what's in the left part of buf after fgets()?

Question

I hope to know what will be in the left part of buf after fgets() been exacted. For example:
char buf[100];
fgets(buf, sizeof(buf), fp);
If one line has just 10 characters + '\n', then what will be in the left part of buf (from but[12] to buf[99])?
If execute fgets() twice, will the second input cover the first input to buf?

Answer 1

When fgets reads data it changes one element of the buffer at a time (simplifying assumption) until it reaches the limit or it finds a terminator in the input. All other elements in the buffer remain the same as before calling fgets (thus, they might have random data or they might leak previously read info).