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calgorithmlinked-listmergesort

Using mergesort to sort linked lists

I am currently working on the mergesort algorithm. I have three functions. list_sort_merge, mergelist and splitlist. list_sort_merge calls the other two to split and merge the list. I am having trouble getting this to work correctly. So what happens when I run GDB is that I get through the split function and get each number by itself such as the following example:

427
42.7
4.2.7

Then the mergesort comes along and segfaults me. What happens is the right_list and left_list are not being passed to mergesort. Meaning when mergesort goes to compare in the function comp_proc, it says that they are both NULL.

I think the problem is coming from the split function.

Can anyone see what I am doing wrong here?

Solution

  • I ended up reimplementing the list functions to suit myself because I couldn't work out how the interfaces to some of the functions in the code were supposed to work, even after the hints in the comments. The new code is still working with a doubly-linked list, but I renamed list_node_t to node_t and shortened current_list_size to size. There's a function to insert at the tail, and another to remove from the head; the other functions are fairly straight-forward.

    There are now three files: the main merge-sort source code (mergelist.c), including a test main() program and the revised versions of the three functions in the question; the list header (list.h) defining the interface to the list_t type; and the list source code (list.c) implementing the list_t type.

    The excitement is mostly in the main code, but the rest was necessary to get things to work. The sort sorts in descending order (largest value first). The crucial changes (that I recall, and other than the interface to the list functions) are highlighted in the code for mergelist.c. The key debugging tool was the dump_list() function. Something similar to that is a sine qua non for debugging sorts and lists and so on.

    Strictly, type names ending in _t are reserved for the implementation (see What does a type followed by _t (underscore t) represent?).

    Sample output

    Compiled with GCC 4.8.1 on Mac OS X 10.8.5, 64-bit pointers.

