C-Programming Execution and Expressions

C Program Execution Process

1. Preprocessing: The C preprocessor performs tasks like macro expansion, file inclusion, and conditional compilation. It processes the #define, #include and #if directives and generates preprocessed source code.

2. Compilation: The preprocessed source code is compiled by the compiler to generate object code. The compiler checks the syntax, and semantics and generates object code (machine code + symbol table).

3. Assembly: The object files generated by the compiler are assembled by the assembler to produce the executable file. The assembler resolves external references and links object files.

4. Linking: The linker combines multiple object files and library files and generates an executable file. It resolves external function calls and variable uses.

5. Program Execution: The generated executable file is executed on the system. The operating system loads the executable into memory and the CPU starts executing the machine instructions.

History

• 1969 - Dennis Ritchie starts working at Bell Labs. He started developing the B programming language as an improved version of Thompson's BCPL language.

• 1970 - Ken Thompson starts developing the Unix operating system at Bell Labs.

• 1971 - The first version of Unix is written in assembly language.

• 1972 - Ritchie develops a C compiler to allow Unix to be written in a higher-level language. The language is initially called "B" and then renamed to "C".

• 1972 - The first C compiler and Unix written in C are released.

• 1973 - Version 6 Unix is the first to be written almost entirely in C. This starts the close relationship between C and Unix.

• 1974 - The C language is described in Ritchie's paper "The Development of the C Language".

• 1978 - Brian Kernighan and Dennis Ritchie publish "The C Programming Language" book. This popularizes C and establishes its syntax and semantics.

• 1983 - C Standardization committee formed. The ANSI X3J11 committee works to standardize C.

• 1989 - The ANSI X3.159-1989 standard for C is published and widely adopted. This makes C a truly portable language.

• 1990 - The ISO C standard ISO/IEC 9899 is published.

• In the 1990s - C became extremely popular and is used to write many important applications and operating systems like Linux kernel.

• 1994 - ISO publishes C9X to add features to C like function prototypes.

• 1999 - ISO publishes the C99 standard with many new features.

• 2011 - The current C11 standard is published with further improvements.

• Today - C remains an important programming language due to its performance, portability and ability to interface with hardware. It is used for systems programming, embedded systems and low-level applications.

Operators and Expressions

Variable

Variables are used to store values that can change during the execution of a program.

• A variable is a named memory location that can store different values at different times during the execution of a program.

  Syntax: datatype variablename;

• They are declared with a name and a data type. Some examples of variable declarations are:

    int x;  
    float y;
    char name;
  

• Variables are then assigned a value using the assignment operator (=). For example:

    x = 5;  
    y = 3.14;
    name = 'John';
  

• The data type of a variable determines the type of values it can store. Common data types are int, float, char, double, boolean, etc.

• Variables are allocated memory during compilation. The amount of memory allocated depends on the data type.

• Variables allow us to store data and manipulate them to perform various computations in a program. They are fundamental to programming.

Data Types

Data types determine

• Type of Data

• Size of memory

• Range of data

• Operations that can be performed on data

Modifiers

Modifiers specify the scope, storage duration and other properties of variables and functions in C.

The main modifiers in C are:

1. static - Specifies that a variable has static storage duration. The variable exists and retains its value for the lifetime of the program.

2. extern - Specifies an external variable or function. It allows a variable/function to be accessed from other files.

3. auto - Specifies a local variable with automatic storage duration. The variable is destroyed when the function exits. This is the default for variables.

4. register - Specifies that a variable should be stored in a register. It hints to the compiler to optimize the variable.

5. const - Specifies a constant variable. The value of a const variable cannot be changed.

6. volatile - Specifies that a variable can be modified by external factors. The compiler should not optimize access to it.

For functions, the main modifiers are:

1. static - Specifies a local function. The function is only visible within the file.

2. inline - Suggests to the compiler to replace calls to the function with the actual code. It optimizes the function.