// C program to illustrate open system call
#include<stdio.h>
#include<fcntl.h>


int main()
{    
    int fd = open("sample.c", O_RDONLY);
     
    printf("\nfd = %d\n", fd);
     
    if (fd ==-1)
    printf("\n fail to open\n");                
    
    return 0;
}

output1(if file exist):

fd=3

output2 (if file doesnot exist):


fd=-1

fail to open




// C program to illustrate close system Call
#include<stdio.h>
#include <fcntl.h>
#include<stdlib.h>
int main()
{
    int fd = open("open.c", O_RDONLY);
    if (fd < 0)
    {
        printf("\n no such file\n");
        //exit(1);
    
    printf("opened the fd = % d\n", fd);
    } 
    // Using close system Call
    if (close(fd) < 0)
     {
         printf("error in closing the file\n");
         exit(1);
    }
    printf("closed the file fd=%d\n",fd);
}

output1:

closed the file fd = 3

output2:

no such file
opened the fd=-1
error in closing the file



// C program to illustrate close system Call
#include<stdio.h>
#include<fcntl.h>
int main()
{
    // assume that kits.txt is already created
    int fd1 = open("kits.txt", O_RDONLY);
    printf("fd1 = % d\n", fd1);
    close(fd1);
     
    // assume that cse.txt is already created
    int fd2 = open("cse.txt", O_RDONLY);
     
    printf("fd2 = % d\n", fd2);
    exit(0);
}

output1:
fd1 = 3
fd2 = 3


// C program to illustrate read system Call
#include<stdio.h>
#include <fcntl.h>
#include<stdlib.h>
int main()
{
int fd, sz;
char c[100];
 
fd = open("cse.txt", O_RDONLY);
if (fd < 0) 
{
printf("error in opening file"); 
exit(1); 
}
 
sz = read(fd, c, 10);
printf("called read(% d, c, 10). returned that  %d bytes were read.\n", fd, sz);
c[sz] = '\0';
printf("Those bytes are as follows: %s\n", c);
}


// C program to illustrate read system Call
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <stdio.h>
int main()
{
 int file=0,sz,i;
char buffer[19];
 if((file=open("testfile.c",O_RDONLY)) <= -1)     
  printf("file opening error");
  printf("\n file descriptor = % d\n", file);
        
 sz=read(file,buffer,19);
for(i=0;i<sz;i++)        
printf("%c",buffer[i]);
printf("\n");
return 0;
}


output1:

file opening error
file descriptor = -1

output2:

create a file named testfile.c with the contents CSE Department KITS Singapur

file descriptor = 3
CSE Department KITS

// C program to illustrate write system Call
#include<stdio.h>
#include <fcntl.h>
#include<string.h>
#include<stdlib.h>
int main()
{
int sz;

int fd = open("foo.txt", O_WRONLY | O_CREAT);
if (fd < 0)
{
    printf("file opening error");
    exit(1);
}

sz = write(fd, "hello CSE A Section\n", strlen("hello CSE A Section\n"));

printf( "%d",sz);

close(fd);
}

output1:
20
check the file foo.txt

output2:
file opening error



// C program to illustrate lseek system Call

#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
int main()
{
int n,f;
char buff[100];
f=open("sample.txt",O_RDWR);
read(f,buff,10);
write(1,buff,10);
printf("\n");
lseek(f,7,SEEK_CUR);//skips 7 characters from the current position
read(f,buff,8);
write(1,buff,8);
printf("\n");
}

create a file named sample.txt with the contents "Computer Science and Engineering"

Output:
"Computer
 and Eng

// C program to illustrate lseek system Call

#include<unistd.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<stdio.h>
int main()
{
int n,f,f1;
char buff[100];
f=open("seeking.txt",O_RDWR);
f1=lseek(f,10,SEEK_SET);
printf("Pointer is at %dth position\n",f1);
read(f,buff,15);
write(1,buff,15);
printf("\n");
}
Output:

