c: freeing an mallocated object error

Question

Okay, so I have a method which I call. The called method returns a char*, which is allocated inside the called method. After having assigned the returned char* to a new char*, I want to free this, after using it. But Xcode complains and throws an error. It says that the object I try to free is not allocated, which I do not get, since I am sure I allocate it.

Here is my code:

void print_r()
{

    char *stateA = isActive(ruter_array[id]->flagg);

    //print out the results etc

    free(stateA);     <-----The program crashes here.


}

char * isActive(unsigned char a_flag)
{
    char *ret = malloc(5);

    if((a_flag & 1) == 1)
    {
        //is active
        ret = "yes";
        return ret;
    }
    ret = "no";
    return ret;

}

Its not important to understand what this does, but why is this freeing crashing?

`isActive` returns a pointer to either the string constant "yes" or the string constant "no", which are not mallocated objects because they're something else (string constants). It also mallocates an object and doesn't return a pointer to that object. (Also, "mallocate" is not a common term, but it works) — user253751, Oct 05 '16 at 20:05

kaylum · Accepted Answer · 2016-10-05T19:41:53.697

3

ret = "no";

That overwrites the pointer. Change to:

strcpy(ret, "no");

Same applies for the "yes" case.

Also, as pointed out by @DavidSchwartz, for your purposes it is probably not necessary to use dynamically allocated memory. Could just return the string literals directly:

char * isActive(unsigned char a_flag)
{
    if((a_flag & 1) == 1)
    {
        //is active
        return "yes";
    }
    return "no";  
}

And of course in that case the caller should not free the returned value.

edited Oct 05 '16 at 19:41

answered Oct 05 '16 at 19:35

kaylum

13,833
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Or don't use `malloc` or `free` at all, since you don't need to. – David Schwartz Oct 05 '16 at 19:38
Wow. I thought I knew everything I needed when it comes to pointers and memory in c. Apparently, that is not the case. Thanx. – Koen Oct 05 '16 at 19:44

score 2 · Answer 2 · answered Oct 05 '16 at 19:41

You allocate some memory and assign the first address of that memory to the pointer ret, but "no" and "yes" are string literals that are stored statically in memory at compile-time. When you do ret = "no", you change the pointer so that it now points to the first character of the string literal "no". So, now you have lost access to the memory that you allocated, and you have a memory leak. But the crash is because you are trying to free memory that was not allocated by malloc() or one of its friends.

score 1 · Answer 3 · edited May 23 '17 at 12:06

1

You can even use strdup:

char * isActive(unsigned char a_flag)
{
    return strdup((a_flag & 1) == 1 ? "yes" : "no");
}

As written here the pointer is still allocated through malloc so it can also be freed.

edited May 23 '17 at 12:06

Community

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answered Oct 05 '16 at 19:47

Marian Dolinský

3,312
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c: freeing an mallocated object error

3 Answers3