ME964High Performance Computing for Engineering Applications“There is no reason for any individual to have a computer in their home.” Ken Olson, president and founder, Digital Equipment Corporation, 1977.© Dan Negrut, 2012ME964 UW-MadisonQuick Overview of C ProgrammingJanuary 26, 2012Before We Get Started… Last time Course logistics & syllabus overview Today Quick overview of C Programming Essential reading: Chapter 5 of “The C Programming Language” (Kernighan and Ritchie) Acknowledgement: Slides on this C Intro include material due to Donghui Zhang and Lewis Girod First assignment will be emailed to you later today Due one week from today, at 11:59 PM Correction: Course website is http://sbel.wisc.edu/Courses/ME964/2012/2The “memory”Memory: similar to a big table of numbered slots where bytes of data are stored.The number of a slot is its Address.One byte Value can be stored in each slot.Some data values span more than one slot, like the character string “Hello\n”A Type provides a logical meaning to a span of memory. Some simple types are:char char char char char [10]char [10]char [10]char [10]intintintintfloatfloatfloatfloatint64_tint64_tint64_tint64_ta single character (1 slot)an array of 10 characterssigned 4 byte integer4 byte floating pointsigned 8 byte integerAddr Value01234 ‘H’ (72)5 ‘e’ (101)6 ‘l’ (108)7 ‘l’ (108)8 ‘o’ (111)9 ‘\n’ (10)10 ‘\0’ (0)11123What is a Variable?char x;char x;char x;char x;char y=‘e’;char y=‘e’;char y=‘e’;char y=‘e’;A Variable names a place in memory where you store a Value of a certain Type.Symbol Addr Value0123x 4Somegarbagey 5 ‘e’ (101)6789101112You first Declare a variable by giving it a name and specifying its type and optionally an initial valuedeclare vs. defineType is single character (char)extern? static? const?NameWhat names are legal?Initial valueVariable x declared but undefinedThe compiler puts x and y somewhere in memory.symbol table?4Multi-byte Variableschar x;char x;char x;char x;char y=‘e’;char y=‘e’;char y=‘e’;char y=‘e’;int z = 0x01020304; int z = 0x01020304; int z = 0x01020304; int z = 0x01020304; Different types require different amounts of memory. Most architectures store data on “word boundaries”, or even multiples of the size of a primitive data type (int, char)Symbol Addr Value0123x 4Some garbagey 5 ‘e’ (101)67z 8 49 310 211 1120x means the constant is written in hexAn int requires 4 bytespadding5In this picture, the architecture uses little-endian convention, since it stores the most significant byte firstMemory, a more detailed view… A sequential list of words, starting from 0. On 32bit architectures (e.g. Win32): each word is 4 bytes. Local variables are stored on the stack Dynamically allocated memory is set aside on the heap (more on this later…) For multiple-byte variables, the address is that of the least significant byte (little endian).6048StackHeapword 0word 1word 2Example…+3 +2 +1 +0V1V2V3V49009049089129169209249289329369409447Another ExampleWhat is the value of:- sizeC- sizeD- sizeDarr#include <iostream>int main() {char c[10];int d[10];int* darr;darr = (int *)(malloc(10*sizeof(int)));size_t sizeC = sizeof(c);size_t sizeD = sizeof(d);size_t sizeDarr = sizeof(darr);free(darr);return 0;}8NOTE: sizeof is a compile-time operator that returns the size, in multiples ofthe size of char, of the variable or parenthesized type-specifier that it precedes.Can a C function modify its arguments?What if we wanted to implement a function pow_assign() that modified its argument? Are these are equivalent?float p = 2.0;float p = 2.0;float p = 2.0;float p = 2.0;/* p is 2.0 here *//* p is 2.0 here *//* p is 2.0 here *//* p is 2.0 here */pow_assignpow_assignpow_assignpow_assign(p, 5);(p, 5);(p, 5);(p, 5);/* /* /* /* Is p Is p Is p Is p is 32.0 here is 32.0 here is 32.0 here is 32.0 here ? */? */? */? */float p = 2.0;float p = 2.0;float p = 2.0;float p = 2.0;/* p is 2.0 here *//* p is 2.0 here *//* p is 2.0 here *//* p is 2.0 here */p = p = p = p = powpowpowpow(p, 5);(p, 5);(p, 5);(p, 5);/* /* /* /* p p p p is 32.0 here is 32.0 here is 32.0 here is 32.0 here */*/*/*/void pow_assign(float x, uint exp)void pow_assign(float x, uint exp)void pow_assign(float x, uint exp)void pow_assign(float x, uint exp){{{{float result=1.0;float result=1.0;float result=1.0;float result=1.0;int i;int i;int i;int i;for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {result = result * x;result = result * x;result = result * x;result = result * x;}}}}x = result;x = result;x = result;x = result;}}}}Would this work?9????Native function, to use you need #include <math.h>In C you can’t change the value of any variable passed as an argument in a function call… void pow_assign(float x, uint exp)void pow_assign(float x, uint exp)void pow_assign(float x, uint exp)void pow_assign(float x, uint exp){{{{float result=1.0;float result=1.0;float result=1.0;float result=1.0;int i;int i;int i;int i;for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {for (i=0; (i < exp); i++) {result = result * x;result = result * x;result = result * x;result = result * x;}}}}x = result;x = result;x = result;x = result;}}}}// a code snippet that uses above // a code snippet that uses above // a code snippet that uses above // a code snippet that uses above // function// function// function// function{{{{float p=2.0;float p=2.0;float p=2.0;float p=2.0;pow_assign(p, 5);pow_assign(p, 5);pow_assign(p, 5);pow_assign(p, 5);// the value of p is 2 here…// the value of p is 2 here…// the value of p is 2 here…// the value of p is 2 here…}}}}In C, all arguments are passed by valueBut, what if the argument is the address of a variable? 10Keep in mind: pass by value requires the variable to be copied. That copy is then passed to the function. Sometime generating a copy can be expensive…C Pointers What is a pointer? A variable that contains the memory address of another variable or of a function In general, it is safe to assume that on 32 bit architectures pointers occupy one word Pointers to int, char, float, void, etc. (“int*”, “char*”, “*float”, “void*”), they all occupy 4 bytes (one word). Pointers: *very* many bugs in C programs are traced back to mishandling of pointers…11Pointers (cont.) The need for pointers