Question

int main()
{
    char *s1, *sTemp;

    s1 = (char*)malloc(sizeof(char)*7);
    *(s1 + 0) = 'a';
    *(s1 + 1) = 'b';
    *(s1 + 2) = 'c';
    *(s1 + 3) = 'd';
    *(s1 + 4) = 'e';
    *(s1 + 5) = 'f';
    *(s1 + 6) = '\0';

    sTemp = (s1 + 3);

    free(sTemp); // shud delete d onwards. But it doesn't !!

    return 0;
}

Hello,

In the C above code sTemp should point to the 3rd cell beyond s1 ( occupied by ‘d’)
So on calling free(sTemp) i expect to have something deleted from this location onwards.

( I purposely mention ‘something‘ as the motive of my experiment initially was to find out till which location the free() ing works )

However i recieve a SIGABRT at the free().

How does free() know that this is not the start of the chunk. and correspondingly can we free up memory only from start of chunks? [ are they only the free-able pointers that free() can accept?? ]

Looking forward to replies 🙂

Answer 1

From the man pages

free() frees the memory space pointed
to by ptr, which must have been
returned by a previous call to
malloc(), calloc() or realloc().
Otherwise, or if free(ptr) has already
been called before, undefined
behaviour occurs.

Source: http://linux.die.net/man/3/free