"The meaning of these flags is exactly as specified in fopen(), except that modes beginning with w shall not cause truncation of the file."

http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html says: "The mode argument points to a string. If the string is one of the following, the file shall be opened in the indicated mode. Otherwise, the behavior is undefined."

I think that's pretty clear. Other platforms might overload 't' to mean something else. Even "yes, please. segfault my process".