fopen fails after numerous successes

Hello,

I have trouble writing an file-copy function.
My fcopy-function should read data from a file into a buffer, then write the buffer to another file.

It works fine for several times in succession (see test_fcopy), but then the sourcefile suddenly can't be opend (fopen returns 0x00).

Heap size is 0x10000 (wich is very big, I guess).
Any ideas?

Marco

int test_fcopy()
{
int rc;
int i;

  for(i=0;i<100;i++)
  {
    rc = fcopy("F:dest.txt", "F:source.txt");
    if(rc != FILESYSTEM_OK)
    {
      //rc != FILESYSTEM_OK after 10 cycles,
      //depending on file size
      break;
    }
  }
}

int fcopy(char *destinationfile, char *sourcefile)
{
#define COPY_BYTES 512
char copy_data[COPY_BYTES];
FILE *dest;
FILE *source;
int c_read;
int c_write;
int rc;

  rc       = FILESYSTEM_OK;
  //open source file for reading
  source = fopen  (sourcefile, "r");
  //open destination file for writing
  dest   = fopen  (destinationfile, "w");

  if((source) && (dest))
  {//if both  files could be opend

    while (!feof (source))
    {
      //read bytes from source file to ram
      c_read  = fread((void*)&copy_data,
                       1,
                       COPY_BYTES,
                       source);
      //write bytes from ram to destination file
      c_write = fwrite((void*)&copy_data,
                        1,
                        c_read,
                        dest);

      if(c_read != c_write)
      {
        rc = FILESYSTEM_ERROR;
      }
    }
    //Close files
    fclose(source);
    fclose(dest);
  }
  else
  {
    rc = FILESYSTEM_ERROR;
  }

  return(rc);
}