Basics, Macros, and Variables

#include<stdio.h>

int main(int argc, char*[] argv){	// char** argv

printf("Hello World!");

return 0;
}

main()	// can be called with any number of parameters
main(void)	// no parameters can be present during call

										Static Libraries
											|
Preprocessor -> Compiler -> Assembler -> Linker -> Loader
	            .i 			.s 		   .o/.obj    .exe

SIGSEGV - Segment Violation (invalid access to valid memory, access to memory we don't have access to)            

SIGBUS - (access to an invalid address)

SIGABRT - an abort() call throws this

SIGFPE - Floating-point exceptions (divide by zero error)  

SIGILL - Illegal statement (CPU tried to execute an instruction it didn't understand)

SIGSYS - invalid argument passed to a system call

SIGTRAP - an exception triggered explicitly by debugger 

Integer - int, octal (0), hex (0x) 
Real - float(f or F), double(d or D)
Character - 'a'
String - "abhi"
Escape Sequences - '\0', '\"'. '\xFF'

Arithmetic - +, -, *, /, %
Relational - <, >, <=, >=, !=, ==
Logical - &&, ||, !
Bitwise - &, |, ~, ^
Assignment - =, compound assignments
Conditional - ?:
Unary - ++, --
Others - comma (,), dereferencing (*), addressof (&)

comma (,) 
{}, (), []

// this comment extends to next line \
printf("hello");
printf("world");

#define REPLACE(a, b) if(a < b) \
						a = b;

Preprocessor directive : #
#include<stdio.h>	// searches in standard list of dir 
#include "myheader" // searches in the current folder first, then standard list of dir

#define PI 3.1415  // macro as object

#define MUL(a, b) a * b // macro as function

#undef
#ifdef
#ifndef
#endif

#if
#endif
#elif
#else

#line n "filename"	//filename is optional

#warning here is a warning message to show	 //shows a compile-time warning message

#error here is your error description //throws compile time error

#pragma once 	//not standard
		warn
		startup
		exit

__LINE__
__TIME__
__DATE__
__FILE__
__FUNC__

#define string_this(a) #a

printf("%s", string_this(Hello));

#define combine_these(a, b) a##b

char* name = "Logan";

printf("%s", combine_these(na, me));

#define foo (foo + 10)
foo
// foo will be replaced by (foo + 10), not further

#define DEF #define A 999

DEF // won't work

#define A 99
printf("%s", A);
#undef  // or #define A 88
printf("%s", A);

#if X == 3
printf("Yes");
#else 
printf("No");
#endif

#define A define
#A B 99		// compile-time error

#if 0
printf("%s", "A");
printf("%s", "B");
printf("%s", "C");
#endif

#include <stdio.h>
#define MULTIPLY(a, b) a*b
int main()
{
    // The macro is expanded as 2 + 3 * 3 + 5, not as 5*8
    printf("%d", MULTIPLY(2+3, 3+5));
    return 0;
}
// Output: 16

#include<stdio.h>
#define A -B
#define B -C
#define C 5

int main()
{
  printf("The value of A is %d\n", A); 
  return 0;
} 

int main()
{
  printf("The value of A is %d\n", - -5);
  return 0;
}

 we want: eprintf ("%s:%d: ", input_file, lineno)
     →  fprintf(stderr, "%s:%d: ", input_file, lineno)

#define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)  

// Input: a.exe my code
#include<stdio.h>

int main(int argc, char* argv[])
{
	printf("%s ", argv[0]);
	printf("%s ", argv[1]);
	printf("%s ", argv[2]);
}

// Output: a.exe my code

int a = 9;		// both
int a;		// both, garbage value assigned

extern int a ;	// declaration only

# include <stdio.h>
  
int x = 0;

int f(){
   return x;
}

int g(){
   int x = 1;
   return f();
}

int main(){
  printf("%d", g());
  printf("\n");
  getchar();
}

// Structural nature of C
#include<stdio.h>

int main()
{
	int a = 9;
	{
		int b = 8;
	}

	printf("%d", a);
	printf("%d", b);

}

// Compile-time error: 'b' undeclared

if(1){
	// block scope
}

// file scope
int a = 5;

int main(){

}

int sum(int a, int b);
// or
int sum(int, int);

int sum(int a, int b)
{
	return a + b;
}

//DEMO 1: Global variable within the same file
#include<stdio.h>

int main()
{
	extern a;		//if we comment this, we get error
	printf("%d", a);
}

int a = 99;

//Output: 99

//DEMO 2: Global variable in another file

//file1.c
#include<stdio.h>

int main()
{
	extern int a;		//if type mismatch happens, 0 is fetched, also commenting leads to error
	printf("%d", a);
}

//file2.c
int a = 88;

// compile as => gcc file1.c file2.c
// Output: 88 

//DEMO 3: Extern works only on globally scoped variables
#include<stdio.h>

int main()
{
	int a = 9;
	{
	extern int a;		
	printf("%d", a);
	}
}

//Compile-time error, undefined reference to 'a'

extern int a;
extern int a;
extern int a;	//can do this mutiple times as its declaration and not definition

extern int a = 9;	//can't do this, compile-time error, extern can't have initializer

typedef long long ll;

typedef unsigned long long ull;

typedef struct* my_structure s;

typedef static int points;	//compile-time error, multiple storage classes are not allowed

const a;	//assumed to be int by default

int const a = 9;	//same as storage class, position doesn't matter
const int a = 9;

volatile int a = 9;
int volatile a = 9;

const volatile int a = 9;	//we can do this, although it makes no sense

//file1.c
#include<stdio.h>

int main()
{
	extern int a;
	printf("%d", a);
}

//file2.c
static int a = 88;

//Output: compile-time error, undefined reference to 'a'

#include<stdio.h>

int a;
int a;
int a = 9;
static int b;
static int b = 9;
int main()
{
	printf("%d", a);
}

//Ouput: 8