One problem that you should take care of when tackling this problem, is that symbolic link can point to a symbolic link. Then symbolic link should also point to symbolic link. And once again, and again and again…

So what can we do about it? First, lets start with lstat() system all. It will tell us is the file we’re interested in, is actually a symbolic link. To be more precise, st_mode field in struct stat will have flag S_IFLNK if specified file is a symbolic link. Note that it should be lstat() and not stat() – the later will return information about file the link points to and not about the link.

Next step is to call readlink(). This system call returns name of the file pointed to by specified symbolic link.

Finally, you should repeat the process recursively, for the pointed to file. Here is the code that does it:

int recursive_deleter(const char* filename)
{
  struct stat st;
  char buffer[1024];

  if (lstat(filename, &st)) {
    perror("stat");
    return -1;
  }     

  if (st.st_mode & S_IFLNK == S_IFLNK) {
    memset(buffer, 0, sizeof(buffer));
    if (readlink(filename, buffer, sizeof(buffer)) < 0) {
      perror("readlink");
      return -1;
    }           

    printf("File %s is a symbolic link to %s\n", filename, buffer);

    if (recursive_deleter(buffer))
      return 0;
  }     

  printf("Deleting %s\n", filename);

  if (unlink(filename)) {
    perror("unlink");
    return -1;
  }     

  return 0;
}

Have fun!

Related posts:

1. Hex dump functions
2. Direct IO in Python
3. How less processes its input

Anyway, here’s something else that I learned during one of the interviews.

I was asked to implement insertion into a linked list in C. Each list node has a number (num). Numbers in the list should be sorted in ascending order – node with lowest number should be first. Here is my naive implementation.

void list_insert(int num)
{
        list_node_t *n, *tmp, *prev;

        n = allocate_node();
        n->num = num;

        if (!head) {
                head = n;
                n->next = NULL;
        } else {
                tmp = head;
                prev = NULL;
                while (tmp) {
                        if (tmp->num >= n->num) {
                                if (prev == NULL) {
                                        head->next = n;
                                        n->next = NULL;
                                } else {
                                        n->next = tmp;
                                        prev->next = n;
                                }

                                break;
                        }

                        prev = tmp;
                        tmp = tmp->next;
                }

                if (!tmp) {
                        n->next = NULL;
                        prev->next = n;
                }
        }
}

Yeah, well it is pretty long, but that’s how you do it, right? You have to make sure that the list is not empty (line 8 ) – if it is empty, then new node should be placed at the head of the list. Then we have to find right place for the new node and insert it into its position (lines 14-29). Finally, if we didn’t find a good position for the new node, we should put the node at the end of the list (lines 31-34).

When I showed this to the examiner he asked me to rethink the routine. In particular he didn’t like number of if statements. After an hour of discussing various optimizations, this is what we came up with.

void list_insert(int num)
{
        list_node_t **tmp, *n;

        n = allocate_node();
        n->num = num;

        tmp = &head;
        while (*tmp) {
                if ((*tmp)->num >= n->num)
                        break;
                tmp = &(*tmp)->next;
        }

/*
        (*tmp) = n;
       n->next = (*tmp)->next;
*/

/*
        The code in the comment above is broken. Thanks to jagan
        for pointing this out. Here's the right code.
*/
        n->next = *tmp;
        *tmp = n;
}

There are several things that make this version better.

First of all, instead of having a pointer to current node (tmp) and previous node (prev), we have one pointer that corresponds to pointer to previous node. This way we can always get to the current node via next field in the previous node. Thus we don’t need two pointers anymore.

Also, this version uses pointer to pointer to node to traverse through the list. We don’t insert the node in the while loop anymore. Purpose of the while loop is to find right place for the new node. This spares if statement that tests if the list is empty and if statement in the loop.

Finally, once tmp points to the right place, we insert the node.

Second version runs faster too. I did a small program that tests both versions and it appears that the later version is something like 5% faster.

Related posts:

1. pthread mutex vs pthread spinlock
2. Todo list

I use two functions below quiet often. Both functions do hexadecimal dump of given buffer. First function is in C. Second function is in Python. Enjoy.

void hex_dump(char *data, int size, char *caption)
{
	int i; // index in data...
	int j; // index in line...
	char temp[8];
	char buffer[128];
	char *ascii;

	memset(buffer, 0, 128);

	printf("---------> %s <--------- (%d bytes from %p)\n", caption, size, data);

	// Printing the ruler...
	printf("        +0          +4          +8          +c            0   4   8   c   \n");

	// Hex portion of the line is 8 (the padding) + 3 * 16 = 52 chars long
	// We add another four bytes padding and place the ASCII version...
	ascii = buffer + 58;
	memset(buffer, ' ', 58 + 16);
	buffer[58 + 16] = '\n';
	buffer[58 + 17] = '\0';
	buffer[0] = '+';
	buffer[1] = '0';
	buffer[2] = '0';
	buffer[3] = '0';
	buffer[4] = '0';
	for (i = 0, j = 0; i < size; i++, j++)
	{
		if (j == 16)
		{
			printf("%s", buffer);
			memset(buffer, ' ', 58 + 16);

			sprintf(temp, "+%04x", i);
			memcpy(buffer, temp, 5);

			j = 0;
		}

		sprintf(temp, "%02x", 0xff & data[i]);
		memcpy(buffer + 8 + (j * 3), temp, 2);
		if ((data[i] > 31) && (data[i] < 127))
			ascii[j] = data[i];
		else
			ascii[j] = '.';
	}

	if (j != 0)
		printf("%s", buffer);
}

And the Python version. Python version is a bit smarter – it allows you to specify output stream.

def DumpBuffer(self, buf, length, caption="", dest=sys.stdout):
	def GetPrintableChar(str):
		if str.isalpha():
			return str
		else:
			return '.'

	dest.write('---------> %s <--------- (%d bytes)\n' % (caption, length))
	dest.write('       +0          +4          +8          +c           0   4   8   c\n')
	i = 0
	while i < length:
		if length - i > 16:
			l = 16
		else:
			l = length - i

		dest.write('+%04x  ' % i)
		s = ' '.join(["%02x" % ord(c) for c in buf[i:i + l]])
		dest.write(s)
		sp = 49 - len(s)
		dest.write(' ' * sp)
		s = ''.join(["%c" % GetPrintableChar(c) for c in buf[i:i + l]])
		dest.write(s)
		dest.write('\n')

		i = i + 16

Related posts:

1. Recursively deleting symbolic link and what it points to
2. Direct IO in Python
3. How to handle SIGSEGV, but also generate a core dump