ungetc() considerations

ungetc() pushes characters back onto the input stream for binary and text files. ungetc() handles only single-byte characters. You can use it to push back as many as four characters onto the ungetc() buffer. For every character pushed back with ungetc(), fflush() backs up the file position by one character and clears all the pushed-back characters from the stream. Backing up the file position may end up going across a record boundary. Remember that for text files, z/OS® XL C/C++ counts the new-lines added to the records as single-byte characters when it calculates the file position.

For example, given the stream you can run the following code fragment:

   fgetc(fp);       /* Returns A and puts the file position at    */
                    /*    the beginning of the character B        */
   ungetc('Z',fp);  /* Logically inserts Z ahead of B             */
   fflush(fp);      /* Moves the file position back by one to A,  */
                    /*    removes Z from the logical stream       */