Ahhh, I've been thinking I should have tried to explain that better, although I do NOT claim to know C! The C Standard Library contains open(). read(), write(). etc. - equivalent (?) to the system calls you're probably used to. It also includes fopen(), fread(), etc. If you learn C "from the book", it probably tells you to use fread(), etc. and may not even mention that the other versions exist. The difference is that the "f" versions are "buffered I/O". These use a "different stdin" (etc.) than the small integer "STDIN" you'd use for system calls. This "stdin" (I think) is the address of a structure which knows where the actual buffer is and the current position in the buffer 0 as well as STDIN=0, I suppose. If you're not aware that you've asked for buffered I/O, the results may not be what you expect.
"cs" is a segment register, but in this case it's a segment override - the address of "stdin" is with respect to section .text rather than .data or .bss. "fgets" itself would be in .text, although the address of it might be found elsewhere(?). I think IDA is adding that (and the underscore on "fgets). I don't think you'd need them if you were writing the code yourself.
Best,
Frank
