Question

This question is definitely related to this and it’s answer is what I based my function off of.

char *get_next_line(FILE *fp) {
char ch = 0;
int CUR_MAX = 4095;
char *buffer = (char*) malloc(sizeof(char) * CUR_MAX); // allocate buffer.
char *temp = (char*) malloc(sizeof(char) * CUR_MAX); // allocate buffer.
int count = 0;
int length = 0;

while ((ch != '\n')) {
  if (ch == '\377') { return NULL; }
  if(count ==CUR_MAX) {
    CUR_MAX *= 2;
    count = 0;
    if ((temp = realloc(buffer, CUR_MAX)) != NULL) {
      buffer = temp;
      free(temp);
    }
  }
  ch = getc(fp);
  buffer[length] = ch;
  length++;
  count++;
}

For some reason, when reading in very large strings I’m faced with:
glibc detected – realloc() invalid next size.

Is there something I’m missing here?

Thanks!

Answer 1

I believe you’re accessing memory outside of your malloced/realloced region.

Not setting count = 0 inside your while look should fix this.

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To try and elaborate…

When starting the formula:

count = 0
length = 0
CUR_MAX = 4095

These will increment until they reach 4095. Once we reach 4095, we will have:

count = 0
length = 4096
CUR_MAX = 8190

We will then increment until count is 8190. Shorly before then, we have:

count = 8100
length = 12196
CUR_MAX = 8190

Your array is only 8190 in length, but you’re dereferencing buffer[12196], which is an invalid index.

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Also, you probably want to handle the else case for when temp == NULL. This probably can be handled by an assert or a big old failure. And don’t free(temp) inside your success side. That’s freeing the memory you just attempted to allocate. And is causing your new segmentation fault.