munmap_chunk(): invalid pointer error

Question

I don't understand how the freeing is working. I know this happens when I try to free memory twice. However, this is really stumping me.

I've tried to post just the relevant parts of the code.

FILE* file = fopen(path, "r");
if (file == NULL)
{
    error(500);
    return;
}

// load file's content
BYTE* content;
size_t length;
if (load(file, &content, &length) == false)
{
    error(500);
    return;
}

This is the load fucntion

bool load(FILE* file, BYTE** content, size_t* length)
{
    printf("\nLOAD STARTED\n");
    content = NULL;
    BYTE *data = NULL;
    int size = 0;
    while(!feof(file))
    {
        char ch = fgetc(file);
        size += 1;
        data = realloc(data, sizeof(BYTE) * (size));
        *(data + (size - 1)) = ch;
    }
    content = &data;
    *length = size;
    printf("\nLOAD ENDED\n");
    return true;
}

A little while later I'm calling free()

printf("\nFREEING CONTENT\n");
// free file's content
free(content);
printf("\nCONTENT FREED\n");

The printf statement FREEING CONTENT works after which I get the

munmap_chunk(): invalid pointer error.

@BLUEPIXY got the problem - you're never changing `size` so your `data` buffer never gets bigger. Also, [`while(!feof(file))` is (almost always) wrong](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) - and it *is* wrong in your code. — Andrew Henle, Apr 16 '16 at 13:10
@AndrewHenle Yeah I noticed that about size. After fixing it, I still get a segmentation fault while freeing. There is something wrong in the memory allocation. Although printing out *data like a string has all the contents of the file. — abhi, Apr 16 '16 at 13:20
You never check the `realloc` return value for `NULL`. Why do you expect it can't fail? — Jens, Apr 16 '16 at 13:23
And `content = NULL;` at the beginning is dangerous bullshit. `*content = NULL;` could work, but is not necessary (since you still have `data = NULL;` at the beginning. And (intend to) do `*content = data;` at the end.) — wildplasser, Apr 16 '16 at 13:33
@AbhishekManiyal *Although printing out *data like a string has all the contents of the file.* Well, you do read and store the data, since`realloc()` does return a pointer to a zero-byte block of memory. But `read()` overruns the end of that buffer and overwrites other memory. This is a perfect example of "undefined behavior" in that it seems to work, but other problems happen later. — Andrew Henle, Apr 16 '16 at 13:35

Jens · Answer 1 · 2016-04-16T13:39:46.660

0

This is the problem:

content = &data;

It assigns the address of a local variable that goes out of scope after the function returns. Since content is a function parameter, nothing gets written to where it points. Did you mean to write

*content = data;

instead? If so you, you should not set content = NULL because you want to use the address passed by the call load(file, &content, &length).

edited Apr 16 '16 at 13:39

answered Apr 16 '16 at 13:26

Jens

69,818
15
125
179

score 0 · Answer 2 · answered Apr 16 '16 at 14:06

0

I managed to fix it. content was basically just a char*. So, I tried this and it worked...

I changed content = &data to content = &(data[0])

It works. Appreciate all the inputs. :D

answered Apr 16 '16 at 14:06

abhi

431
5
11

munmap_chunk(): invalid pointer error

2 Answers2