Introduction to C CMSC 104 Spring 2014, Section 02, Lecture 6 Jason Tang

Introduction to C CMSC 104 Spring 2014, Section 02, Lecture 6 Jason Tang

Topics History of Programming Languages Compilation Process Anatomy of C CMSC 104 Coding Standards

Machine Code In the beginning, people wrote programs in machine code (i.e., binary) Error prone Not exactly fun Nobody does this anymore Machine code (obviously) only ran on specific machines 01111100111000110001100111010110 01111101000001000010000111010110 01111100011001110100001000010100 01001110100000000000000000100000

Assembly As a shortcut, programmers created pneumonic words for certain binary patterns Computer converts these words to binary code Still not fun, but better than nothing Still specific to particular machines Still in use today -- including by your friendly lecturer! MULLW R7,R3,R3 MULLW R8,R4,R4 ADD R3,R7,R8 BLR

High-Level Languages Fortran - scientific computing Oldest programming language still in use (1957) Lisp - based on lambda calculus Second oldest language still used today (1958) S1 = 3.0 Language no longer tied to S2 = 4.0 specific machine H2 = (S1 * S1) + (S2 * S2) (defun sumsquare (s1 s2) (+ (* s1 s1) (* s2 s2)))

C Derived from language B, 1972 Fairly efficient, close to the machine Language that Linux is written in Most widely used computer language today Portable assembly int sumsquare(int s1, int s2) { return ((s1*s1) + (s2*s2)); }

Why Different Computer Languages? Each language designed to meet certain goal(s): Speed Size (in bytes of memory) Ease of learning Ease of writing

Compilation Process In the end, computers still operate in binary Programmers use a text editor, such as nano, to create a source file containing source code They then run a compiler Compiler reads source file Converts source code within into machine code

Sample C Program Source File #include <stdio.h> int main(void) { printf("hello, world!\n"); return 0; } (This is a valid, syntactically and semantically correct C program.)

Actual File Contents #include <stdio.h> int main(void) { printf("hello, world!\n"); return 0; } ASCII encoding 0x23 0x69 0x6e 0x63 0x6c 0x75 0x64 0x65 0x20 0x3c 0x73 0x74 0x64 0x69 0x6f 0x2e...

Compilation Stage 1: Preprocessing Performed by preprocessor Performs textual substitutions, in RAM, according to preprocessor directives Does not modify original source code on disk Include files have filenames that end in.h More about include files over the course of semester

Compilation Stage 2: Translation Performed by compiler Translates preprocessor-modified code into object code Displays syntax errors [Can] save syntactically correct object code to object file Object files filenames end in.o

Compilation Stage 3: Linking Performed by linker Combines ( links together ) all object files to produce executable Displays semantic errors Can add object code from runtime libraries Saves executable to disk

Invoking the UMBC Compiler Suppose previous program was saved to file named foo.c -Wall means show To run preprocessor and compiler: all warnings gcc -Wall foo.c -c -o foo.o To run linker: gcc foo.o -o foo To execute program:./foo Note the./ Or combine these two steps: gcc -Wall foo.c -o foo

Compilation Summary you, the programmer foo.c preprocessed code in RAM text editor preprocessor foo.o foo compiler linker shell foo program is executed

Anatomy of a C Program #include <stdio.h> int main(void) { printf("hello, world!\n"); return 0; } 1. Copy and paste the above into a file named foo.c 2. Compile it: gcc -o foo foo.c 3. Run program:./foo

Anatomy of a C Program Preprocessor directive #include <stdio.h> int main(void) { printf("hello, world!\n"); return 0; } Entry point into program (i.e., algorithm name) Algorithm contents Boundary of algorithm

Another C Program /** * This is my second program. **/ #include <stdio.h> int main(void) { printf("will\nit\nblend?"); return 0; }

Another C Program /** * This is my second program. **/ #include <stdio.h> int main(void) { printf("will\nit\nblend?"); return 0; } Comment This printf line displays something to the screen

Comments Descriptive text used to aid the reader to understand contents of program Ignored by preprocessor (and compiler and linker) Begin with /* (forward slash, star) and ends upon next instance of */ (star, forward slash) All of your homework submissions must have file header comments

Preprocessor Directives Lines that begin with a # #include <stdio.h> means include contents of a special file named stdio.h at this point in code This is needed for the pre-made printf algorithm

int main(void) {... } Every algorithm in C must have a name This function is named main, takes 0 (void) inputs, and has 1 output (an integer) Every C program must have one, and only one, main() function

Function Bodies Beginning of function denoted by left brace ( { ) End of function denoted by right brace ( } ) By convention, statements within a function are indented by a tab See course website for other acceptable indentation styles

printf("hello, World!\n"); A single C statement Calls pre-made C algorithm (i.e., function) named printf with a single argument (i.e., input) That argument is the string Hello, World!\n Every C statement ends with a semicolon

"Hello, World!\n" String Sequence of characters enclosed by double-quotes Backslash introduces an escape sequence \n represents a newline (as if hitting Enter key) So what does "Will\nit\nblend?" represent?

return 0; Another C statement Because it is a statement, it must end with a semicolon Sets output of this function to the value zero Recall that main() function has a single integer output For this class, your main() always returns 0

General Layout of a C Program 1. File header comment 2. Preprocessor directive(s) 3. int main(void) { 3.1.statement 1; 3.2.statement 2;... 3.3.return 0; 4. }

File Header Comment At top of every source file Comment that contains: Your name and email address If a project, your partner s name and email address as well A description of what the program does

Coding Style See course web site for C programming standards and indentation guidelines All homework submissions must conform to these standards And thus bad style will negatively impact your grade Note: comments and indentation are not required during exams