CODEWITHSUNDEEP
c
msc
msc

Reputation: 34598

Why can't we define function inside the main function?

In the following program, I try to call the function n() and inside n(), try to call m() function which is defined in the main function, but when I compile, I get the error below:

In function `n':
(.text+0xa): undefined reference to `m'
error: ld returned 1 exit status

Why do I get an error? Please explain.

The code is here:

#include <stdio.h>
void m();
void n()
{
    m();
}

void main()
{
    n();
    void m()
    {
        printf("hi");
    }
}

Upvotes: 2

Views: 23346

Answers (3)

maxdev
maxdev

Reputation: 2566

You can not implement a function within the scope of another function in standard C. Move the implementation of m() out of your main.

The code you posted should not compile at all; though, the error you get is because the linker ld can not find the implementation of m. The function can be used because you declared it, but the implementation is missing and thus can not be linked.

Also note that your main function shall return a value of type int. Using void will make your program return an arbitrary value from which the operating system/shell can't conclude whether execution was successful.

#include <stdio.h>

void m();

void n() {
    m();
}

int main() {
    n();
    return 0;
}

void m() {
    printf("hi");
}

Upvotes: 6

Rommel
Rommel

Reputation: 1

Move the function declaration of m() outside of the main() method.

Upvotes: 0

melpomene
melpomene

Reputation: 85767

m is defined inside of main. In standard C, that's not allowed (you can't define a function within another function).

Some compilers (e.g. gcc) allow it as an extension. But then the function is local, i.e. m only exists within main and can't be seen from the outside. Similarly, variables defined within a function are local to that function and can't be seen from the outside.

Your void m(); declaration at the top claims that a (global) function called m exists, but it doesn't. That's why you get the linker error.

Upvotes: 8

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