Cannot copy an entire file to another one using write system call

Question

I have to copy the content of file1 into a buffer (of size 23 bytes), then, I have to copy data from the buffer to file2.

I have trouble to make sure that file1 is entirely copied into the buffer. When the buffer is copied to file2, file2 contents only part of the content of file1, and the output says that only 4 bytes of data have been copied into file2.

I tried to figure out what I did wrong, but I have no luck so far. Your help will be very appreciated.

I am using Oracle VM VirtualBox, where I have Ubuntu installed.

I am also using make (MakeFile) to update all files at once on the command prompt.

My code is below in C/POSIX.

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

#define  My_Full_Name "AAA!"

int PrintSentence()
{

  size_t buffersize = (size_t) (4 * 5.75);  //or (4 bytes * 5.75) = 23 bytes
  char buffer[buffersize];
  char source_file[200];
  char destination_file[200];

  ssize_t bytes_read;

  int fdSource, fdDestination;

  mode_t mode = S_IRUSR | S_IWUSR;

  printf("Welcome to File Copy by %s\n", My_Full_Name);

  printf("Enter the name of the source file: ");

  scanf("%s", source_file);

  printf("Enter the name of the destination file: ");

  scanf("%s", destination_file);

  fdSource = open(source_file, O_RDONLY);

  if (fdSource < 0)
  {
    perror("Open failed!!");
    return 1;
  }

  else
  {

    bytes_read = read(fdSource, buffer, sizeof(buffer));

    fdDestination = open(destination_file, O_CREAT | O_WRONLY | mode);

    if (fdDestination < 0)
    {
      perror("Oups!! cannot create file again!!");

      return 1;
    }
    else
    {
      write(fdDestination, buffer, sizeof(buffer));
      printf("current content of buffer: %s\n", buffer); //just to check
      printf("current value of buffer size = %zd  \n", buffersize); //just to check
      printf("File copy was successful, with %d byte copied\n", fdDestination); //the output says only 4 bytes are copied

    }

  }

  return;

}

Answer 1

Here:

printf("File copy was successful, with %d byte copied\n", fdDestination );

fdDestination is a file descriptor, it's not the number of bytes written. 0, 1 and 2 are your three standard streams, 3 will be your input file which is opened first, so 4 will be your output file, that's why it's always outputting 4.

You want to save the return value from write(), and use the value of that instead (after checking that return value for errors, of course, which you should be doing for read() as well).

EDIT: Slightly modifying your code:

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

#define  My_Full_Name "AAA!"

int main(void) {
    size_t buffersize = (size_t) (4 * 5.75);
    char buffer[buffersize];
    char source_file[200];
    char destination_file[200];

    ssize_t bytes_read, bytes_written;
    int fdSource, fdDestination;

    mode_t mode = S_IRUSR | S_IWUSR;

    printf("Welcome to File Copy by %s\n", My_Full_Name);
    printf("Enter the name of the source file: ");
    scanf("%s", source_file);

    printf("Enter the name of the destination file: ");
    scanf("%s", destination_file);

    fdSource = open(source_file, O_RDONLY);

    if (fdSource < 0) {
        perror("Open failed!!");
        return 1;
    } else {
        bytes_read = read(fdSource, buffer, sizeof(buffer));
        fdDestination = open(destination_file, O_CREAT | O_WRONLY | mode);

        if (fdDestination < 0) {
            perror("Oups!! cannot create file again!!");
            return 1;
        } else {
            bytes_written = write(fdDestination, buffer, sizeof(buffer));
            printf("current content of buffer: %s\n", buffer);
            printf("current value of buffer size = %zd  \n", buffersize);
            printf("File copy was successful, with %d byte copied\n",
                    bytes_written);
        }
    }

    return 0;
}

gives me this:

paul@local:~/src/c/fpc$ cat infile
12345678901234567890123
paul@local:~/src/c/fpc$ ./fpc
Welcome to File Copy by AAA!
Enter the name of the source file: infile
Enter the name of the destination file: outfile
current content of buffer: 12345678901234567890123
current value of buffer size = 23
File copy was successful, with 23 byte copied
paul@local:~/src/c/fpc$ cat outfile; echo ""
12345678901234567890123
paul@local:~/src/c/fpc$

Answer 2

How can you expect that a file will be entirely written into the buffer when your buffer is but 23 bytes long? With call to read you are reading only 23 bytes and leave the rest of the contents of the file1 untouched. Or, do is it your expected behaviour that your program should copy only 23 bytes of its contents to the target file?