Question

UNIX is all about manipulating files and input/output streams fluidly, so it is important to get a strong grasp of how this fundamentally works at the system call level to understand higher-level system programming concepts. Every program automatically has three file descriptors opened by the shell standard input standard output standard error 1 2 One can use read and write other open file. Normally, standard input and output on the terminal are line-buffered, so, for example, the specified number of bytes if a newline character appears in the input on these file descriptors just like with any read call will return before it has read a The standard header errno.h provides a set of system error numbers and an integer identifier called errno. This integer (located in a static memory library functions, including the unbuffered I/0 calls, with these system error numbers when an error occurs a read location) is automatically populated by some This allows us to see what specifically went wrong when, for example, write call returns -1. or For human readability, string.h provides strerror , which takes errno as an argument and returns a string.h) to get the length of that string, directly to standard error string describing the error. Using strlen ) (also from write the error message one can PART 1: cat Write your own takes whatever is passed to its standard input and writes it to its standard output. simple implementation of the cat utility in C. Recall that cat Use the following enum as your buffer size for normal read and write operations: BUFFER SIZE = 16 : enum Note that standard input from the terminal will normally be line-buffered, and lines may be longer than BUFFER SIZE, so you will need to read and write in loops to process all the data If at any point read or write return an error (-1), your program should output that error to standard error using errno, strerror, strlen, and write (see the example ioerr.c on the course website for inspiration) and exit with a unique non-zero status. Test your program with input redirection on the command line, e.g.: ./cat < input-file

Answer 1

Please find the program to read and write to output file. Also i have added necessary comments in it.

Program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>

enum { BUFFER_SIZE = 16 };
int main(int argc, char *argv[])
{
   int n;
   unsigned char read_char[BUFFER_SIZE]={'\0'};
   int fptr, fptw;

   /* check for format */
   if(argc != 3)
   {
     printf("Usage: %s <input_file> <output_file>\n", argv[0]);
     return -1;
   }

   /* open the file */
   if ((fptr = open(argv[1], O_RDONLY )) < 0)
   {
      printf("Error: %s %s\n",argv[1], strerror(errno));
      return -1;
   }

   /* new file wil be created if output file is not found,
    * if output file found, content will be appended to outfile. */
   if ((fptw = open(argv[2], O_CREAT |O_APPEND|O_WRONLY) ) < 0)
   {
      printf("Error: %s %s\n",argv[2], strerror(errno));
      return -1;
   }

   if(fchmod(fptw, 0) == -1)
   {
      printf("Error: %s %s\n",argv[1], strerror(errno));
      return -1;
   }

   /* change the file mode to -rw-r----- */
   if(fchmod(fptw, S_IRUSR |S_IWUSR |S_IRGRP ) == 1)
   {
      printf("Error: %s %s\n",argv[1], strerror(errno));
      return -1;
   }
   /* read the file until read returns 0 */
   n= read(fptr, read_char, BUFFER_SIZE);

   if (n == -1)
   {
      printf("Error: %s\n", strerror(errno));
      return -1;
   }

   /* writing to the file */
   while (n != 0)
   {
      if(n < 16)
      write(fptw, read_char, n);
      else
      write(fptw, read_char, BUFFER_SIZE);

      bzero(read_char, BUFFER_SIZE);

      /* read the object file until fread returns less than 1 */
      n= read(fptr, read_char, BUFFER_SIZE);

      if (n == -1)
      {
         printf("Error: %s\n", strerror(errno));
         return -1;
      }
   }

/* close both the files */

   close(fptr);
   close(fptw);
   return 0;
}

Output:

USER>./filecat exoplanets.dat
Usage: ./filecat <input_file> <output_file>
USER>

USER>cc file_cat.c -o filecat

USER>./filecat exoplanets.dat output_file

USER>ls -l output_file
-rw-r----- 1 asgu warp 100 Jun 4 10:03 output_file

USER>cat output_file
aaaaaa 10 3.4
bbbbbb 20 5.6
cccccc 340 7.86
dddd 230 4.574
eeee 5 7.8
ggggg 344 5.2
hhhhh 780 3.45


USER>cat exoplanets.dat
aaaaaa 10 3.4
bbbbbb 20 5.6
cccccc 340 7.86
dddd 230 4.574
eeee 5 7.8
ggggg 344 5.2
hhhhh 780 3.45

#following output shows file contents are appended

USER>./filecat exoplanets.dat output_file
USER>
USER>
USER>ls -l output_file
-rw-r----- 1 asgu warp 200 Jun 4 10:07 output_file
USER>
USER>cat output_file
aaaaaa 10 3.4
bbbbbb 20 5.6
cccccc 340 7.86
dddd 230 4.574
eeee 5 7.8
ggggg 344 5.2
hhhhh 780 3.45
aaaaaa 10 3.4
bbbbbb 20 5.6
cccccc 340 7.86
dddd 230 4.574
eeee 5 7.8
ggggg 344 5.2
hhhhh 780 3.45
USER>