Which functions in the C standard library commonly encourage bad practice? [closed]

Answer 1

Any of the functions that manipulate global state, like gmtime() or localtime(). These functions simply can't be used safely in multiple threads.

EDIT: rand() is in the same category it would seem. At least there are no guarantees of thread-safety, and on my Linux system the man page warns that it is non-reentrant and non-threadsafe.