CMSC 104, Lecture 111 Introduction to C Topics l Compilation l Using the gcc Compiler l The Anatomy of a C Program l 104 C Programming Standards and Indentation Styles Reading l Sections ,

CMSC 104, Lecture 112 Writing C Programs l A programmer uses a text editor to create or modify files containing C code. l Code is also known as source code. l A file containing source code is called a source file. l After a C source file has been created, the programmer must invoke the C compiler before the program can be executed (run).

CMSC 104, Lecture 113 Using the C Compiler at UMBC l Invoking the compiler is system dependent. oAt UMBC, we have two C compilers available, cc and gcc. oFor this class, we will use the gcc compiler as it is the compiler available on the Linux system. (in fact cc is just a link to gcc)

CMSC 104, Lecture 114 Invoking the gcc Compiler At the prompt, type gcc -ansi -Wall pgm.c where pgm.c is the C program source file. l -ansi is a compiler option that tells the compiler to adhere to the ANSI C standard. l -Wall is an option to turn on all compiler warnings (best for new programmers).

CMSC 104, Lecture 115 The Result : a.out l If there are no errors in pgm.c, this command produces an executable file, which is one that can be executed (run). l The gcc compiler names the executable file a.out l To execute the program, at the prompt, type a.out l Although we call this process “compiling a program,” what actually happens is more complicated.

CMSC 104, Lecture Stages of Compilation Stage 1: Preprocessing o Performed by a program called the preprocessor o Modifies the source code (in RAM) according to preprocessor directives (preprocessor commands) embedded in the source code oStrips comments and whitespace from the code oThe source code as stored on disk is not modified.

CMSC 104, Lecture Stages of Compilation (con’t) Stage 2: Compilation oPerformed by a program called the compiler oTranslates the preprocessor-modified source code into object code (machine code) oChecks for syntax errors and warnings oSaves the object code to a disk file, if instructed to do so (we will not do this). oIf any compiler errors are received, no object code file will be generated. oAn object code file will be generated if only warnings, not errors, are received.

CMSC 104, Lecture Stages of Compilation (con’t) The Compiler consists of the following sub- systems: oParser - checks for errors oCode Generator - makes the object code oOptimizer - may change the code to be more efficient

CMSC 104, Lecture Stages of Compilation (con’t) Stage 3: Linking oCombines the program object code with other object code to produce the executable file. oThe other object code can come from the Run-Time Library (a collection of object code with an index so that the linker can find the appropriate code), other libraries, or object files that you have created. oSaves the executable code to a disk file. On the Linux system, that file is called a.out. oIf any linker errors are received, no executable file will be generated.

CMSC 104, Lecture 1110 Program Development Using gcc Source File pgm.c Program Object Code File pgm.o Executable File a.out Preprocessor Modified Source Code in RAM Compiler Linker Other Object Code Files (if any) Editor

CMSC 104, Lecture 1111 An algorithm for writing code Write the code using a text editor (eg emacs, vi, pico) Try to compile the code While there are still syntax errors Fix errors Try to compile the code Test the program Fix any semantic errors Compile the code Test the program

CMSC 104, Lecture 1112 Incremental Approach to Writing Code l Tips about writing code. l Write your code in incomplete but working pieces. l For instance: For your project oDon’t write the whole program at once. oJust write enough that you display the prompt to the user on the screen. oGet that part working first. oNext write the part that gets the value from the user, and then just print it out.

CMSC 104, Lecture 1113 Incremental Approach to Writing Code (continued) oGet that working. oNext change the code so that you use the value in a calculation and print out the answer. oGet that working. oMake program modifications:  perhaps additional instructions to the user  a displayed program description for the user  add more comments. oGet the final version working.

CMSC 104, Lecture 1114 A Simple C Program /* Filename: hello.c Author: Brian Kernighan & Dennis Ritchie Date written: ?/?/1978 Description: This program prints the greeting “Hello, World!” */ #include int main ( ) { printf (“Hello, World!\n”) ; return 0 ; }

CMSC 104, Lecture 1115 Anatomy of a C Program program header comment preprocessor directives (if any) int main ( ) { statement(s) return 0 ; }

CMSC 104, Lecture 1116 The Program Header Comment l A comment is descriptive text used to help a reader of the program understand its content. l All comments must begin with the characters /* and end with the characters */. These are called comment delimiters l The program header comment always comes first l The program header comment should include the filename, author, date written and a description of the program. Look at the class web page for details of the required contents of our header comment.

CMSC 104, Lecture 1117 Preprocessor Directive l Lines that begin with a # in column 1 are called preprocessor directives (comands) l Example: the #include directive causes the preprocessor to include a copy of the standard input/output header file stdio.h at this point in the code. l This header file was included because it contains information about the printf ( ) function that’s used in this program.

CMSC 104, Lecture 1118 int main ( ) l Every program must have a function called main. This is where program execution begins. l main() is placed in the source code file as the first function for readability. l The reserved word “int” indicates that main() returns an integer value. l The parenthesis following the reserved word “main” indicate that it is a function.

CMSC 104, Lecture 1119 The Function Body l A left curly brace -- { -- begins the body of every function. A corresponding right curly brace -- } -- must end the function. l The style is to place these braces on separate lines in column 1 and to indent the entire function body 3 to 5 spaces.

CMSC 104, Lecture 1120 printf (“Hello, World!\n”) ; l This line is a C statement. l It is a call to the function printf ( ) with a single argument (parameter), namely the string “Hello, World!\n”. l Even though a string may contain many characters, the string itself should be thought of as a single quantity. l Notice that this line ends with a semicolon. All statements in C end with a semicolon.

CMSC 104, Lecture 1121 return 0 ; l Because function main() returns an integer value, there must be a statement that indicates what this value is. l The statement return 0 ; indicates that main() returns a value of zero to the operating system. l A value of 0 indicates that the program successfully terminated execution. l Do not worry about this concept now. Just remember to use the statement.

CMSC 104, Lecture 1122 Good Programming Practices l C programming and indentation styles are available on the 104 course homepage. l You are expected to conform to these standards for all programming projects in this class and in CMSC 201. (This will be part of your grade for each project!) l These standards include: oNaming conventions oUse of whitespace oUse of Braces oComments

CMSC 104, Lecture 1123 Examples of Comment Styles /* a comment */ /*** another comment ***/ /*****/ /*A comment can be written in this * fashion to set it off from the * surrounding code. */

CMSC 104, Lecture 1124 More Comments /*******************************************\ * If you wish, you can put comments * * in a box. This is typically used for * * program header comments and for * * function header comments * \*******************************************/

CMSC 104, Lecture 1125 Use of Whitespace l Use blank lines to separate major parts of a source file or function l Separate declarations from executable statements with a blank line l Preprocessor directives, main(), and the braces are in column 1

CMSC 104, Lecture 1126 Use of Whitespace (continued) l All executable statements are indented one tab stop. How deep should my tabs be ? l Either 3 or 4 spaces. Choose whichever you like and use that same number consistently throughout your program. l 2 spaces are not enough for good readability, more than 4 causes the indentation to be too deep.

CMSC 104, Lecture 1127 Another C Program /***************************************** ** File: proj1.c ** Author: Joe Student ** Date: 9/15/01 ** SSN: ** Section: 0304 ** ** ** This program prompts the user for two integer values then displays ** their product. ** ***********************************************/

CMSC 104, Lecture 1128 Another C Program (con’t) #include int main() { int value1, value2, product ; printf(“Enter two integer values: “) ; scanf(“%d%d”, &value1, &value2) ; product = value1 * value2 ; printf(“Product = %d\n”, product) ; return 0 ; }