i have a problem with free();

i have this simple program

Code:

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

char *foo(char *a, int n){
   
    char *d,*c;
    int i;
    d = (char *)malloc(n*sizeof(char));
    c=d;
    n=strlen(a)-n;
    for (i=n;i<strlen(a);i++){
        *(d+i-n) = *(a+i);
        }
        *(d+i-n) = '\0';
   
        return c;
   
    }
   
int main(void){
   
    char b[]="test";
   
    printf("%s\n",foo(b,2));

    getchar();
   
    return 0;
   
}

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if i put free(d) before return c; it will erase the contents of the array and so no string will be returned, i cant use free(d) in main since we have no d in main, what should i do? how can i free the allocated memory without losing the data that is stored?

Quote:

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Originally Posted by nik2

i cant use free(d) in main since we have no d in main, what should i do? how can i free the allocated memory without losing the data that is stored?

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Have a pointer in the main function to store the return value of foo, then use free on it after printing.

Quote:

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Originally Posted by laserlight

Have a pointer in the main function to store the return value of foo, then use free on it after printing.

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thanks i made this

Code:

---

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

char *foo(char *a, int n){
   
    char *d,*c;
    int i;
    d = (char *)malloc(n*sizeof(char));
    c=d;
    n=strlen(a)-n;
    for (i=n;i<strlen(a);i++){
        *(d+i-n) = *(a+i);
        }
        *(d+i-n) = '\0';
   
        return c;
   
    }
   
int main(void){
   
    char b[]="test";
    char *res;
    res = foo(b,3);
   
    printf("%s\n",res);
   
    free(res);
    printf("%s\n",res);

    getchar();
   
    return 0;
   
}

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why does it print the same thing after using free()
i thought it is supposed to print nothing since we free the allocated memory

thanks

Quote:

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Originally Posted by nik2

why does it print the same thing after using free()

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It so happens that the data is still in memory. What you did actually results in undefined behaviour.

Quote:

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Originally Posted by nik2

i thought it is supposed to print nothing since we free the allocated memory

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Freeing the allocated memory does not make its contents nothing.

Freeing the allocated memory does not generally change the memory. It just marks it for reuse. Accessing that memory is undefined behavior, and can result in an access violation.

Some compilers like MVS will mark freed memory in a debug build, but generally the behavior cannot be relied upon.

so should I free the allocated memory or not?

You should free it and not try to access it after it has been freed.

ok thanks everyone :)

Basically, you're just telling the OS: we're done with this memory.
Now, what the OS does with it after that is up to the OS. We don't care nor should care. We know the OS does its job, and as far as we are concerned, it is free memory--memory that no longer exists.

You need to include <stdlib.h> and get rid of the cast on the return value of malloc.

I have the same problem so i didnt make a new thread.

On the other hand one thing is still not clear.

Quote:

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Originally Posted by laserlight

Have a pointer in the main function to store the return value of foo, then use free on it after printing.

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But wouldnt it still leave "char* d" in foo() unfreed?

Thank you!

Quote:

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Originally Posted by Ducky

But wouldnt it still leave "char* d" in foo() unfreed?

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It is not the pointer that is being freed, but rather the memory for what the pointer points to. d points to what c points to, and since the value of c is returned, the pointer in the caller would point to the same thing.

It makes perfect sense.

Thank you Laserlight! :)