Difference between scanf() and strtol() / strtod() in parsing numbers

Question

Note: I completely reworked the question to more properly reflect what I am setting the bounty for. Please excuse any inconsistencies with already-given ans

Answer 1

I am not sure how implementing scanf() may be related to ungetc(). scanf() can use up all bytes in the stream buffer. ungetc() simply pushes a byte to the end of buffer and the offset is also changed.

scanf("%d", &x);
ungetc('9', stdin);
scanf("%d", &y);
printf("%d, %d\n", x, y);

If the input is "100", the output is "100, 9". I do not see how scanf() and ungetc() may interfere with each other. Sorry if I added a naive comment.