binary file copy has extra character

**leonv** · 03-22-2010

I need to modify strings in a binary file (c-tree database file on Windows). I thought it would be easy to write a stream editor to read the file a byte at a time, look for and replace the string, and write the stream to a new file.

I've hit a couple of snags. The first is the file size. To simplify the code, I modified to fread and fwrite each byte. The file ends up one character (0xFF) longer. Here's the simplified code:

Code:

#include <stdio.h>
#include <windows.h>

char currentChar;
char holdStr[25];
FILE *in;
FILE *out;
char newString[20]="\\\\SERVER\\";
char oldString[20]="\\\\server1\\";
char space[10]=" ";

int createFile()
{
  int result;
  int inCnt=0, outCnt=0;
  int matchCntr=0;
  int charCnt;
  char char_buffer[500];
  char char_dir[500];
  char shortpath[500];
  WIN32_FIND_DATA FindFileData;
  HANDLE hFind;

  do
  {
	fread(&currentChar, 1, 1, in);
	inCnt++;

	fwrite( &currentChar, 1, 1, out );
	outCnt++;

  } while (!feof(in));
  return(0);
  
  
}

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

  in = fopen(".\\images.dat", "rb");
  if ( in == NULL)
  {
    printf("could not open .\\images.dat\n");
    exit(1);
  }
  
  out = fopen(".\\images_new.dat", "wb");
  if ( out == NULL)
  {
    printf("could not open .\\images_new.dat\n");
    exit(1);
  }
  
  createFile();
  fclose(in);
  fclose(out);
  exit(0);
}

Can someone help me understand what is happening? Does fclose write an EOF?

Thanks,
Leon

**C_ntua** · 03-22-2010

You mean the output file ends up one character longer than the input file, right?
I don't think fclose() adds and EOF. I would guess fopen() does that when it creates the file.
In any case, when fread() reads the EOF then the do-while loop doesn't end. Instead you write the currentChar in the outpute file (out). Whatever the currentChar is you write an additional character. A quick fix might be:

Code:

  while (1) {
	fread(&currentChar, 1, 1, in);
	if (feof(in) break;
        inCnt++;
	fwrite( &currentChar, 1, 1, out );
	outCnt++;
  }

**vart** · 03-22-2010

I think better yet

Code:

while (fread(&currentChar, 1, 1, in) == 1)
{
    inCnt++;
    fwrite( &currentChar, 1, 1, out );
    outCnt++;
}

or

Code:

unsigned char buf[BUF_SIZ];
size_t len;
while((len = fread(buf, 1, sizeof buf, in) ) > 0)
{
    inCnt += len;
    len = fwrite( buf, 1, len, out );
    outCnt += len;
}

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