    List Before (0x7FEC21C039A0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03AE0, Size 10
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x7FEC21C03A20, Prev 0x7FEC21C039E0, Data 9
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 2
 5: Node 0x7FEC21C03A40, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A20, Data 7
 6: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x7FEC21C03A40, Data 5
 7: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 8: Node 0x7FEC21C03AA0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A80, Data 6
 9: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 0
10: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List -->>list_sort_merge() (0x7FEC21C039A0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03AE0, Size 10
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x7FEC21C03A20, Prev 0x7FEC21C039E0, Data 9
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 2
 5: Node 0x7FEC21C03A40, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A20, Data 7
 6: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x7FEC21C03A40, Data 5
 7: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 8: Node 0x7FEC21C03AA0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A80, Data 6
 9: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 0
10: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List Split-L (0x7FEC21C03B20)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03A40, Size 5
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x7FEC21C03A20, Prev 0x7FEC21C039E0, Data 9
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 2
 5: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 7
List Split-R (0x7FEC21C03B00)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03AE0, Size 5
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 3: Node 0x7FEC21C03AA0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A80, Data 6
 4: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 0
 5: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List -->>list_sort_merge() (0x7FEC21C03B20)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03A40, Size 5
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x7FEC21C03A20, Prev 0x7FEC21C039E0, Data 9
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 2
 5: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 7
List Split-L (0x7FEC21C03B60)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03A00, Size 3
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 9
List Split-R (0x7FEC21C03B40)
Head 0x7FEC21C03A20, Tail 0x7FEC21C03A40, Size 2
 1: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x000000000000, Data 2
 2: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 7
List -->>list_sort_merge() (0x7FEC21C03B60)
Head 0x7FEC21C039C0, Tail 0x7FEC21C03A00, Size 3
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C03A00, Prev 0x7FEC21C039C0, Data 3
 3: Node 0x7FEC21C03A00, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 9
List Split-L (0x7FEC21C03BA0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C039E0, Size 2
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x000000000000, Prev 0x7FEC21C039C0, Data 3
List Split-R (0x7FEC21C03B80)
Head 0x7FEC21C03A00, Tail 0x7FEC21C03A00, Size 1
 1: Node 0x7FEC21C03A00, Next 0x000000000000, Prev 0x000000000000, Data 9
List -->>list_sort_merge() (0x7FEC21C03BA0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C039E0, Size 2
 1: Node 0x7FEC21C039C0, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 1
 2: Node 0x7FEC21C039E0, Next 0x000000000000, Prev 0x7FEC21C039C0, Data 3
List Split-L (0x7FEC21C03BE0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C039C0, Size 1
 1: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x000000000000, Data 1
List Split-R (0x7FEC21C03BC0)
Head 0x7FEC21C039E0, Tail 0x7FEC21C039E0, Size 1
 1: Node 0x7FEC21C039E0, Next 0x000000000000, Prev 0x000000000000, Data 3
-->>list_sort_merge()
List List-L (0x7FEC21C03BE0)
Head 0x7FEC21C039C0, Tail 0x7FEC21C039C0, Size 1
 1: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x000000000000, Data 1
List List-R (0x7FEC21C03BC0)
Head 0x7FEC21C039E0, Tail 0x7FEC21C039E0, Size 1
 1: Node 0x7FEC21C039E0, Next 0x000000000000, Prev 0x000000000000, Data 3
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03BA0)
Head 0x7FEC21C039E0, Tail 0x7FEC21C039C0, Size 2
 1: Node 0x7FEC21C039E0, Next 0x7FEC21C039C0, Prev 0x000000000000, Data 3
 2: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 1
-->>list_sort_merge()
List List-L (0x7FEC21C03BA0)
Head 0x7FEC21C039E0, Tail 0x7FEC21C039C0, Size 2
 1: Node 0x7FEC21C039E0, Next 0x7FEC21C039C0, Prev 0x000000000000, Data 3
 2: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 1
List List-R (0x7FEC21C03B80)
Head 0x7FEC21C03A00, Tail 0x7FEC21C03A00, Size 1
 1: Node 0x7FEC21C03A00, Next 0x000000000000, Prev 0x000000000000, Data 9
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B60)
Head 0x7FEC21C03A00, Tail 0x7FEC21C039C0, Size 3
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C039C0, Prev 0x7FEC21C03A00, Data 3
