Write a C program to convert a string from upper$-$case characters to lower$-$case characters. Use the library function strlen$()$ to iterate through all characters of the string. Since strings in C are null$-$terminated character sequences, strlen can only determine the length of a string by stepping through the sequence until it hits a null character. Use the clock$()$ function to measure program performance.$\backslash$

$$ Define string as: char str$[6000]="$TO MAXIMISE SYSTEM PERFORMANCE OF A PROGRAM, BOTH THE PROGRAMMER AND COMPILER REQUIRE A MODEL OF THE TARGET MACHINE SPECIFYING HOW INSTRUCTIONS ARE PROCESSED AND THE TIMING CHARACTERISTICS OF DIFFERENT OPERATIONS. FOR EXAMPLE, THE COMPILER MUST KNOW THE TIMING INFORMATION TO BE ABLE TO DECIDE WHETHER IT SHOULD USE MULTIPLICATION OR A SERIES OF ADDITION AND SHIFT OPERATIONS."

$$ Repeat the above for double, triple, and quadruple string lengths

$$ Can this code be optimized? What is the optimization method called?

$$ Plot the string length vs time for both the normal and O$2$ optimization level with the gcc compiler. Comment on the results and nature of the graph.