C-Programming Execution and Expressions

C-Programming Execution and Expressions

The C Program Execution Process

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C Program Execution Process

Explain program execution process in C. - Sarthaks eConnect | Largest  Online Education Community

  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.

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