realloc and buffer overflow

Question

Why does result appear to not get reallocated?

  while (loc) {
    char nextLine[MAX_PATH_LEN + 30];
    sprintf(nextLine, "%s:%d\n", loc->item.pathname, loc->item.offset);
    DPRINTF('h', ("got next line\n"));
    while (spaceUsedUp + strlen(nextLine) > allocatedSize) {
      allocatedSize *= 2;
    }
    if (realloc(result, allocatedSize) == NULL) {
      perror("realloc");
    }
    DPRINTF('h', ("Next line length is %d\n", strlen(nextLine)));
    DPRINTF('h', ("Allocated size is %d\n", allocatedSize));
    DPRINTF('h', ("The size of the result is %d\n", strlen(result)));

    strcat(result, nextLine); // THIS LINE CAUSES THE BUFFER OVERFLOW                         

    spaceUsedUp += strlen(nextLine);
    DPRINTF('h', ("SpaceUsedUp is %d\n", spaceUsedUp));
    loc = loc->nextLocation;
  }

The output is:

got next line
Next line length is 21
Allocated size is 100
The size of the result is 0
SpaceUsedUp is 21
got next line
Next line length is 21
Allocated size is 100
The size of the result is 21
SpaceUsedUp is 42
got next line
Next line length is 21
Allocated size is 100
The size of the result is 42
SpaceUsedUp is 63
got next line
Next line length is 21
Allocated size is 100
The size of the result is 63
SpaceUsedUp is 84
got next line
Next line length is 21
Allocated size is 200
The size of the result is 84
*** buffer overflow detected ***: ./proj3/disksearch terminated

Answer 1

You are discarding the result returned by realloc. You need to assign that value to result. The typical usage looks like:

if ((tmp = realloc(result, allocatedSize)) == NULL) {
      perror("realloc");
      /* more error handling here, including (usually) freeing result or exiting */
} else {
      result = tmp;
}