problem with writing to a file

Hi

I am new here. Here I was trying to write a program to copy one file to another.

Code:

---

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


int main(int argc, char *argv[])


{
    FILE *fp , *ft;


    if ((fp = fopen(argv[1], "rb")) == NULL)
        {
          printf("Can't open %s for reading\n", argv[1]);
        exit(EXIT_FAILURE);
        }


    if ((ft = fopen(argv[2], "wb")) == NULL)
        {
          printf("Can't open %s\n", argv[2]);
        exit(EXIT_FAILURE);
        }


    int ch;


    while( (feof(fp) == 0) )     
        { 
        fread(&ch, sizeof(int),1, fp);
       
            fwrite(&ch, sizeof(int),1, ft);
               
        }


    if (ferror(fp) != 0)
        fprintf(stderr,"Error in reading file %s.\n", argv[1]);


    if (ferror(ft) != 0)
        fprintf(stderr,"Error in  writing file %s.\n", argv[2]);


    fclose(ft);


    fclose(fp);
   
    return 9;


}

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This program takes two filenames on the command line. e.g

Quote:

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./a.out test1 test2

---

.

So it should copy test1 to test2. Now I created test1 in gedit. I just put one character "c" in it and closed it. And I ran the program. When I check sizes of test1 and test2 , using

Quote:

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ls -l

---

command, I see that test2 has 4 bytes and test1 has 2 bytes.
So something strange is happening here. I just put a single character in test1, so shouldn't it show 1 byte ? And why test2 is showing 4 bytes ? Any help appreciated. I have Win XP on my laptop. And I use Ubuntu 12 inside VirtualBox. So this is Linux environment...

thanks
ratnakar

Code:

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fwrite(&ch, sizeof(int),1, ft);

---

You're writing an int each time. That means your "to" file will always be a multiple of sizeof(int), probably 4 or 8 on most systems. Try reading and writing one character at a time:

Code:

---

char ch;
...
fread(&ch, sizeof(ch), 1, fp);
fwrite(&ch, sizeof(ch), 1, ft);

---

Notice, I used ch in my sizeof. That way, whatever type ch is, I read and write only the correct amount of data for that variable.

Oh, and read this link: FAQ > Why it's bad to use feof() to control a loop - Cprogramming.com.

Note, you don't want to use feof to control your loop. Instead, use the return value of fread. From the documentation:

Code:

---

RETURN VALUE
      fread()  and  fwrite()  return the number of items successfully read or written (i.e., not
      the number of characters).  If an error occurs, or the end-of-file is reached, the  return
      value is a short item count (or zero).


      fread()  does  not distinguish between end-of-file and error, and callers must use feof(3)
      and ferror(3) to determine which occurred.

---

It does suggest you use feof/ferror to check why it didn't read, however, what may not be obvious from that, is you should do it after fread fails. That is, after your loop:

Code:

---

char ch;
while (fread(&ch, sizeof(ch), 1, fp) != 1)
{
    if (fwrite(...) != 1)
    {
        // break
    }
}
if (ferror(fp))
    // handle error in read file
if (ferror(ft))
    // handle error in write file

---

fwrite has the same return value behavior as fread, so check it's return value too

I see that people already replied to this. I just want to add that, for debugging purpose, you could look at the file with a hex editor. That way you can compare the two files and see what's in one file that's not in the other.
There are a lot of Hex editors out there, I use Ghex (in Ubuntu 12.04).

Comparison of hex editors - Wikipedia, the free encyclopedia

thanks. I didn't get email notification , so I saw this late. I know that char type takes 1 byte at a time, but is 'char' not limited from 0 to 255 ?

anduril, I changed int to char in my program and I tried copying a pdf file. I saw the results of two files by 'ls -l' command. I still get 1 more byte in the new file. Why is that ?

Are you still using feof to control the while loop?

Oh I still am using the feof. Let me read that link posted by anduril. I will post here what I discover

Ok. I understood the significance of not using feof. Thanks everybody.............

You'd be much better off reading and writing as much data as you can in a single call, instead of handling a single byte at a time. Try allocating a large array of char, then make the calls in the following manner:

Code:

---

n = fread(buf, 1, sizeof buf, stream);
fwrite(buf, 1, n, stream);
if (n == sizeof buf)
 continue;

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That's usually the fastest way to copy data in standard C. If you only write one byte at a time, then it's more practical just to use getc/putc instead.

Thanks Barney......will keep that in mind.....