px4_autopilot/apps/systemlib/uthash/doc/utlist.txt

utlist: linked list macros for C structures
===========================================
Troy D. Hanson <thanson@users.sourceforge.net>
v1.9.5, November 2011

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Introduction
------------
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A set of general-purpose 'linked list' macros for C structures are included with
uthash in `utlist.h`.  To use these macros in your own C program, just
copy `utlist.h` into your source directory and use it in your programs.

  #include "utlist.h"

These macros support the basic linked list operations: adding and deleting
elements, sorting them and iterating over them.

Download
~~~~~~~~
To download the `utlist.h` header file, follow the link on the
http://uthash.sourceforge.net[uthash home page]. 

BSD licensed
~~~~~~~~~~~~
This software is made available under the 
link:license.html[revised BSD license]. 
It is free and open source. 

Platforms
~~~~~~~~~
The 'utlist' macros have been tested on:

 * Linux, 
 * Mac OS X, and 
 * Windows, using Visual Studio 2008, Visual Studio 2010, or Cygwin/MinGW.

Using utlist
------------

Types of lists
~~~~~~~~~~~~~~
Three types of linked lists are supported:

- *singly-linked* lists,
- *doubly-linked* lists, and
- *circular, doubly-linked* lists

Efficiency
^^^^^^^^^^
For all types of lists, prepending elements and deleting elements are
constant-time operations. Appending to a singly-linked list is an 'O(n)'
operation but appending to a doubly-linked list is constant time using these
macros.  (This is because, in the utlist implementation of the doubly-linked
list, the head element's `prev` member points back to the list tail, even when
the list is non-circular). Sorting is an 'O(n log(n))' operation. Iteration
and searching are `O(n)` for all list types.

List elements
~~~~~~~~~~~~~
You can use any structure with these macros, as long as the structure 
contains a `next` pointer. If you want to make a doubly-linked list, 
the element also needs to have a `prev` pointer.

  typedef struct element {
      char *name;
      struct element *prev; /* needed for a doubly-linked list only */
      struct element *next; /* needed for singly- or doubly-linked lists */
  } element;

You can name your structure anything. In the example above it is called `element`.
Within a particular list, all elements must be of the same type. 

List head
~~~~~~~~~
The list head is simply a pointer to your element structure. You can name it
anything. *It must be initialized to `NULL`*.

  element *head = NULL;

