Introduction to Computer Science and Programming in CSession 16: October 28, 2008Columbia UniversityAnnouncementsHomework 3 is out. Due November 6th before classEverybody check your homework 2 submission files. If something is wrong with your tar file, go to office hours and get help submitting.2ReviewPointer: variable that stores memory addressDeclare using:int * x_ptr; /* a pointer called x_ptr to an int*/Pointer operations:* <pointer> – the thing <pointer> points to & <variable> – the address of <variable>3TodayPointers and Arrays(correction on argv)Memory Management4Some vocabulary* operator is also known as dereferencea pointer references a variable in memory5Pointers and ArraysC blurs the distinction between pointers and arraysWhen we declare an arraychar A[10];what is A?A can be treated as a pointer to the first element of A6Pointers and ArraysIn other words, the following two lines are equivalent:char * array_ptr = &A[0];char * array_ptr = A;This also means the following:A[0] == *array_ptrA[1] == *(array_ptr+1)7Pointers and ArraysWhen we want a function to be able to modify the value of a variable, we pass it by referencesscanf(price, “$%f”, &dollars);Because arrays are basically pointers, this happens automatically when we pass arrays to functions.For example: strcpy(stringA, stringB);8Pointer ArithmeticWhat if A was an array of ints?A[1] == *(array_ptr+1) ??Yes. C automatically keeps pointer arithmetic in terms of the size of the variable type being pointed to.Be careful to keep track of what C does for you and what it does not.9*argv[]int main(int argc, char *argv[])Last class we were unsure if *argv[] is a pointer to an array or an array of pointers: If it was a pointer to an array, it would just be an array. So (char *) argv[](*argv)[1] points to the first character of the first word10Memory ManagementWe discussed before that C does not like to initialize arrays with variable sizes.To get around this, you can use stdlib.h’s malloc() command.malloc() stands for memory allocation.malloc(N) returns a pointer to an allocated block of memory of N bytes.11malloc()Typical usage:int N = 40000;char *giantString = malloc(N*sizeof(char));Returns a null pointer if malloc fails.When we are done with the memory, we can free it with:free(giantString);12ManagementWith malloc() and free(), we are able to use arbitrary amounts of memory and able to clear memory to save space.This is one aspect of C that makes some people consider C too powerful.Many other languages have automated memory management.13Memory Leaksint N = 40000;char *giantString = malloc(N*sizeof(char));strcpy(giantString, argv[1]);giantString = malloc(N*sizeof(char));Now a huge block of memory is allocated but the program has no way of finding it. If this code runs a lot, the amount of memory the program is using will keep