Where the nested function's body kept?

Question

Please check the following code,

#include <stdio.h>
#include <string.h>

void* make_adder_function(int a)
{

    char str[64] ={};

    sprintf(str,"this function adds %d with input",a);

    void adder(int b) {
        int x = a;
        char ptr[64] = {};
        strcpy(ptr,str);
        printf("%s\n", ptr);
        printf("%d + %d = %d\n", x, b, x+b);
    }

    return adder;
}


int main()
{
    void (*adder_ten)(int);
    void (*adder_twenty)(int);

    adder_ten = make_adder_function(10);
    adder_twenty = make_adder_function(20);

    adder_ten(20);
    adder_twenty(20);

    return 0;
}

While Running the code I am getting the following output,

ajith@chat:~/Desktop$ ./a.out 
this function adds 20 with input
20 + 20 = 40
Segmentation fault (core dumped)

It shows like the scope of function adder() is only inside the make_adder_function(), I assumes this happens because the function body of adder() is kept in the stack frame of make_adder_function(). Can anybody give an explanation for this? How can I persist a nested function's lifetime throughout the program?

C doesn't have nested functions. Nested functions is a non-portable compiler-specific extension of GCC. Please avoid such extensions. — Some programmer dude
– Some programmer dude, Commented Feb 25, 2020 at 8:54
The problem is not the function, the problem is the printf("%s\n", str);, where str refers to a buffer that has reached its lifetime. The local variable a, that you use, also, btw, but it doesn't lead to a crash. It can contain some arbitrary value, however. — Ctx
– Ctx, Commented Feb 25, 2020 at 8:56
Please to not make changes to your code after receiving comments and answers. Your changes make them useless. — Gerhardh
– Gerhardh, Commented Feb 25, 2020 at 9:01
Info on how to correctly emulate a closure in C. Summary: it's a pita. — user3386109
– user3386109, Commented Feb 25, 2020 at 9:07

Some programmer dude · Accepted Answer · 2023-05-16 13:59:15Z

1

Your problem (the non-standard extension aside) is that the local variable str inside the make_adder_function function will have ended its life-time when the make_adder_function function ends. And this happens before you call the adder function, which then tries to use the non-existing str variable.

The nested-function extension only nests functions, it doesn't have closures.

As a simple workaround you could make str a static variable. Its life-time would then be for the full runtime of the program. The problem then is that there's only one variable, which will be shared between all calls to the make_adder_function function.

edited May 16, 2023 at 13:59

answered Feb 25, 2020 at 8:56

Some programmer dude

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3 Comments

Ajith C Narayanan Over a year ago

now I added local variables for out of scope values

Ctx Over a year ago

@AjithCNarayanan That doesn't change anything, you still use variables, that are past their lifetime, when you copy str and a inside adder(). And please do not make edits that invalidate existing answers, if you want to show current code, do it below the original code marked with "Edit:" or similar.

Some programmer dude Over a year ago

@AjithCNarayanan Your change doesn't help, you still attempt to use str after its life-time. The copying you now do is done when adder is called, not when make_adder_function is called.

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