 3: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 1
List -->>list_sort_merge() (0x7FEC21C03B40)
Head 0x7FEC21C03A20, Tail 0x7FEC21C03A40, Size 2
 1: Node 0x7FEC21C03A20, Next 0x7FEC21C03A40, Prev 0x000000000000, Data 2
 2: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 7
List Split-L (0x7FEC21C03B80)
Head 0x7FEC21C03A20, Tail 0x7FEC21C03A20, Size 1
 1: Node 0x7FEC21C03A20, Next 0x000000000000, Prev 0x000000000000, Data 2
List Split-R (0x7FEC21C03BA0)
Head 0x7FEC21C03A40, Tail 0x7FEC21C03A40, Size 1
 1: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x000000000000, Data 7
-->>list_sort_merge()
List List-L (0x7FEC21C03B80)
Head 0x7FEC21C03A20, Tail 0x7FEC21C03A20, Size 1
 1: Node 0x7FEC21C03A20, Next 0x000000000000, Prev 0x000000000000, Data 2
List List-R (0x7FEC21C03BA0)
Head 0x7FEC21C03A40, Tail 0x7FEC21C03A40, Size 1
 1: Node 0x7FEC21C03A40, Next 0x000000000000, Prev 0x000000000000, Data 7
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B40)
Head 0x7FEC21C03A40, Tail 0x7FEC21C03A20, Size 2
 1: Node 0x7FEC21C03A40, Next 0x7FEC21C03A20, Prev 0x000000000000, Data 7
 2: Node 0x7FEC21C03A20, Next 0x000000000000, Prev 0x7FEC21C03A40, Data 2
-->>list_sort_merge()
List List-L (0x7FEC21C03B60)
Head 0x7FEC21C03A00, Tail 0x7FEC21C039C0, Size 3
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C039E0, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C039E0, Next 0x7FEC21C039C0, Prev 0x7FEC21C03A00, Data 3
 3: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C039E0, Data 1
List List-R (0x7FEC21C03B40)
Head 0x7FEC21C03A40, Tail 0x7FEC21C03A20, Size 2
 1: Node 0x7FEC21C03A40, Next 0x7FEC21C03A20, Prev 0x000000000000, Data 7
 2: Node 0x7FEC21C03A20, Next 0x000000000000, Prev 0x7FEC21C03A40, Data 2
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B20)
Head 0x7FEC21C03A00, Tail 0x7FEC21C039C0, Size 5
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C03A40, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C03A40, Next 0x7FEC21C039E0, Prev 0x7FEC21C03A00, Data 7
 3: Node 0x7FEC21C039E0, Next 0x7FEC21C03A20, Prev 0x7FEC21C03A40, Data 3
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C039C0, Prev 0x7FEC21C039E0, Data 2
 5: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 1
List -->>list_sort_merge() (0x7FEC21C03B00)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03AE0, Size 5
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 3: Node 0x7FEC21C03AA0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A80, Data 6
 4: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 0
 5: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List Split-L (0x7FEC21C03B40)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03AA0, Size 3
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 3: Node 0x7FEC21C03AA0, Next 0x000000000000, Prev 0x7FEC21C03A80, Data 6
List Split-R (0x7FEC21C03B60)
Head 0x7FEC21C03AC0, Tail 0x7FEC21C03AE0, Size 2
 1: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x000000000000, Data 0
 2: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List -->>list_sort_merge() (0x7FEC21C03B40)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03AA0, Size 3
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A60, Data 8
 3: Node 0x7FEC21C03AA0, Next 0x000000000000, Prev 0x7FEC21C03A80, Data 6
List Split-L (0x7FEC21C03BA0)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03A80, Size 2
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x000000000000, Prev 0x7FEC21C03A60, Data 8
List Split-R (0x7FEC21C03B80)
Head 0x7FEC21C03AA0, Tail 0x7FEC21C03AA0, Size 1
 1: Node 0x7FEC21C03AA0, Next 0x000000000000, Prev 0x000000000000, Data 6
List -->>list_sort_merge() (0x7FEC21C03BA0)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03A80, Size 2
 1: Node 0x7FEC21C03A60, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 5
 2: Node 0x7FEC21C03A80, Next 0x000000000000, Prev 0x7FEC21C03A60, Data 8
List Split-L (0x7FEC21C03BE0)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03A60, Size 1
 1: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x000000000000, Data 5
List Split-R (0x7FEC21C03BC0)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A80, Size 1
 1: Node 0x7FEC21C03A80, Next 0x000000000000, Prev 0x000000000000, Data 8
-->>list_sort_merge()
List List-L (0x7FEC21C03BE0)
Head 0x7FEC21C03A60, Tail 0x7FEC21C03A60, Size 1
 1: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x000000000000, Data 5
List List-R (0x7FEC21C03BC0)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A80, Size 1
 1: Node 0x7FEC21C03A80, Next 0x000000000000, Prev 0x000000000000, Data 8
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03BA0)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A60, Size 2
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03A60, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x7FEC21C03A80, Data 5
-->>list_sort_merge()
List List-L (0x7FEC21C03BA0)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A60, Size 2
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03A60, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x7FEC21C03A80, Data 5
