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GO Classes DPP | GATE CS | C Programming | Multi-file Extern Linkage

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11 11 votes

Given the following three files, what will be the output after compiling and linking them together?

file1.c

extern int i;

void f() {
    i++;
}

file2.c

int i = 0;

void f();

void g() {
    f();
}

main.c

#include <stdio.h>

extern int i;

void f();
void g();

int main() {
    f();
    g();

    printf("%d", i);

    return 0;
}
  1. $\texttt{0}$
     
  2. $\texttt{1}$
     
  3. $\texttt{2}$
     
  4. Linker error

3 Answers

2 2 votes

In $\texttt{file2.c}$, the statement $\texttt{int i = 0;}$ is the actual definition of the global variable $\texttt{i}$.

In $\texttt{file1.c}$, the statement $\texttt{extern int i;}$ declares that $\texttt{i}$ is defined somewhere else.

So, $\texttt{file1.c}$ uses the same global $\texttt{i}$ from $\texttt{file2.c}$.

In $\texttt{main.c}$ also, $\texttt{extern int i;}$ refers to the same global variable $\texttt{i}$.

Initially, $\texttt{i = 0}$.

In $\texttt{main()}$, first $\texttt{f()}$ is called.

Function $\texttt{f()}$ does $\texttt{i++}$, so $\texttt{i}$ becomes $\texttt{1}$.

Then $\texttt{g()}$ is called.

Inside $\texttt{g()}$, function $\texttt{f()}$ is called again.

So, $\texttt{i++}$ runs again and $\texttt{i}$ becomes $\texttt{2}$.

Therefore, the output is $\texttt{2}$.

Answer: C

• edited by
0 0 votes
After independent compilaion of the each file, the object files are generated and all the object files are linked to one file and the extern keyword is resolved in all places.
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