Dynamic Allocation

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Dynamic Allocation
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Outline

• Dynamic Memory Allocation
• Dynamically Sized Arrays

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Dynamic Allocation

• Used when space requirements are unknown at
  compile time.
• Most of the time the amount of space required is
  unknown at compile time.
• Example
• Need space to allow user to enter some data
• 80 bytes? 132? 1? 256? 255?
• Three Friends
• malloc
• realloc
• free

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malloc

• int ip
• ip malloc(10 sizeof(int))
• if(ip NULL)
• / Handle Error! /
• Options for handling error
• Abort
• Ask again
• Save user data
• Ask for less
• Free up something

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malloc

• int ip
• ip malloc(10 sizeof(int))
• if(ip NULL)
• / Handle Error! /
• Note how incredibly bad it would be to use the
  assignment operator!
• The pointer would be set to NULL
• The error code would be skipped
• Some programmers use NULL ip

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malloc -- What happens?

• int foo(int n)
• int ip
• ip malloc(n sizeof(int))
• if(ip NULL)
• / Handle Error! /
• ...

Stack
Heap
40 bytes
Data
Code
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Anticipating Problems

• ip malloc(10 sizeof(int))
• if(ip NULL)
• What might go wrong?
• Be paranoid!

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Welcome to my nightmare!

• ip malloc(10 sizeof(int))
• Code foolishly inserted here!
• if(ip NULL)
• Be afraid.
• Be very afraid!

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Tricky but safe

• if((ip malloc(10sizeof(int))) NULL)
• / Handle Error Here /

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Using the space

• int sample(int n)
• int i
• int ip
• if((ip malloc(nsizeof(int))) NULL)
• / Handle Error Here /
• for(i 0 i lt n i)
• ipi 0
• ...

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Flexibility

• define MAX 10
• int ip
• ip malloc(MAX sizeof(int))
• What if we change the type of int ip???
• define MAX 10
• int ip
• ip malloc(MAX sizeof(ip))

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After some calls to malloc
Stack
Heap
Non-constant data
Constant data
Code
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What does runtime track?

• Address
• 1400
• 2400
• 3000
• 4000

• Size
• 200
• 300
• 100
• 500

Notice that no record is made of the name of any
pointer
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What happens?

• int ip
• ip malloc(...)
• .
• .
• .
• free(ip)

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Prototypes

• void malloc(size_t n)
• void free(void p)
• void realloc(void p, size_t n)
• What is this mysterious void pointer?

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void pointer

• Not originally in c
• Relatively recent addition
• Basically a generic pointer
• Intended for use in applications like free where
  the block of memory located at some address will
  be freed without any necessity of defining the
  type

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Powerful Dangerous

• void vp
• char cp
• int ip
• ip cp / illegal /
• Instead
• ip (int )cp
• Or
• vp cp / Legal, powerful and /
• ip vp / dangerous!!! /
• Why is this being done?

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Casting

• Usually casting is not required
• May be masking a problem
• int ip
• char cp
• ...
• cp x
• ip ???
• ip 42
• cp ???

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Warnings

• Using void pointers as a crutch to get around
  casting is a bad thing!
• Malloc doesnt care what you are doing with a
  block of memory it allocates to you. What you do
  with the memory is your responsibility
• Passing in random values to free is a bad thing!
• Free does not change contents of block that was
  freed
• Free does not change pointer
• After a call to free it is possible to do
  anything to the freed memory that was possible
  before the call!!!
• Definitely a bad thing!!!

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Persistent Data I

• char foo(void)
• static char ca10
• return ca
• Anyone calling this function now has access to
  this block. Could be dangerous. Why?
• Note that this approach is not dynamic

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Example

• char strFromUnsigned(unsigned u)
• static char strDigits ?????
• char pch
• pch strDigits5
• do
• --pch (u 10) '0'
• while((u / 10) gt 0)
• return pch

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Problem in use

• strHighScore strFromUnsigned(HighScore)
• .
• .
• .
• strThisScore strFromUnsigned(ThisScore)

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Persistent Data II

• char foo(void)
• char ca10
• return ca
• Since ca was allocated on stack during function
  call pointer returned is now pointing to who
  knows what
• Bad

