They do not explicitly call out that the systems default character is implementation specific and that the function as is might be incompatible with default beh

They do not explicitly call out that the systems default character is implementation specific and that the function as is might be incompatible with default behavior (on as it turns out the vast majority of systems). Just saying it needs to be able to fit an unsigned char and call it a day is hardly clear documentation when it isn't even obvious that the signness of the character type is implemention defined for most c-programers. It almost reads as a deliberate trap (of course it isn't, but imo it is not good documentation).