Functions & Pointers in C

Jordan Erenrich

erenrich@cs.cornell.edu

Functions

• Similar to Javafloat multiply(float a, float b) {

float ans = a * b;

reutrn ans;

}

Forward References• C compiler is one pass• Functions must be declared before they are used.

Typically:#include <stdlib.h>

float multiply(float, float); /* Forward reference */

int main (int argc, char* argv[]) {int x, y, z;x = 5;y = 2;z = multiply(x, y);

}

float multiply(float a, float b) {return a*b;

}

Passing by Value

• Function parameters are unchanged, outside of the function. i.e.,

void swap(float a, float b) {float temp;temp = a;a = b;b = temp

}

• This function is useless• How can we modify external data? Use pointers

What is a pointer?

• A reference to a physical memory address

Memory Address~~~~ 0x0000~~~~ 0x0001

0x0002 ~~~~ 0x0002~~~~ .~~~~ .~~~~ .~~~~ .~~~~ 0xffff

Using Pointersint x = 42;

int* iPtr; /* int* is a pointer type */iPtr = &x; /* & gets a pointer to a variable*/

/* Outputs “42 = 42” */printf(“%d = %d\n”, x, *iPtr);

x = 137;

/* Outputs “137 = 137” */printf(“%d = %d\n”, x, *iPtr);

Passing by Reference

• Use the * operator to dereference a pointervoid swap(float* a, float* b) {

float temp;temp = *a;*a = *b;*b = temp

}

• Therefore:int x = 42, y = 137;printf(“%d - %d”, x, y); /*Prints “42 –137” */

swap(&x, &y);printf(“%d - %d”, x, y); /*Prints “137 – 42” */

C array implementation

• Pointer to a contiguous block of memorychar myStr[50] = “Hello World”;

H e l l o w o r l d \0 ~ ~ ~

myStr

0x~~~

Declaring Arrays

• Static length is simplest #define MAXSTRING 1000 char myString[MAXSTRING]

• But, it’s size is…static

• Arrays can not be resized after a static declaration

Dynamically Allocation Arrays

• Explicitly allocate memory with mallocchar *myStr;int length = 1234;myStr = malloc(sizeof(char)*length)

• Must explicitly de-allocate memoryfree(myStr);

Arrays of Arrayschar **arrOfStrs; /*Double pointer*/int i, strLen = 1024, numStrings = 5;

/*Allocate the memory*/arrOfStrs = malloc(sizeof(char*)*numStrings);for(i = 0; i < numStrings; i++) {

arrOfStrs[i] = malloc(sizeof(char)*strLen);}. . ./*Free the memory*/for(i = 0; i < numStrings; i++) {

free(arrOfStrs[i]);}free(arrOfStrs);