Remove relative directory symbols from path (Unix/Linux)

#include <string.h>
#include <stdlib.h>
#include <errno.h>

static char *path_remove_reldir(const char * path, char *normalized_path)
{
#ifdef WIN32
    // Not implemented (I think nobody needs it)
    errno = EINVAL;
    return NULL;
#else
    int ilastsep = -1;
    int iread = 0;
    int iwrite = 0;

    if (path == NULL || normalized_path == NULL)
    {
        errno = EINVAL;
        return NULL;
    }

    do
    {
        if (path[iread] == PATH_SEPARATOR_CHAR || path[iread] == '\0')
        {
            int len_dir = iread - (ilastsep + 1);

            if (len_dir == 2 && path[ilastsep + 1] == '.' && path[ilastsep + 2] == '.')
            {
                // ".."

                // Find the separator of parent directory.
                iwrite -= 2;
                while (iwrite >= 0 && normalized_path[iwrite] != PATH_SEPARATOR_CHAR)
                {
                    iwrite--;
                }
                // Raise an error if not available.
                if (iwrite < 0)
                {
                    normalized_path[0] = '\0';
                    errno = EINVAL;
                    return NULL;
                }

                // Prevent erasing root directory symbol by NUL.
                if (iwrite == 0 && normalized_path[iwrite] == PATH_SEPARATOR_CHAR &&
                    path[iread] == '\0')
                {
                    iwrite++;
                }

                // overwrite separator/NUL character.
                normalized_path[iwrite] = path[iread];
                iwrite++;
            }
            else
            {
                if (len_dir == 1 && path[ilastsep + 1] == '.')
                {
                    // note that "./" must not be converted to "/".
                    if (iwrite > 0)
                    {
                        iwrite--;

                        // prevent erasing root directory symbol by NUL.
                        if (iwrite == 0 && normalized_path[iwrite] == PATH_SEPARATOR_CHAR &&
                            path[iread] == '\0')
                        {
                            iwrite++;
                        }

                        normalized_path[iwrite] = path[iread];
                        iwrite++;
                    }
                }
                else
                {
                    memcpy(&normalized_path[iwrite], &path[ilastsep + 1], len_dir);
                    iwrite += len_dir;
                    normalized_path[iwrite] = path[iread];
                    iwrite++;
                }
            }

            ilastsep = iread;
        }
    } while (path[iread++] != '\0');

    return normalized_path;
#endif
}