List operations
~~~~~~~~~~~~~~~
The lists support inserting or deleting elements, sorting the elements and 
iterating over them.

[width="100%",cols="10<m,10<m,10<m",grid="cols",options="header"]
|===============================================================================
|Singly-linked             | Doubly-linked              | Circular, doubly-linked
|LL_PREPEND(head,add);     | DL_PREPEND(head,add);      | CDL_PREPEND(head,add;
|LL_APPEND(head,add);      | DL_APPEND(head,add);       | 
|LL_CONCAT(head1,head2);   | DL_CONCAT(head1,head2);    | 
|LL_DELETE(head,del);      | DL_DELETE(head,del);       | CDL_DELETE(head,del);
|LL_SORT(head,cmp);        | DL_SORT(head,cmp);         | CDL_SORT(head,cmp);
|LL_FOREACH(head,elt) {...}| DL_FOREACH(head,elt) {...} | CDL_FOREACH(head,elt) {...}
|LL_FOREACH_SAFE(head,elt,tmp) {...}| DL_FOREACH_SAFE(head,elt,tmp) {...} | CDL_FOREACH_SAFE(head,elt,tmp1,tmp2) {...}
|LL_SEARCH_SCALAR(head,elt,mbr,val);| DL_SEARCH_SCALAR(head,elt,mbr,val); | CDL_SEARCH_SCALAR(head,elt,mbr,val);
|LL_SEARCH(head,elt,like,cmp);| DL_SEARCH(head,elt,like,cmp); | CDL_SEARCH(head,elt,like,cmp);
|===============================================================================

'Prepend' means to insert an element in front of the existing list head (if any),
changing the list head to the new element. 'Append' means to add an element at the
end of the list, so it becomes the new tail element. 'Concatenate' takes two
properly constructed lists and appends the second list to the first.  (Visual
Studio 2008 does not support `LL_CONCAT` and `DL_CONCAT`, but VS2010 is ok.)

The 'sort' operation never moves the elements in memory; rather it only adjusts
the list order by altering the `prev` and `next` pointers in each element. Also
the sort operation can change the list head to point to a new element.

The 'foreach' operation is for easy iteration over the list from the head to the
tail. A usage example is shown below. You can of course just use the `prev` and
`next` pointers directly instead of using the 'foreach' macros.
The 'foreach_safe' operation should be used if you plan to delete any of the list
elements while iterating. 

The 'search' operation is a shortcut for iteration in search of a particular
element. It is not any faster than manually iterating and testing each element.
There are two forms: the "scalar" version searches for an element using a
simple equality test on a given structure member, while the general version takes an
element to which all others in the list will be compared using a `cmp` function.


The parameters shown in the table above are explained here:

head::
  The list head (a pointer to your list element structure).
add::
  A pointer to the list element structure you are adding to the list.
del::
  A pointer to the list element structure you are deleting from the list.
elt::
  A pointer that will be assigned to each list element in succession (see
  example) in the case of iteration macros; also, the output pointer from
  the search macros.
like::
  An element pointer, having the same type as `elt`, for which the search macro 
  seeks a match (if found, the match is stored in `elt`). A match is determined
  by the given `cmp` function.
cmp::
  pointer to comparison function which accepts two arguments-- these are
  pointers to two element structures to be compared. The comparison function
  must return an `int` that is negative, zero, or positive, which specifies
  whether the first item should sort before, equal to, or after the second item,
  respectively. (In other words, the same convention that is used by `strcmp`).
  Note that under Visual Studio 2008 you may need to declare the two arguments
  as `void *` and then cast them back to their actual types.
tmp::
  A pointer of the same type as `elt`. Used internally. Need not be initialized.
mbr::
  In the scalar search macro, the name of a member within the `elt` structure which
  will be tested (using `==`) for equality with the value `val`.
val::
  In the scalar search macro, specifies the value of (of structure member
  `field`) of the element being sought.

Example
~~~~~~~
This example program reads names from a text file (one name per line), and
appends each name to a doubly-linked list. Then it sorts and prints them.

.A doubly-linked list
--------------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "utlist.h"

#define BUFLEN 20

typedef struct el {
    char bname[BUFLEN];
    struct el *next, *prev;
} el;

int namecmp(el *a, el *b) {
    return strcmp(a->bname,b->bname);
}

el *head = NULL; /* important- initialize to NULL! */

int main(int argc, char *argv[]) {
    el *name, *elt, *tmp, etmp;

    char linebuf[BUFLEN];
    FILE *file;

    if ( (file = fopen( "test11.dat", "r" )) == NULL ) {
        perror("can't open: "); 
        exit(-1);
    }

    while (fgets(linebuf,BUFLEN,file) != NULL) {
        if ( (name = (el*)malloc(sizeof(el))) == NULL) exit(-1);
        strncpy(name->bname,linebuf,BUFLEN);
        DL_APPEND(head, name);
    }
    DL_SORT(head, namecmp);
    DL_FOREACH(head,elt) printf("%s", elt->bname);

    memcpy(&etmp.bname, "WES\n", 5);
    DL_SEARCH(head,elt,&etmp,namecmp);
    if (elt) printf("found %s\n", elt->bname);

    /* now delete each element, use the safe iterator */
    DL_FOREACH_SAFE(head,elt,tmp) {
      DL_DELETE(head,elt);
    }

    fclose(file);

    return 0;
}
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