am-ash-or-am-i / c-programs

C Programming

Code syntax

Main function

void main() {
    // code
}

Header files

#include <stdio.h>

Input and output

#include <stdio.h>

void main() {
  int n;
  scanf("%d", &n);
  printf("Hello world!\n");
}

Structure in C

struct name {
    type member;
    type member;
    ...
};

e.g.

struct Student {
    int regNo;
    float marks;
    char section;
};

• struct Student is the name of the structure
• It occupies 4 + 8 + 4 = 16 bytes of memory

Using typedef

typedef struct Student {
    int regNo;
    float marks;
    char section;
} Student;

Union vs Struct

// All can be assigned and used at the same time
struct struct_example {
  int integer;
  float decimal;
  char name[20];
};

// Only any 1 of them can be assigned/used at a time
union union_example {
  int integer;
  float decimal;
  char name[20];
};

int main() {
  struct struct_example s;
  union union_example u;

  s.integer = 10;
  s.decimal = 10.5;
  strcpy(s.name, "Hello World!");

  u.integer = 10;
  u.decimal = 10.5;
  strcpy(u.name, "Hello World!");

  printf("struct_example: %d %f %s\n", s.integer, s.decimal, s.name);
  printf("union_example: %d %f %s\n", u.integer, u.decimal, u.name);

  return 0;
}

Storage classes in c

• Auto (Used by default)

  • Slowest access

  • Stored in stack

    int a = 32; 

• Register

  • Fastest access

  • Used for variables that are accessed more times during program

    register int x = 324;

• Static

  • Faster than auto

  • Uses data segment

  • Initialized only one during compilation

    static int x=324;

• Extern

  • Global variable, can be accessed from any function

  • Can be used to access global variable from another file

    • file1.c

    int x = 324;
    int main(){
        extern int val;
        printf("Global var %d", val);
    }

    • file2.c

    # include <stdio.h>
    # include "file1.c"
    extern int x;
    int main(){
        printf("Global var %d", x);
    }

Pointers

• Wild pointer: pointer that has not been initialized

  • Uninitialized pointers are known as wild pointers because they point to some arbitrary memory location and may cause a program to crash or behave unexpectedly.

    int *ptr;
    *ptr = 32523;
    printf("%d", *ptr);

• Dangling pointer: pointer that points to a memory location that has been deleted (or freed)

    int *ptr = (int *)malloc(sizeof(int));
    *ptr = 32523;
    free(ptr);
    printf("%d", *ptr);

• Null pointer: pointer that has been initialized to NULL

  • NULL is a macro defined in <stdio.h> header file. It is an implementation-defined null pointer constant. It may be defined as 0 or ((void *)0). It is guaranteed that if NULL is defined as 0 then it will always be a null pointer constant.

    int *ptr = NULL;
    printf("%d", *ptr);