CS2110 Dynamic Allocation Chapter 19 Section 19.4 This file is called Mike-Ch19.ppt

CS2110 Outline We will learn how to allocate dynamic (that is, runtime-controllable) amounts of memory in C This will basically be making dynamically sized arrays

CS2110 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

CS2110 Every time a C programmer uses malloc incorrectly, a kitten explodes Only your C skills can save me! Picture CC license

CS2110 Mike’s Kitten Saving Challenge I count 16 different common ways to screw up malloc in this lecture Use the paper I’ve given you, and write down every one you can find as we go I am absolutely not kidding when I tell you that a malloc bug can waste numerous programmer-weeks 0

CS2110 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 1

CS2110 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 2

CS2110 malloc -- What happens? int foo(int n) { int *ip; ip = malloc(n * sizeof(int)); if(ip == NULL) { /* Handle Error! */ }... Stack Heap Data Code 40 bytes

CS2110 Anticipating Problems ip = malloc(10 * sizeof(int)); if(ip == NULL) What might go wrong? Be paranoid!

CS2110 Welcome to my nightmare! ip = malloc(10 * sizeof(int)); Code foolishly inserted here! if(ip == NULL) Be afraid. Be very afraid! 3

CS2110 Tricky but safe if((ip = malloc(10*sizeof(int))) == NULL) { /* Handle Error Here */ }

CS2110 Using the space int sample(int n) { int i; int *ip; if((ip = malloc(n*sizeof(int))) == NULL) { /* Handle Error Here */ } for(i = 0; i < n; i++) ip[i] = 0;...

CS2110 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)); 4

CS2110 After some calls to malloc Stack Heap Non-constant data Constant data Code

CS2110 What does runtime track? Address Size Notice that no record is made of the name of any pointer

CS2110 What happens? int *ip; ip = malloc(...);. free(ip);

CS2110 Prototypes void *malloc(size_t n); void free(void *p); void *realloc(void *p, size_t n); voidWhat is this mysterious void pointer?

CS2110 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

CS2110 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? 6

CS2110 Casting Usually casting is not required May be masking a problem int *ip; char *cp;... *cp = ‘x’; *ip = ??? *ip = 42; *cp = ??? ip cp

CS2110 Warnings badUsing void pointers as a crutch to get around casting is a “ bad ” thing! Malloc doesn’t care what you are doing with a block of memory it allocates to you. What you do with the memory is your responsibility badPassing in non-malloced 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!!! badDefinitely a bad thing!!! 67

CS2110 Persistent Data I char *foo(void) { static char ca[10]; return ca; } Anyone calling this function now has access to this block. Could be dangerous. Why? Note that this approach is not dynamic

CS2110 Example char *strFromUnsigned(unsigned u) { /* this is the important part */ static char strDigits[] = “?????”; char *pch; pch = &strDigits[5]; /* you can trust us that this really will convert an unsigned to a string */ do *--pch = (u % 10) + '0'; while((u /= 10) > 0); return pch; }

CS2110 Problem in use strHighScore = strFromUnsigned(HighScore);. strThisScore = strFromUnsigned(ThisScore); 8

CS2110 Persistent Data II char *foo(void) { char ca[10]; return ca; } Since ca was allocated on stack during function call pointer returned is now pointing to who knows what Bad 9

CS2110 Persistent Data III char *foo(void) { char *ca = malloc(...); /* error checking but no free */ return ca; }

CS2110 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 10

CS2110 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.

CS2110 Memory Management Some functions that call malloc –calloc –strdup –regcmp –others... C doesn’t do automatic memory management for efficiency reasons –If you want to manage memory...do it yourself!

CS2110 Memory Management Ok? int *ip, *ip2; ip = malloc(...); ip2 = ip; free(ip); *ip2 = 42; 1.Yes 2.No 3.Maybe

CS2110 Memory Management Ok? int *ip, *ip2; ip = malloc(...); ip2 = ip; free(ip); *ip2 = 42; 1.Yes 2.No 3.Maybe 11

CS2110 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? 1312

CS2110 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

CS2110 Realloc ptr in-use Before After

CS2110 Realloc: What might happen ptr unused Before After

CS2110 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?

CS2110 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... 14

CS2110 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

CS2110 Realloc Realloc may return –same pointer –different pointer –NULL Is this a good idea? cp = realloc(cp, n); 1.Yes 2.No 3.Sometimes

CS2110 Realloc Is this a good idea? cp = realloc(cp, n); No! If realloc returns NULL cp is lost Memory Leak! 15

CS2110 How to do it void *tmp; if((tmp = realloc(cp,...)) == NULL) { /* realloc error */ } else { cp = tmp; free(tmp); } NO! 16

CS2110 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.

CS2110 Kitty Challenge Reprise