QDPLF 5HVL]LQJlumetta.web.engr.illinois.edu/220-F20/slides/557-fully... · 2018. 3. 28. · ï l î...

3/27/2018

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University of Illinois at Urbana-ChampaignDept. of Electrical and Computer Engineering

ECE 220: Computer Systems & Programming

Fully Dynamic Allocation

ECE 220: Computer Systems & Programming © 2018 Steven S. Lumetta. All rights reserved. slide 1

Summary of Dynamic Resizing Pros and Cons

dynamically resized array◦ start with small constant◦multiply size by a constant as necessary

pros◦ easy to implement◦array uses contiguous memory

cons (quantified for 2× multiplier)◦ copying cost (≤ 2N for N players)◦waste space (~38%)

© 2018 Steven S. Lumetta. All rights reserved. slide 2ECE 220: Computer Systems & Programming

Player Deletion with Dynamic Resizing

What about deletion?

If order doesn’t matter,◦ copy last element over deleted element,◦ then reduce count◦ (requires constant time).

If order matters, deletion can be expensive.


Allocating Individual Players Requires a Pointer for Each

Can we use dynamic allocation ◦ to allocate one thing (a player) at a time◦ instead of resizing an array?

Yes, but first, we need to solve a problem:◦Every call to malloc returns a pointer.◦These pointers have no predictable relationship to one another.

◦So we need to store a pointer to each player.


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Where Can All the Pointers Go?

With dynamic resizing, we used one player pointer in the global data area:

static player_t* player_list = NULL;

Where can we put more pointers?

We can use ◦a dynamically resized array of pointers.◦But … have we really solved the problem in that case? (An array of pointers does reducing copying and waste space.)


Where Can We Put More Pointers?

Can we do something else?


player

player_list

playerplayer

player

Solution: Add a Pointer to the Player Struct!

What if we add a player_t*to the player struct?


player

player_list

playerplayer

player

Mark the End of the List by Pointing to Nothing

What about the last player’s pointer?

Set it to NULL.


player

player_list

playerplayer

player

NULL

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Singly-Linked Lists are Common for Unordered Groups

The data structure shown◦ is called a singly-linked list◦ (or, frequently, just a linked list).◦Singly-linked lists are usually used when order is not important.

How do we insert into a linked list?◦Specifically, where should we insert a new element: at the start, or the end?

Insert at the start: it’s faster.


Inserting into a Singly-Linked List Requires Two Changes

Make two changes. In what order?


player

player_list

playerplayer

player

NULL

new_player

1

2

Correct Ordering of Changes is Important

First, change the next field of the new player.

Otherwise, the old list is lost!

new_player->next = player_list;

player_list = new_player;

That’s all.


How Can We Remove Player p from the List?

Need to change pointer(link) marked in blue.


player

player_list

playerplayer

player

NULL

p

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Singly-Linked List Deletion is Linear in Size of List

Deletion is slower:◦ to delete player p◦ from a list that starts at player_list,◦we must walk over the list to find p,◦ then change pointer to p to p->next.

In general,◦with N things in the list,◦we examine on average N/2.


Modify Player Structure to Use Dynamic Allocation

Before writing player_delete,◦ let’s modify our player structure◦ to use dynamic allocation ◦ for the name* field.

*We treated the password field as a normalstring before, but technically it should be

hashed or encrypted to a fixed-length string.


Review: Example Player Structure

struct player_t {char name[32];char password[20]; int32_t age;int32_t num_games;int32_t score_dist[16];struct game_t* game;

};


char* name;

namepoints to a

dynamically allocated block of memory.player_t* next;

next is used for the linked list.

Modify player_init to Dynamically Allocate the Name

Then, in player_init, we can write…p->name = malloc (strlen (n) + 1);if (NULL == p->name) { return 0; }strcpy (p->name, n);

orp->name = strdup (n);if (NULL == p->name) { return 0; }

(recall that n is the new player’s name).


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First, Remove Player to Be Deleted from the List

Questions for you:

To delete “Y,” what needs to change?

The next field of player “Z.”


player_list

player

name

“Z”

player

name

“Y”

player

name

“X”

NULL

Free All Dynamically Allocated Data for the Player

Questions for you:

What needs to be freed to delete “Y?”

Both the player structure and the name.


player_list

player

name

“Z”

player

name

“Y”

player

name

“X”

NULL

Do Not Use Dynamic Data After Freeing It

Questions for you:

In what order?

First the name, then the player structure.


player_list

player

name

“Z”

player

name

“Y”

player

name

“X”

NULL

1

2

Ready to Write a Function to Delete a Player

Now we can write player_delete.

The function signature is:int32_t player_delete (player_t* p);◦p points to the player structure to remove from the list and free

◦ function returns 1 on success, or 0 on failure


3/27/2018

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Using a player_t**makes the code

simpler.

