Hello World Program in C

Intermediate Programming Course Day 02_1

Keypoints

• Compiling and running a simple C program

Hello world

// hello_world.c
#include <stdio.h>

// Print "Hello, world!" followed by newline and exit
int main(void){
    printf("Helllo, world\n");
    return 0;
}

// Compilation command line 
$ gcc hello_world.c -std=c99 -pedantic -Wall -Wextra
$ ./a.out
// Output 
Hello, world!

• #include is a preprocessor directive, similar to import in Python
• **** means it's defined in stdio.h(standard input output package); **.h** is "header file"
• main is a function, every program has exactly one main
• int is the return type
• (void) says the main function takes no inputs/parameters
• printf prints a string to “standard out” (terminal)
• \n denotes the newline character
• return 0 means “program completed without error”
• gcc is the compiler; hello_world.c is the cpp file name;
• Switches in compilation command: -std=c99 is to use the c99 standard from 1999 year; -pedantic -Wall -Wextra is to get full feedback, i.e. error and warning (potential error)
• ./ is the execution sign; . is current directory
• a.out is the compilation default file name

Basic C/C++ programming workflow

• Edit file (using EMACS or VIM)
• Compile using GNUplot C compiler (gcc) to compile, link, and create excut
  • If compile-time errors reported, edit .c file and re-compile
• Run the excutable file
  • If run-time errors reported/detected, go back to edit step

Inside the compiler

• Step 1：preprocessor
  • Bring together all the code that belongs together
  • Process the directives that start with #, such as #include
    • also #define (to ensure library exists)
• Step 2：compiler
  • Turn human-readable source code into object code
  • Might yield warnings and errors if mistakes are “visible” to compiler
• Step 3：linker
  • Bring together all the relevant object code into a single excutable file
  • Might yield warnings and errors if relevant code is missing, naming conflicts, etc

    many .c files --compiler--> many .o files --linker--> one excutable file