fread and fwrite.. extra data being written to file

[benjammin]

hi all,

I've been trying to write a simple C program to use fread and fwrite to read some data from a file and write it into another file. It works, though i always seem to get some extra text at the end. Usually part of the original text. Heres my code. Any help is much appreciated.

thanks,
ben

**************************************************
#include <stdio.h>

int main( void)
{
unsigned char *buff;
FILE *fptr;
FILE *fptr2;

if (( fptr = fopen("file.txt", "rb" )) == NULL || (fptr2 = fopen("file2.txt", "wb")) == NULL){
printf( "File could not be opened.\n");
}

else {
while ( !feof(fptr)){

fread (buff , 1 , sizeof(buff) , fptr );
fwrite (buff , 1 , sizeof(buff) , fptr2 );


}
fclose(fptr);
fclose(fptr2);
}
}

[firedraco]

It is because you are looping on eof. feof() will return true after the last fread() doesn't work. For example:

File:
abc

If you use your code with this file, you will get an 'a', a 'b', and a 'c', but the eof is still not reached until you attempt to read again, and since the buffer is not modified, you will write an extra 'c' to the output file.

[benjammin]

thanks firedraco,

I used the file "abc" and it returned an extra "abc" on the next line. how do i modify the buffer?

[Duthomhas]

The fread() function returns the number of items it read. You should be looping against that, and not using feof() at all.

_{Why doesn't anyone ever use code tags?}

        while (1) {
            /* Read as many bytes as we can from the file.   *
             * Keep track of how many bytes we actually got. */
            size_t readcount = fread(buff, 1, sizeof(buff), fptr);

            /* If there weren't any more bytes to read, then we are done. */
            if (!readcount) break;

            /* Otherwise, write the bytes back out to the output file. */
            fwrite(buff, 1, readcount, fptr2);
        }

        /* At this point, all bytes from the input file have been copied to the output file. */
        fclose(fptr);
        fclose(fptr2);

Hope this helps.