NOTIFY(2) NOTIFY(2) NAME notify, noted, atnotify - handle asynchronous process notification SYNOPSIS int notify(void (*f)(void*, char*)) int noted(int v) int atnotify(int (*f)(void*, char*), int in) DESCRIPTION When a process raises an exceptional condition such as dividing by zero or writing on a closed pipe, a note is posted to communicate the exception. A note may also be posted by a write(see read(2)) to the process's /proc/n/note file or to the /proc/m/notepg file of a process in the same process group (see proc(3)). When the note is received the behavior of the process depends on the origin of the note. If the note was posted by an external process, the process receiving the note exits; if generated by the system the note string, preceded by the name and id of the process and the string "suicide: ", is printed on the process's standard error file and the process is suspended in the Broken state for debugging. These default actions may be overridden. The notify func- tion registers a notification handler to be called within the process when a note is received. The argument to notify replaces the previous handler, if any. An argument of zero cancels a previous handler, restoring the default action. A fork(2) system call leaves the handler registered in both the parent and the child; exec(2) restores the default behavior. After a note is posted, the handler is called with two argu- ments: the first is a pointer to a Ureg structure (defined in /$objtype/include/ureg.h) giving the current values of registers; the second is a pointer to the note itself, a null-terminated string with no more than `ERRLEN' characters in it including the terminal NUL. The Ureg argument is usu- ally not needed; it is provided to help recover from traps such as floating point exceptions. Its use and layout are machine- and system-specific. A notification handler must finish by calling noted; if the handler returns the behavior is undefined and probably erro- neous. The argument to noted defines the action to take: NDFLT instructs the system to perform the default action as if the handler had never been registered; NCONT instructs NOTIFY(2) NOTIFY(2) the system to resume the process at the point it was noti- fied. In neither case does noted return to the handler. If the note interrupted an incomplete system call, that call returns an error (with error string interrupted) after the process resumes. A notification handler can also jump out to an environment set up with setjmp using the notejmp func- tion (see setjmp(2)). Regardless of the origin of the note or the presence of a handler, if the process is being debugged (see proc(3)) the arrival of a note puts the process in the Stopped state and awakens the debugger. Rather than using the system calls notify and noted, most programs should use atnotify to register notification han- dlers. The parameter in is non-zero to register the func- tion f, and zero to cancel registration. A handler must return a non-zero number if the note was recognized (and resolved); otherwise it must return zero. When the system posts a note to the process, each handler registered with atnotify is called with arguments as described above until one of the handlers returns non-zero. Then noted is called with argument NCONT. If no registered function returns non- zero, atnotify calls noted with argument NDFLT. The set of notes a process may receive is system-dependent, but there is a common set that includes: Note Meaning interrupt user interrupt (DEL key) hangup I/O connection closed alarm alarm expired sys: breakpoint breakpoint instruction sys: bad address system call address argument out of range sys: odd address system call address argument unaligned sys: bad sys call system call number out of range sys: odd stack system call user stack unaligned sys: write on closed pipe write on closed pipe sys: fp: fptrap floating point exception sys: trap: trap other exception (see below) The notes prefixed sys: are generated by the operating sys- tem. They are suffixed by the user program counter in for- mat pc=0x1234. If the note is due to a floating point exception, just before the pc is the address of the offend- ing instruction in format fppc=0x1234. Notes are limited to ERRLEN bytes; if they would be longer they are truncated but the pc is always reported correctly. The types and syntax of the trap and fptrap portions of the notes are machine-dependent. NOTIFY(2) NOTIFY(2) SEE ALSO intro(2), notejmp in setjmp(2) BUGS Since exec(2) discards the notification handler, there is a window of vulnerability to notes in a new process.