Pointer is at 10th Position
Science and Eng



/* C program to illustrate opendir system call  */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <dirent.h>

int main()
{
  
  DIR *d;
 
  d = opendir("/home/kits/CSE2021");
  if (d == NULL) 
{
    printf("Couldn't open \n");
    exit(1);
  }
else
printf("\ndirectory opened successfully\n");

    closedir(d);
  return 0;
}

output1:
Couldn't open


output2:  if /home/kits/CSE2021 directory exists
directory opened successfully


// Unix system calls opendir, readdir

#include <stdio.h>
#include <dirent.h>

int main()
{
          struct dirent *de; 

          DIR *dr = opendir(".");

          if (dr == NULL)
          {
                   printf("Could not open current directory" );
                   return 0;
          }

          while ((de = readdir(dr)) != NULL)
                  printf("%s\n", de->d_name);

          closedir(dr);
          return 1;


}
output:
list the contents  of the current directory


/* C program to illustrate readdir system call  */

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
#include <errno.h>
#include <string.h>

int main(int c, char *v[]) {
    DIR *myDirectory;
    struct dirent *myFile;

    if (c == 2) {
        myDirectory = opendir(v[1]);
        if (myDirectory) {
            puts("OK the directory is opened, let's see its files:");
            while ((myFile = readdir(myDirectory)))
                printf("%s\n", myFile->d_name);
            /*
             ** closedir
             */
            if (closedir(myDirectory) == 0)
                puts("The directory is now closed.");
            else
                puts("The directory can not be closed.");
        } else if (errno == ENOENT)
            puts("This directory does not exist.");
        else if (errno == ENOTDIR)
            puts("This file is not a directory.");
        else if (errno == EACCES)
            puts("You do not have the right to open this folder.");
        else
            puts("That's a new error, check the manual.");
    } else
        puts("Sorry we need exactly 2 arguments.");
    return (0);
}


Output1:

./a.out .
OK the directory is opened, let's see its files:

list the contents of the current directory

The directory is now closed.

Output2:

create a directory test and a file named example in that test directory
./a.out test
OK the directory is opened, let's see its files:
.
..
example
The directory is now closed.

Output3:

create a subdirectory ctest in that test directory
./a.out test
OK the directory is opened, let's see its files:
.
ctest
..
example
The directory is now closed.


Output4:

./a.out

Sorry we need exactly 2 arguments.

//C program to illustrate stat system call
/* C program to find file permission, size, creation and last modification date of a given file. */

#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <time.h>


void printFileProperties(struct stat stats);


int main()
{
    char path[100];
    struct stat stats;

    printf("Enter source file path: ");
    scanf("%s", path);


    // stat() returns 0 on successful operation,
    // otherwise returns -1 if unable to get file properties.
    if (stat(path, &stats) == 0)
    {
        printFileProperties(stats);
    }
    else
    {
        printf("Unable to get file properties.\n");
        printf("Please check whether '%s' file exists.\n", path);
    }

    return 0;
}

/**
 * Function to print file properties.
 */
void printFileProperties(struct stat stats)
{
    struct tm dt;

    // File permissions
    printf("\nFile access: ");
    if (stats.st_mode & R_OK)
        printf("read ");
    if (stats.st_mode & W_OK)
        printf("write ");
    if (stats.st_mode & X_OK)
        printf("execute");

    // File size
    printf("\nFile size: %ld", stats.st_size);

    // Get file creation time in seconds and
    // convert seconds to date and time format
    dt = *(gmtime(&stats.st_ctime));
    printf("\nCreated on: %d-%d-%d %d:%d:%d", dt.tm_mday, dt.tm_mon, dt.tm_year + 1900,
                                              dt.tm_hour, dt.tm_min, dt.tm_sec);

    // File modification time
    dt = *(gmtime(&stats.st_mtime));
    printf("\nModified on: %d-%d-%d %d:%d:%d", dt.tm_mday, dt.tm_mon, dt.tm_year + 1900,
                                              dt.tm_hour, dt.tm_min, dt.tm_sec);
}

output1:
Enter source file path: /home/kits/test/example
File access: read
File size: 11
created on: 30-06-2021 11:40:16
modified on: 30-06-2021 11:40:16

output2:
Unable to get file attributes
Please check whether /home/kits/test/nofile file exists.