Use a player_t** to Find the Link to Change

player_t** find;for (find = &player_list;

p != *find;find = &(*find)->next) {if (NULL == *find) {

return 0;}

}


Point find first tothe pointer to thehead of the list.

Initialize find to Point to the Pointer to the Head



return 0;}

}


Once p == *find,we have found

the link to change.

Advance Until find Points to Pointer to Player to Delete



return 0;}

}


Advance by pointing find to thenext field of the structure to whichthe pointer find points to points.

Move find from next Field to next Field



return 0;}

}


3/27/2018

7

If we reach the end of thelist, p is not in the list, so fail.

For Safety, Check for End of List in Loop Body



return 0;}

}


Remember that find points to the pointer to be changed.

Free the name,then the player.

Return success.

Remove the Player, Free the Blocks, and Return Success

*find = p->next;

free (p->name);

free (p);

return 1;

}


Examine How player_delete Works in Detail

Let’s do a detailed example◦of player_delete execution◦ on a linked list of three players◦with variables shown in LC-3 memory.

Let’s first identify where each variable resides:◦ in the global data area,◦ in the heap, or◦ in the stack.


Dynamically Allocated Data Reside in the …

The linked list is shown below (head on left).

Where are these data(global data, heap, or stack)?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924

“Charlie”x5783 “Baddie”xABCDheap

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File-Scope Variables Reside in the …

The file-scope variableplayer_list points tothe head of the list.

Where is player_list stored?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924


player_listx6770x3998

global data

Local Variables Reside in the …

And where is local variable find?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findbitsxE743

stack

Parameter p is Close to Local Variable find


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findbitsxE743

stack

px8424xE748

Start the Function by Initializing find

Here’s the loop again.for (find = &player_list;


return 0;}

}


Start by initializing find.

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Initialize find to &player_list


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findbitsxE743

stack

px8424xE748

x3998

Continue Executing the Loop

What happens next?for (find = &player_list;


return 0;}

}


Execute the loop test.

Is *find Equal to p?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx3998xE743

stack

px8424xE748

*find is x6770




return 0;}

}


Execute the loop body.

3/27/2018

10

Is *find Equal to NULL?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx3998xE743

stack

px8424xE748

*find is x6770




return 0;}

}


Execute the loop update.

Where is (*find)->next?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx3998xE743

stack

px8424xE748

*find is x6770

Compiler Can Calculate Offsets for Each Field

struct player_t {char* name;char password[20]; int32_t age;int32_t num_games;int32_t score_dist[16];struct game_t* game;player_t* next;

};


+x00+x01+x15+x17+x19+x39+x3A

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Set find to &(*find)->next


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx3998xE743

stack

px8424xE748

x67AA

x67AA


And then …for (find = &player_list;


return 0;}

}


Back to the loop test.

Is *find Equal to p? What About NULL?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx67AAxE743

stack

px8424xE748

x67AA

*find is x9924


After the loop test and the loop body…for (find = &player_list;


return 0;}

}


Execute the loop update.

3/27/2018

12

Where is (*find)->next?


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx67AAxE743

stack

px8424xE748

x67AA

*find is x9924

x995E

Set find to &(*find)->next


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx67AAxE743

stack

px8424xE748

x67AA x995E

x995E


And then …for (find = &player_list;


return 0;}

}


Back to the loop test.

Is *find Equal to p? Yes! Loop Test Fails…


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx995ExE743

stack

px8424xE748

x67AA x995E

*find is x8424

3/27/2018

13

Overwrite *find with p->next

Here’s the code after the loop.

*find = p->next;

free (p->name);

free (p);

return 1;

}

Notice that we overwrite *find.


Set the Bits at *find to p->next


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x8424

x5783x9924



global data

findx995ExE743

stack

px8424xE748

x67AA x995Ex0000

“Baddie” is No Longer in the List!


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x0000

x5783x9924



global data

findx995ExE743

stack

px8424xE748

x67AA x995E

Finish the Rest of the Function

What’s next?

*find = p->next;

free (p->name);

free (p);

return 1;

}


Free the name, then player p.

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14

Free the Two Blocks of Dynamically Allocated Data


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

xABCDx8424

player

name

next x0000

x5783x9924



global data

findx995ExE743

stack

px8424xE748

x67AA x995E

1

2

The Function is Done

What’s next?

*find = p->next;

free (p->name);

free (p);

return 1;

}


Return success!

The List After the Function has Returned


“Gao”x8772

player

name

next x9924

x8772x6770

player

name

next x0000

x5783x9924

“Charlie”x5783heap


global data

stack

x67AA x995E

QDPLF 5HVL]LQJlumetta.web.engr.illinois.edu/220-F20/slides/557-fully... · 2018. 3. 28. · ï l î...

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