Modifying contents in the middle of a file

Usage of lseek() system call.

1. Create a test input file

                                               
   echo 0123456789 > numbers.txt
                                               
                                           

2. You can start with the following program:

                                               
   #include <stdio.h>
   #include <unistd.h>
   #include <fcntl.h>
   
   int main(int argc, char const *argv[])
   {
   	int fd = open("numbers.txt", O_RDONLY);
   
   	long l;
   	l = lseek(fd, 0L, SEEK_END);
   	printf("Length of the file is %ld bytes.\n", l);
   
   	close(fd);
   	return 0;
   }
                                               
                                           

3. Look at the documentation page of the lseek() system call and attempt to explain the example program above.
4. Change the program to modify the input file into 012abcd789 (use lseek() function and open the output file only for writing)
5. Reset the file into its original contents 0123456789
6. Change the program to modify the input file into 012abcd789 and then to 012abcd7xyz (use lseek() function and open the output file only for writing)
7. Reset the file into its original contents 0123456789
8. Change the program to modify the input file into: 0123456789 \0\0\0 ... \0\0\0 xyz , where the \0 represent byte with value zero. Make sure that the sequence of \0 characters is at least 10000 bytes.
   1. Accomplish this task by usage of the write system call. Save output in a file named output1 .
   2. Accomplish this task by usage of lseek system call. Save output in a file named output1 .
9. Observe number of blocks in the output files (use program stat)

Listing directory contents

1. Examine example at a manpage of the stat() system call
2. Examine the following program:

                                               
   /*
    * C Program to List Files in Directory
    */
   #include <dirent.h>
   #include <stdio.h>
    
   int main(void)
   {
   	DIR *d;
   	struct dirent *dir;
   	d = opendir(".");
   	if (d)
   	{
   		while ((dir = readdir(d)) != NULL)
   		{
   			printf("%s\n", dir->d_name);
   		}
   		closedir(d);
   	}
   	return(0);
   }
                                               
                                           

/*
* C Program to List Files in Directory
*/
#include <dirent.h>
#include <stdio.h>

int main(void)
{
    DIR *d;
    struct dirent *dir;
    d = opendir(".");
    if (d)
    {
        while ((dir = readdir(d)) != NULL)
        {
            printf("%s\n", dir->d_name);
        }
        closedir(d);
    }
    return(0);
}

1. Modify the program above to use first command argument line parameter as a directory name similarly as the ls command does.
2. Merge the file stat and directory listing programs into one which would emulate functionality of the ls -l command.