CS61C Machine Structures Lecture 5 C Structs Memory Mangement 1 27 2006 John Wawrzynek www cs berkeley edu johnw www inst eecs berkeley edu cs61c CS 61C L05 C Structs 1 Wawrzynek Spring 2006 UCB C String Standard Functions int strlen char string compute the length of string int strcmp char str1 char str2 return 0 if str1 and str2 are identical how is this different from str1 str2 char strcpy char dst char src copy the contents of string src to the memory at dst The caller must ensure that dst has enough memory to hold the data to be copied CS 61C L05 C Structs 2 Wawrzynek Spring 2006 UCB Pointers 1 4 review Sometimes you want to have a procedure increment a variable What gets printed void AddOne int x x x 1 y 5 int y 5 AddOne y printf y d n y CS 61C L05 C Structs 3 Pointers 2 4 Wawrzynek Spring 2006 UCB review Solved by passing in a pointer to our subroutine Now what gets printed void AddOne int p p p 1 y 6 int y 5 AddOne y printf y d n y CS 61C L05 C Structs 4 Wawrzynek Spring 2006 UCB Pointers 3 4 But what if what you want changed is a pointer What gets printed void IncrementPtr int p p 1 p int A 3 50 60 70 int q A IncrementPtr q printf q d n q q 50 Aq 50 CS 61C L05 C Structs 5 60 70 Wawrzynek Spring 2006 UCB Pointers 4 4 Solution Pass a pointer to a pointer declared as h Now what gets printed void IncrementPtr int h q 60 h h 1 q Aq int A 3 50 60 70 int q A IncrementPtr q printf q d n q CS 61C L05 C Structs 6 50 60 70 Wawrzynek Spring 2006 UCB Dynamic Memory Allocation 1 4 C has operator sizeof which gives size in bytes of type or variable Assume size of objects can be misleading and is bad style so use sizeof type Many years ago an int was 16 bits and programs were written with this assumption What is the size of integers now sizeof is not a real function it does its work at compile time So char foo 3 sizeof int is valid But neither of these is valid in C char foo 3 myfunction int char foo 3 myfunction 7 CS 61C L05 C Structs 7 Wawrzynek Spring 2006 UCB Dynamic Memory Allocation 2 4 To allocate room for something new to point to use malloc with the help of a typecast and sizeof ptr int malloc sizeof int Now ptr points to a space somewhere in memory of size sizeof int in bytes int simply tells the compiler what will go into that space called a typecast malloc is almost never used for 1 var ptr int malloc n sizeof int This allocates an array of n integers CS 61C L05 C Structs 8 Wawrzynek Spring 2006 UCB Dynamic Memory Allocation 3 4 Once malloc is called the memory location contains garbage so don t use it until you ve set its value After dynamically allocating space we must dynamically free it free ptr Use this command to clean up CS 61C L05 C Structs 9 Wawrzynek Spring 2006 UCB Dynamic Memory Allocation 4 4 The following two things will cause your program to crash or behave strangely later on and cause VERY VERY hard to figure out bugs free ing the same piece of memory twice calling free on something you didn t get back from malloc The runtime does not check for these mistakes memory allocation is so performance critical that there just isn t time to do this and the usual result is that you corrupt the memory allocator s internal structures but you won t find out until much later on in some totally unrelated part of your code CS 61C L05 C Structs 10 Wawrzynek Spring 2006 UCB C structures Overview A struct is a data structure composed from simpler data types Like a class in Java C but without methods or inheritance struct point int x int y type definition void PrintPoint point p As always in C the argument is passed by value a copy is made printf d d p x p y struct point p1 0 10 PrintPoint p1 CS 61C L05 C Structs 11 Wawrzynek Spring 2006 UCB C structures Pointers to them Usually more efficient to pass a pointer to the struct The C arrow operator dereferences and extracts a structure field with a single operator The following are equivalent struct point p code to assign to pointer printf x is d n p x printf x is d n p x CS 61C L05 C Structs 12 Wawrzynek Spring 2006 UCB How big are structs Recall C operator sizeof which gives size in bytes of type or variable How big is sizeof p struct p char x int y 5 bytes 8 bytes Compiler may word align integer y CS 61C L05 C Structs 13 Wawrzynek Spring 2006 UCB Linked List Example Let s look at an example of using structures pointers malloc and free to implement a linked list of strings node structure for linked list struct Node char value struct Node next CS 61C L05 C Structs 14 Wawrzynek Spring 2006 UCB Linked List Example add a string to an existing list Node list add Node list char string struct Node node struct Node malloc sizeof struct Node node value char malloc strlen string 1 strcpy node value string node next list return node char s1 abc s2 cde Node theList NULL theList list add theList s1 theList list add theList s2 CS 61C L05 C Structs 15 Wawrzynek Spring 2006 UCB Linked List Example add a string to an existing list 2nd call Node list add Node list char string struct Node node struct Node malloc sizeof struct Node node value char malloc strlen string 1 strcpy node value string node next list return node node list NULL string cde CS 61C L05 C Structs 16 Wawrzynek Spring 2006 UCB Linked List Example add a string to an existing list Node list add Node list char string struct Node node struct Node malloc sizeof struct Node node value char malloc strlen string 1 strcpy node value string node next list return node list node NULL string cde CS 61C L05 C Structs 17 Wawrzynek Spring 2006 UCB Linked List Example add a string to an existing list Node list add Node list char string struct Node node struct Node malloc sizeof struct Node node value char malloc strlen string 1 strcpy node value string node next list return node list node CS 61C L05 C Structs 18 NULL string cde Wawrzynek Spring 2006 UCB Linked List Example add a string to an existing list Node list add Node list char string struct Node node struct Node malloc sizeof struct Node node value char malloc strlen string 1 strcpy node value string node next list return node list node NULL string cde cde CS 61C …
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