/**********************************************************************
 * fileio.c
 * Tom Kelliher
 *
 * This program demonstrates how to do low-level file I/O in C.  This
 * program implements a simple version of Unix's cp command.  Two
 * filename are expected on the command line.  The first file is the
 * name of the file to be copied, the second is the file to be created.
 **********************************************************************/


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


/* Prototypes. */
void pdie(const char *);
void die(const char *);


#define BUFFER_SIZE 1024   /* Size of the read/write buffer. */


/**********************************************************************
 * main
 **********************************************************************/

int main(int argc, char* argv[])
{
   int rfd;   /* Read file descriptor. */
   int wfd;   /* Write file descriptor. */
   char buffer[BUFFER_SIZE];   /* Read/Write buffer. */
   char *bp;   /* Pointer into write buffer. */
   int bufferChars;   /* Number of bytes remaining to be written. */
   int writtenChars;  /* Number of bytes written on last write. */

   if (argc != 3)
   {
      printf("Two filenames expected.\n");
      exit(1);
   }

   /* Open file to be copied. */
   if ((rfd = open(argv[1], O_RDONLY, 0)) < 0)
      pdie("Open failed");

   /* Open file to be created. */
   if ((wfd = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC,
                   S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) < 0)
      pdie("Open failed");

   while (1)
   {
      /* Normal case --- some number of bytes read. */
      if ((bufferChars = read(rfd, buffer, BUFFER_SIZE)) > 0)
      {
         bp = buffer;   /* Pointer to next byte to write. */

         /* 
            Since we can't guarantee that all the bytes will be written
            in a single write(), this code must be written such that
            several write()'s can possibly be called.
         */
         while (bufferChars > 0)
         {
            if ((writtenChars = write(wfd, bp, bufferChars)) < 0)
               pdie("Write failed");

            bufferChars -= writtenChars;   /* Update. */
            bp += writtenChars;
         }
      }
      else if (bufferChars == 0)   /* EOF reached. */
         break;
      else   /* bufferChars < 0 --- read failure. */
         pdie("Read failed");
   }

   close(rfd);
   close(wfd);

   return 0;
}


/**********************************************************************
 * pdie --- Print error message, call perror, and die.
 **********************************************************************/

void pdie(const char *mesg) {

   perror(mesg);
   exit(1);
}


/**********************************************************************
 * die --- Print error message and die.
 **********************************************************************/

void die(const char *mesg) {

   fputs(mesg, stderr);
   fputc('\n', stderr);
   exit(1);
}