Question

I’m trying to dynamically allocate memory for (what is essentially) a 2-dimensional array of chars – i.e – an array of strings.

My code is as follows:

typedef char LineType[MAX_CHARS+1];
LineType* lines;

int c = 0;
int N = 2;

lines = (LineType *) malloc (N * sizeof( LineType) );
do { 
    if (c > N ) {
        N *=2;
        lines = (LineType*) realloc (lines, N * sizeof( LineType));
    }

    .
    .
    .
    c++;

} while ( . . . )

This compiles fine but fails at runtime, giving a warning about possible HEAP CORRUPTION and breaking at dbgheap.c (in : _CrtIsValidHeapPointer)

What am I doing wrong? I figured it’s probably due to the mix of a fixed/dynamic dimensions in the data structure… But what is then the best way to declare and then dynamically allocate (and reallocate) memory for an array (of varying size) of strings (each of which is of a fixed size)?

Thanks a lot in advance

UPDATE 26/8/2012

I changed the code a bit to adjust it to people’s comments and suggestions. The problem still persists…

Answer 1

Assuming c is used to index into lines, you need to test for c >= N, not c > N.

As an aside, I suggest using a typedef to make your code more readable. I would also avoid the redundant allocation code:

typedef char fixed_string[MAX_CHARS + 1];

int c = 0;
int N = 0;
fixed_string *lines = NULL;

do {
    if (c >= N ) {
        N = N ? 2*N : 2;
        lines = (fixed_string*) realloc (lines, N * sizeof(fixed_string));
    }
    ⋮
    c++;
} while (…);

As a further aside, be careful when using a growth factor of 2. It leaves behind holes that can never be reused by the same array. A factor of 1.5 (3*N/2) is safer.

EDIT: I note from other comments that you experience the crash at the point of reallocation. This is consistent with writing past the end of the array. A debug memory allocator will fill the space immediately surrounding an allocated block of memory with special bytes and check that those bytes are preserved the next time it does something with that block of memory. The HEAP CORRUPTION message signals that you have corrupted those surrounding bytes by writing outside the memory you were given.