#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

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

#define READBUFSIZE 1024

/* Copy a file one byte at a time */

int main(int argc, char **argv) {
  int  fin, fout;
  char readbuf[READBUFSIZE];

  // Take the filename and number of bytes from command line arguments
  if (argc != 3) {
    fprintf(stderr, "usage: ./cp_example outfile nBytes\n");
    return EXIT_FAILURE;
  }

  // Open the input file
  fin = open("/dev/zero", O_RDONLY);
  if (fin == -1) {
    perror("open for read failed");
    return EXIT_FAILURE;
  }

  // Open the output file
  fout = open(argv[1], O_WRONLY | O_CREAT);
  if (fout == -1) {
    perror("open for write failed");
    return EXIT_FAILURE;
  }

  // Read number of bytes to transfer
  long nBytes;
  if ( sscanf(argv[2], "%ld", &nBytes) != 1 ) {
    fprintf(stderr, "Invalid number of bytes: %s\n", argv[2]);
    return EXIT_FAILURE;
  }


  // Read from the file, write to fout.
  size_t readLen = 0;
  int bytesRemaining = 0;
  for (bytesRemaining=nBytes; bytesRemaining>0; bytesRemaining -= readLen ) {
    size_t bytesToRead = READBUFSIZE < bytesRemaining ? READBUFSIZE : bytesRemaining;
    if ( (readLen = read(fin, readbuf, bytesToRead)) == 0) break; // EOF
    // Test to see if we encountered an error while reading.
    if ( readLen < 0 ) {
      if ( errno == EINTR ) {
	readLen = 0; // try again
      } else {
	perror("read failed");
	return EXIT_FAILURE;
      }
    }
    if (write(fout, readbuf, readLen) < readLen) {
      perror("fwrite failed");
      return EXIT_FAILURE;
    }
  }

  close(fin);
  close(fout);

  return EXIT_SUCCESS;
}