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Persistent Data III

• char foo(void)
• char ca malloc(...)
• / error checking but no free /
• return ca

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Memory Leaks

• Memory leaks occur when the programmer loses
  track of memory allocated by malloc or other
  functions that call malloc
• void foo(void)
• char ca malloc(...)
• / no free /
• return
• Bad

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Memory Leaks

• Obviously a program that runs for a fraction of a
  second and then terminates will not cause a
  problem if it leaks a few bytes of memory.
• However, for real world programs memory leaks are
  not acceptable.
• For the educational purposes of this class your
  programs will be required to leak no memory!
• Programs submitted with memory leaks may incur
  severe penalties.

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Memory Management

• Some functions that call malloc
• calloc
• strdup
• regcmp
• others...
• C doesnt do automatic memory management for
  efficiency reasons
• If you want to manage memory...do it yourself!

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Memory Management

• Ok?
• int ip, ip2
• ip malloc(...)
• ip2 ip
• free(ip)
• ip2 42
• Yes
• No
• Maybe

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More...

• int ip, ip2 / Line 1 /
• ip malloc(...) / Line 2 /
• / Error checking here /
• ip2 ip / Line 3 /
• ip 4 / Line 4 /
• free(ip) / Line 5 /
• free(ip2) / Line 6 /
• Problem line?

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Realloc

• ptr realloc(ptr, num_bytes)
• What it does (conceptually)
• Find space for new allocation
• Copy original data into new space
• Free old space
• Return pointer to new space

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Realloc
ptr
in-use
Before
in-use
After
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Realloc What might happen
Before
ptr
unused
After
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Dynamic Allocation

• int ip malloc(...)
• malloc may allocate more space than requested
• Why?
• Efficiency
• Typically if you ask for 1 byte you will get 8.
• Given this line of code
• char cp malloc(1)
• Which is more likely
• Program will probably keep this memory as is
• Program will eventually realloc
• How much can you safely use?

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Safety

• Program should only use memory actually requested
• Big problem! Why?
• Program that oversteps bounds may work...
• Sometimes!
• Note...
• char cp malloc(1)
• ADDR SIZE
• cp 8 (maybe!)
• Now...
• realloc(cp,6)
• will return same pointer thus...

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Realloc

• May return same pointer passed to it without
  indicating any problem.
• Using memory beyond that which has been allocated
  may work
• Some of the time
• Normally it will work when tested by a student
  but will fail when tested by a TA (for a grade)
• Go figure
• Thus the No Mercy rule

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Realloc

• Realloc may return
• same pointer
• different pointer
• NULL
• Is this a good idea?
• cp realloc(cp, n)
• Yes
• No
• Sometimes

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Realloc

• Is this a good idea?
• cp realloc(cp, n)
• No!
• If realloc returns NULL cp is lost
• Memory Leak!

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How to do it

• void tmp
• if((tmp realloc(cp,...)) NULL)
• / realloc error /
• else
• cp tmp
• free(tmp)

NO!
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Additional Information

• realloc(NULL, n) ? malloc(n)
• realloc(cp, 0) ? free(cp)
• These can be used to make realloc work in a
  single loop design to build a dynamic structure
  such as a linked list.

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Example

• int size 0 / Size of "array" /
• int ip NULL / Pointer to "array" /
• int temp
• int i
• char buffer80
• while(fgets(buffer, 80, stdin) ! NULL)
• bufferstrlen(buffer) - 1 '\0'
• size
• if( (temp realloc(ip,
  sizesizeof(temp))) NULL)
• fprintf(stderr, "Realloc failure\n")
• exit(EXIT_FAILURE)
• ip temp
• ipsize - 1 strtol(buffer, NULL, 10)

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Dynamic AllocationWhat can go wrong?

• Allocate a block of memory and use the contents
  without initialization
• Free a block but continue to use the contents
• Call realloc to expand a block of memory and then
  once moved continue to use the old address
• Allocate a block and lose it by losing the value
  of the pointer
• Read or write beyond the boundries of the block
• FAIL TO NOTICE ERROR CONDITIONS

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Questions?
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