void door_bell(void* dummy, char* note) { print("look who's there: %s\n", note); noted(NCONT); } int main() { char buf[256]; int fd,i; sprint(buf, "/proc/%d/note", getpid()); fd = open(buf, OREAD); notify(door_bell); print("starting up: %d\n", fd); for (;;) { if ((i = read(fd, buf, sizeof(buf))) < 0) break; if (i) print("selfserving: %s\n", buf); } return 0; } term% 8c test.c ; 8l test.8 ; ./8.out EVERYTHING IS DEAD AT THIS POINT --Boundary_(ID_Ay6Rx+jMTAjnVL7SYFMMiA) Content-type: text/html; charset=US-ASCII Content-transfer-encoding: QUOTED-PRINTABLE Guys,

I'= ve been experimenting with how Plan 9 handles notes for processes and=

I must confess that I'm now confused and in need you= r help.

First of all, the proc(3) man page s= ays that "A read [from /proc/n/note] of at least ERRLEN

cha= racters will retrieve the oldest note posted to the process and pr= event its delivery to

the process" and= for some reason I have always assumed that the read would be a block= ing

one. Yet, it doesn't seem to be the case:

&nb= sp; term% dd -if /proc/1/note -bs 256

&nb= sp; 0+0 record in

0+0 records out<= /div>

A visit to /sys/src/9/port/devproc.c confirms that if = there are no notes any read immediately

return with 0. At t= his point the whole idea of letting an external process read notes su= ddenly

becomes much less appealing: the only option left to= the reader is constant polling(*). On

top of that the= re always seems to be a race condition between somebody reading on /p= roc/n/note

and the scheduler actually delivering= a note via the call to a handler. These two things

make me= the following question: what is the point of reading /proc/n/note fo= r anything but a

stopped/borken process?

Thanks,

Roman.

(= *) Speaking of constant polling: the following hangs 9vx for good on = my system:

term% cat test.c

#= include <u.h>

#include <libc.h>

<= div>void door_bell(void* dummy, char* note)

{

&nb= sp; print("look who's there: %s\n", note);

&nbs= p; noted(NCONT);

}

int= main()

{

char buf[256];

&= nbsp; int fd,i;

sprint(bu= f, "/proc/%d/note", getpid());

fd =3D open(buf= , OREAD);

notify(door_bell);

&= nbsp; print("starting up: %d\n", fd);

for (;;)= {

if ((i =3D read(fd, buf, sizeo= f(buf))) < 0)

br= eak;

if (i)

= ; print("selfserving: %s\n", buf);

}

return 0;

}

<= div>

term% 8c test.c ; 8l test.8 ; ./8.out

<= div>

EVERYTHING IS DEAD AT THIS POINT

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