List List-R (0x7FEC21C03B80)
Head 0x7FEC21C03AA0, Tail 0x7FEC21C03AA0, Size 1
 1: Node 0x7FEC21C03AA0, Next 0x000000000000, Prev 0x000000000000, Data 6
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B40)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A60, Size 3
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A80, Data 6
 3: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x7FEC21C03AA0, Data 5
List -->>list_sort_merge() (0x7FEC21C03B60)
Head 0x7FEC21C03AC0, Tail 0x7FEC21C03AE0, Size 2
 1: Node 0x7FEC21C03AC0, Next 0x7FEC21C03AE0, Prev 0x000000000000, Data 0
 2: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x7FEC21C03AC0, Data 4
List Split-L (0x7FEC21C03B80)
Head 0x7FEC21C03AC0, Tail 0x7FEC21C03AC0, Size 1
 1: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x000000000000, Data 0
List Split-R (0x7FEC21C03BA0)
Head 0x7FEC21C03AE0, Tail 0x7FEC21C03AE0, Size 1
 1: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x000000000000, Data 4
-->>list_sort_merge()
List List-L (0x7FEC21C03B80)
Head 0x7FEC21C03AC0, Tail 0x7FEC21C03AC0, Size 1
 1: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x000000000000, Data 0
List List-R (0x7FEC21C03BA0)
Head 0x7FEC21C03AE0, Tail 0x7FEC21C03AE0, Size 1
 1: Node 0x7FEC21C03AE0, Next 0x000000000000, Prev 0x000000000000, Data 4
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B60)
Head 0x7FEC21C03AE0, Tail 0x7FEC21C03AC0, Size 2
 1: Node 0x7FEC21C03AE0, Next 0x7FEC21C03AC0, Prev 0x000000000000, Data 4
 2: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C03AE0, Data 0
-->>list_sort_merge()
List List-L (0x7FEC21C03B40)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03A60, Size 3
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A80, Data 6
 3: Node 0x7FEC21C03A60, Next 0x000000000000, Prev 0x7FEC21C03AA0, Data 5
List List-R (0x7FEC21C03B60)
Head 0x7FEC21C03AE0, Tail 0x7FEC21C03AC0, Size 2
 1: Node 0x7FEC21C03AE0, Next 0x7FEC21C03AC0, Prev 0x000000000000, Data 4
 2: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C03AE0, Data 0
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C03B00)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03AC0, Size 5
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A80, Data 6
 3: Node 0x7FEC21C03A60, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 5
 4: Node 0x7FEC21C03AE0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A60, Data 4
 5: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C03AE0, Data 0
-->>list_sort_merge()
List List-L (0x7FEC21C03B20)
Head 0x7FEC21C03A00, Tail 0x7FEC21C039C0, Size 5
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C03A40, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C03A40, Next 0x7FEC21C039E0, Prev 0x7FEC21C03A00, Data 7
 3: Node 0x7FEC21C039E0, Next 0x7FEC21C03A20, Prev 0x7FEC21C03A40, Data 3
 4: Node 0x7FEC21C03A20, Next 0x7FEC21C039C0, Prev 0x7FEC21C039E0, Data 2
 5: Node 0x7FEC21C039C0, Next 0x000000000000, Prev 0x7FEC21C03A20, Data 1
List List-R (0x7FEC21C03B00)
Head 0x7FEC21C03A80, Tail 0x7FEC21C03AC0, Size 5
 1: Node 0x7FEC21C03A80, Next 0x7FEC21C03AA0, Prev 0x000000000000, Data 8
 2: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A80, Data 6
 3: Node 0x7FEC21C03A60, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 5
 4: Node 0x7FEC21C03AE0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A60, Data 4
 5: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C03AE0, Data 0
<<--list_sort_merge()
List <<--list_sort_merge() (0x7FEC21C039A0)
Head 0x7FEC21C03A00, Tail 0x7FEC21C03AC0, Size 10
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 8
 3: Node 0x7FEC21C03A40, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A80, Data 7
 4: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A40, Data 6
 5: Node 0x7FEC21C03A60, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 5
 6: Node 0x7FEC21C03AE0, Next 0x7FEC21C039E0, Prev 0x7FEC21C03A60, Data 4
 7: Node 0x7FEC21C039E0, Next 0x7FEC21C03A20, Prev 0x7FEC21C03AE0, Data 3
 8: Node 0x7FEC21C03A20, Next 0x7FEC21C039C0, Prev 0x7FEC21C039E0, Data 2
 9: Node 0x7FEC21C039C0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A20, Data 1
10: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C039C0, Data 0
List After (0x7FEC21C039A0)
Head 0x7FEC21C03A00, Tail 0x7FEC21C03AC0, Size 10
 1: Node 0x7FEC21C03A00, Next 0x7FEC21C03A80, Prev 0x000000000000, Data 9
 2: Node 0x7FEC21C03A80, Next 0x7FEC21C03A40, Prev 0x7FEC21C03A00, Data 8
 3: Node 0x7FEC21C03A40, Next 0x7FEC21C03AA0, Prev 0x7FEC21C03A80, Data 7
 4: Node 0x7FEC21C03AA0, Next 0x7FEC21C03A60, Prev 0x7FEC21C03A40, Data 6
 5: Node 0x7FEC21C03A60, Next 0x7FEC21C03AE0, Prev 0x7FEC21C03AA0, Data 5
 6: Node 0x7FEC21C03AE0, Next 0x7FEC21C039E0, Prev 0x7FEC21C03A60, Data 4
 7: Node 0x7FEC21C039E0, Next 0x7FEC21C03A20, Prev 0x7FEC21C03AE0, Data 3
 8: Node 0x7FEC21C03A20, Next 0x7FEC21C039C0, Prev 0x7FEC21C039E0, Data 2
 9: Node 0x7FEC21C039C0, Next 0x7FEC21C03AC0, Prev 0x7FEC21C03A20, Data 1
10: Node 0x7FEC21C03AC0, Next 0x000000000000, Prev 0x7FEC21C039C0, Data 0

