x);
 
return 0;
}

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This is what return values and output parameters (i.e., a pointer parameter that points to an object in the caller to which the function can write the result) are for.

Quote:

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Originally Posted by laserlight

This is what return values and output parameters (i.e., a pointer parameter that points to an object in the caller to which the function can write the result) are for.

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I do not understand your answer program should supposed to give result 3 but it's give error so how to get result 3

Quote:

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Originally Posted by Player777

I do not understand your answer program should supposed to give result 3 but it's give error so how to get result 3

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Try this program then:

Code:

---

#include <stdio.h>

int foo(void)
{
    int x = 2; // we don't really need this, but as an example...
    return x;
}

int main(void)
{
    int x = foo();
    x++;
    printf("x = %d\n", x);
    return 0;
}

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or this one:

Code:

---

#include <stdio.h>

void foo(int *result)
{
    *result = 2;
}

int main(void)
{
    int x;
    foo(&x);
    x++;
    printf("x = %d\n", x);
    return 0;
}

---

Quote:

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Originally Posted by laserlight

Try this program then:

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Thank you ! The program gives me the output that I wanted But my understanding has been failed to understand the extern keywords

I though if any variable is declared in function and If we want to use that variable in main function we can do that by making it extern variable. I tried to do that but I couldn't do this in my program

your example proves that extern is used in two different source files not in same source file.

The extern keyword is concerned with linkage. Linkage is about deciding, given an identifer such as a variable name: does this identifer refer to the same object or function as that identifier? Where "this" and "that" could be in the same translation unit (internal linkage) or in separate translation units (external linkage). A "translation unit" means a source file and its included headers.

A local variable has no linkage: it is clear from the local scope that "this" or "that" local variable name refers to exactly which object.

Quote:

---

Originally Posted by laserlight

The extern keyword is concerned with linkage. Linkage is about deciding, given an identifer such as a variable name: does this identifer refer to the same object or function as that identifier? Where "this" and "that" could be in the same translation unit (internal linkage) or in separate translation units (external linkage). A "translation unit" means a source file and its included headers.

A local variable has no linkage: it is clear from the local scope that "this" or "that" local variable name refers to exactly which object.

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Can the extern be used only in the different source file or can it be used in the same source file?

You can use it in the same source file, but why would you? The variable has already been declared.

Frankly, I don't think you should dwell so much on this topic. In general, global state makes it more difficult to reason about your program, hence global variables should be sparingly used as they result in global state. Therefore, dedicating so much time to learn about them when you could learn about other things that are more commonly used is not good time allocation.