Answer:

Modify the program so that some normally legitimate character is bad. Do this by altering the if statement.

if ( (isprint( ch ) || ch == '\t' || ch == '\n' || ch == '\r') && ch != '%' )

Now, the character % will be regarded as bad. Create a test file and insert some % characters into it.

Bad Files

Better testing could be done by creating a program, similar to the above, that inserts bad characters using the hex value. Do something like

putchar( 0xFF )

to put a non-ascii value into a file. Use file redirection to create a bad file from a good text file.

#include <stdio.h>

/* Replace '@' in a text file with byte not allowed in text files */
void main( void )
{
  int ch;

  while ( (ch = getchar()) != EOF )
  {
    if ( ch == '@' )
      putchar( 0xFF );
    else
      putchar( ch );
  }
}

Often to test a program you have to write additional programs that produce testing data.