    mergelist.c

    The biggest problem in splitlist() was that the lists were not cleanly severed. If you tracked down the left list, you traversed the right list too. This could lead to problems — lots of problems, in fact.

    #include "list.h"
#include <assert.h>

static void mergelist(list_t *list_ptr, list_t *left_list, list_t *right_list);
static void splitlist(list_t *list_ptr, list_t *left_list, list_t *right_list);

static int comp_proc(data_t d1, data_t d2)
{
    if (d1 > d2)
        return +1;
    else if (d1 < d2)
        return -1;
    else
        return 0;
}

void list_sort_merge(list_t *list_ptr)
{
    if (list_ptr->size > 1)  // 1, not 0 — do not try splitting a singleton list.
    {
        dump_list(stdout, "-->>list_sort_merge()", list_ptr);  // Debug
        list_t *right_list = list_construct();
        list_t *left_list = list_construct();
        splitlist(list_ptr, left_list, right_list);
        dump_list(stdout, "Split-L", left_list);  // Debug
        dump_list(stdout, "Split-R", right_list); // Debug
        list_sort_merge(left_list);
        list_sort_merge(right_list);
        dump_list(stdout, "Sort-L", left_list);  // Debug
        dump_list(stdout, "Sort-R", right_list); // Debug
        list_ptr->head = NULL;
        list_ptr->tail = NULL;
        list_ptr->size = 0;    // Additional
        mergelist(list_ptr, left_list, right_list);
        list_destruct(right_list);
        list_destruct(left_list);
        dump_list(stdout, "<<--list_sort_merge()", list_ptr); // Debug
    }
}

static void mergelist(list_t *list_ptr, list_t *left_list, list_t *right_list)
{
    node_t *holder;
    fprintf(stdout, "-->>list_sort_merge()\n");
    dump_list(stdout, "List-L", left_list);
    dump_list(stdout, "List-R", right_list);
    while (left_list->size > 0 && right_list->size > 0)
    {
        assert(left_list->head != 0 && right_list->head != 0);
        /* Sort into descending order */
        if (comp_proc(left_list->head->data, right_list->head->data) > 0)
        {
            holder = list_remove_head(left_list);
            list_insert_tail(list_ptr, holder);
        }
        else
        {
            holder = list_remove_head(right_list);
            list_insert_tail(list_ptr, holder);
        }
    }
    while (left_list->size > 0)
    {
        holder = list_remove_head(left_list);
        list_insert_tail(list_ptr, holder);
    }
    while (right_list->size > 0)
    {
        holder = list_remove_head(right_list);
        list_insert_tail(list_ptr, holder);
    }
    fprintf(stdout, "<<--list_sort_merge()\n");
}

static void splitlist(list_t *list_ptr, list_t *left_list, list_t *right_list)
{
    if (list_ptr->size > 1)
    {
        size_t temp = 0;
        node_t *holder = list_ptr->head;
        right_list->size = list_ptr->size / 2;
        left_list->size = list_ptr->size - right_list->size;

        left_list->head = list_ptr->head;
        right_list->tail = list_ptr->tail;

        while (temp < left_list->size)
        {
            temp++;
            holder = holder->next;
        }

        /* Make sure the two lists are separate — a major issue */
        right_list->head = holder;
        left_list->tail = holder->prev;
        left_list->tail->next = NULL;
        left_list->head->prev = NULL;
        right_list->tail->next = NULL;
        right_list->head->prev = NULL;
    }
}

int main(void)
{
    int values[] = { 1, 3, 9, 2, 7, 5, 8, 6, 0, 4 };
    enum { NUM_VALUES = sizeof(values)/sizeof(values[0]) };
    list_t *list = list_construct();

