What is C?

• General-purpose, procedural computer programming language
• C supports structured programming and recursion
• Static type system
• Creates highly efficient and performant machine code
• Wikipedia: C (programming language)
• Open Hub Comparison
• TIOBE Index

Application Areas

• Operating systems (eg. Linux, Android)
• Implementation of programming languages (eg. cpython)
• High performance computations
• Application software (eg. GNOME Desktop, GIMP, ...)
• Embedded systems
• "Green Computing"

History

1969-1973

Development at Bell Labs as successor to B

In parallel with the beginnings of the Unix operating system

1978

Brian Kernighan and Dennis Ritchie (K&R) published
"The C Programming Language"

C99

Still not supported by all common C compilers

C11, C17

#include <stdio.h>

/* Hello World program written in C. */
int main(void) {
  printf("Hello, World!\n");  // output some text
  return 0;  // 0 means success
}

Visualize execution

Preprocessor Directives

Allow to include code declared elsewhere

#include <stdio.h>

/* Hello World program written in C. */
int main(void) {
  printf("Hello, World!\n");  // output some text
  return 0;  // 0 means success
}

Comments

// single line comment
int x = 5;  // comments can be added at the end of a line

/*
 * multiline comment
 * ...
 */

Main Function

Serves a special purpose in C programs:
program execution begins with main(.)

#include <stdio.h>

/* Hello World program written in C. */
int main(void) {
  printf("Hello, World!\n");  // output some text
  return 0;  // 0 means success
}

Return Value

Indicate success (0) or a failure code to the calling operating system shell

#include <stdio.h>

/* Hello World program written in C. */
int main(void) {
  printf("Hello, World!\n");  // output some text
  return 0;  // 0 means success
}

Executable program files are created from readable source code.

main.c

→

executable

Creating a C executable is a four step process.

1. Preprocessor
2. Compiler
3. Assembler
4. Linker

Example

Download hello.c

By default, all four stages (preprocessor, compiler, assembler and linker)
are done

clang hello.c  # or `gcc hello.c`
./a.out

clang hello.c -o hello
./hello

clang $INFILE -o $PROGRAM_NAME
gcc $INFILE -o $PROGRAM_NAME

1. Preprocessor

• Essentially a pure text processor
• Removes comments, performs includes etc.
• Usually never done separately

clang -E hello.c

2. Compiler

• Translates high level code to a lower level (assembly language)
• Parses the source code
• Performs type checking
• Usually never done separately
• Must recognize all identifiers (function declarations etc.)

clang -S hello.c

3. Assembler

• Translates assembly language to object code (machine language)
• Creates *.o files
• Usually done separately when working with multiple input files
  • Impossible to create executables without main(.)
  • Impossible to create executables with multiple main(.)

clang -c hello.c

4. Linker

• Combines object files into a unified executable program, resolving all symbols
• Should be run separately when working with multiple input files

clang -o hello hello.o