NOTIFY(2) NOTIFY(2)
NAME
notify, noted, atnotify - handle asynchronous process
notification
SYNOPSIS
#include <u.h>
#include <libc.h>
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 either by exiting the
program or by calling noted; if the handler returns the
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NOTIFY(2) NOTIFY(2)
behavior is undefined and probably erroneous. Until the
program calls noted, any further externally-generated notes
(e.g. hangup or alarm) will be held off, and any further
notes generated by erroneous behavior by the program (such
as divide by zero) will kill the program. 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 the system to resume the
process at the point it was notified. 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 function (see setjmp(2)),
which is implemented by modifying the saved state and call-
ing noted(NCONT).
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.
Noted has two other possible values for its argument. NSAVE
returns from the handler and clears the note, enabling the
receipt of another, but does not return to the program.
Instead it starts a new handler with the same stack, stack
pointer, and arguments as the original, at the address
recorded in the program counter of the Ureg structure. Typ-
ically, the program counter will be overridden by the first
note handler to be the address of a separate function; NSAVE
is then a `trampoline' to that handler. That handler may
executed noted(NRSTR) to return to the original program,
usually after restoring the original program counter. NRSTR
is identical to NCONT except that it can only be executed
after an NSAVE. NSAVE and NRSTR are designed to improve the
emulation of signals by the ANSI C/POSIX environment; their
use elsewhere is discouraged.
The set of notes a process may receive is system-dependent,
but there is a common set that includes:
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NOTIFY(2) NOTIFY(2)
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.
Alef
Because the run-time system does not protect itself, Alef
programs require extreme care to recover from notes. Notes
are, however, useful for shutting down multi-process pro-
grams cleanly. There is no atnotify in Alef.
SOURCE
/sys/src/libc/9syscall
/sys/src/libc/port/atnotify.c
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.
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