    //dump_list(stdout, "Empty list", list);
    for (size_t i = 0; i < NUM_VALUES; i++)
    {
        node_t *node = node_construct(values[i]);
        list_insert_tail(list, node);
        //dump_list(stdout, "Inserting", list);
    }

    dump_list(stdout, "Before", list);
    list_sort_merge(list);
    dump_list(stdout, "After", list);

    list_destruct(list);

    return 0;
}

    list.h

    #ifndef LIST_H_INCLUDED
#define LIST_H_INCLUDED

#include <stdio.h>

typedef int data_t;

typedef struct node_t node_t;
struct node_t
{
    node_t *next;
    node_t *prev;
    data_t  data;
};

typedef struct list_t list_t;
struct list_t
{
    node_t *head;
    node_t *tail;
    size_t  size;
};

extern node_t *node_construct(data_t data);
extern void    node_destruct(node_t *node);

extern size_t  list_size(list_t *list);
extern void    list_insert_tail(list_t *list, node_t *node);
extern node_t *list_remove_head(list_t *list);
extern void    list_destruct(list_t *list);
extern list_t *list_construct(void);
extern void    dump_list(FILE *fp, const char *tag, list_t *list);

extern void list_sort_merge(list_t *list_ptr);

#endif /* LIST_H_INCLUDED */

    list.c

    #include "list.h"
#include <assert.h>
#include <inttypes.h>
#include <stdlib.h>

extern node_t *list_head(list_t *list);
extern node_t *list_tail(list_t *list);
extern void    list_destruct(list_t *list);
extern list_t *list_construct(void);

size_t list_size(list_t *list)
{
    return list->size;
}

void list_insert_tail(list_t *list, node_t *node)
{
    assert(list != 0);
    assert(node != 0);
    if (list->tail == 0)
    {
        assert(list->tail == 0 && list->head == 0 && list->size == 0);
        list->tail = node;
        list->head = node;
        node->prev = 0;
        node->next = 0;
        list->size = 1;
    }
    else
    {
        list->tail->next = node;
        node->prev = list->tail;
        node->next = 0;
        list->size++;
        list->tail = node;
    }
}

node_t *list_remove_head(list_t *list)
{
    assert(list != 0);
    node_t *node = list->head;
    if (list->head != 0)
    {
        assert(list->size > 0);
        list->head = node->next;
        if (node->next != 0)
            node->next->prev = 0;
        node->prev = 0;
        node->next = 0;
        list->size--;
    }
    return node;
}

void list_destruct(list_t *list)
{
    assert(list != 0);
    node_t *next;
    for (node_t *node = list->head; node != 0; node = next)
    {
        next = node->next;
        node_destruct(node);
    }
    free(list);
}

void dump_list(FILE *fp, const char *tag, list_t *list)
{
    assert(list != 0);
    fprintf(fp, "List %s (0x%.12" PRIXPTR ")\n", tag, (uintptr_t)list);
    fprintf(fp, "Head 0x%.12" PRIXPTR ", Tail 0x%.12" PRIXPTR ", Size %zu\n",
            (uintptr_t)list->head, (uintptr_t)list->tail, list->size);
    size_t i = 0;
    for (node_t *node = list->head; node != 0; node = node->next)
        fprintf(fp, "%2zu: Node 0x%.12" PRIXPTR ", Next 0x%.12" PRIXPTR ", Prev 0x%.12" PRIXPTR ", Data %d\n",
            ++i, (uintptr_t)node, (uintptr_t)node->next, (uintptr_t)node->prev, node->data);
}

list_t *list_construct(void)
{
    list_t *list = malloc(sizeof(*list));
    if (list != 0)
    {
        list->head = 0;
        list->tail = 0;
        list->size = 0;
    }
    return list;
}

node_t *node_construct(data_t data)
{
    node_t *node = malloc(sizeof(*node));
    if (node != 0)
    {
        node->data = data;
        node->next = 0;
        node->prev = 0;
    }
    return node;
}

void node_destruct(node_t *node)
{
    free(node);
}