access internal GPIO registers

I want to access the GPIO registers and write data to them and control them
directly. How do I do this in C? Are there any sites or any resources that
help with this, or explain it?

Accessing these registers directly seems to be everybody wants to do... ;)
Search for the mmap() system call to map this physical address space into
your process.

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

FILE *fp; 

int main(int argc, char** argv)
{

  printf("\n**********************************\n"
      "*  Welcome to PIN Blink program  *\n"
      "**********************************\n");
  
  //create a variable to store whether we are sending a '1' or a '0'
  char set_value[20]; 
  //Integer to keep track of whether we want on or off
  int toggle = 0;
  
  //Using sysfs we need to write "160" to /sys/class/gpio/export
  //This will create the folder /sys/class/gpio/gpio160
  if ((fp = fopen("/sys/class/gpio/export", "ab")) == NULL)
    {
      printf("Cannot open export file.\n");
      exit(1);
    }
  //Set pointer to begining of the file
    rewind(fp);
    //Write our value of "160" to the file
    strcpy(set_value,"160");
    fwrite(&set_value, sizeof(char), 3, fp);
    fclose(fp);
  
  printf("...export file accessed, new pin now accessible\n");
  
  //SET DIRECTION
  //Open the LED's sysfs file in binary for reading and writing, store file
pointer in fp
  if ((fp = fopen("/sys/class/gpio/gpio160/direction", "rb+")) == NULL)
  {
    printf("Cannot open direction file.\n");
    exit(1);
  }
  //Set pointer to begining of the file
  rewind(fp);
  //Write our value of "out" to the file
  strcpy(set_value,"out");
  fwrite(&set_value, sizeof(char), 3, fp);
  fclose(fp);
  printf("...direction set to output\n");
  
  
    strcpy(set_value,"1");
    

int ch;

while(ch!=3)
  {
printf("1:LED ON\n2:LED OFF\n3:exit");
printf("\nenter your choice\n");
scanf("%d",&ch);  
switch(ch){

case 1:  
  {if ((fp = fopen("/sys/class/gpio/gpio160/value", "rb+")) == NULL)
    {
      printf("Cannot open value file.\n");
      exit(1);
    }
      //Set pointer to begining of the file
      rewind(fp);
      //Write our value of "1" to the file 
      strcpy(set_value,"1");
      fwrite(&set_value, sizeof(char), 1, fp);
      fclose(fp);
      printf("...value set to 1...\n");
  
  } break;
    case 2:
    {if ((fp = fopen("/sys/class/gpio/gpio160/value", "rb+")) == NULL)
    {
      printf("Cannot open value file.\n");
      exit(1);
    }
      //Set pointer to begining of the file
      rewind(fp);
      //Write our value of "0" to the file 
      strcpy(set_value,"0");
      fwrite(&set_value, sizeof(char), 1, fp);
      fclose(fp);
      printf("...value set to 0...\n");
  
  } break;
    
}  
  }
  return 0;
}