Re: Portabilty of poll trick
Bastien ROUCARIES wrote: > - close the end of the pipe, but it does not break poll(2), and > moreover in multithread context > it is not safe due to fd reuse Yes, close() in multithreaded applications needs to be done carefully, to avoid that unintended operations get done to the next file that is connected to the same file descriptor. However, I don't see how a library / abstraction can help here. It's just careful coding of the application that is needed. Bruno
Re: Portabilty of poll trick
Le mar. 27 avr. 2021 à 22:51, Ben Pfaff a écrit : > > On Tue, Apr 27, 2021 at 3:47 PM Bastien ROUCARIES > wrote: > > > > Le mar. 27 avr. 2021 à 22:40, Bruno Haible a écrit : > > > > > > Hi Bastien, > > > > > > > I want to assess the safety and portability of the following trick, > > > > > > I would want to help you with this, but I can't. You have not stated: > > > - What is this code supposed to do? > > I want to shutdown (2) a pipe, in a multithread application, in order > > to get out a poll(2) wait state > > > - Why is it a "trick"? What advantages does it have compared to the code > > > a naïve developer would write? > > The naive delevopper will: > > - close the end of the pipe, but it does not break poll(2), and > > moreover in multithread context > > it is not safe due to fd reuse > > -use timeout but in this case why use poll... > > > > The goal is also to shutdown an eventfd but without kernel support, I > > suppose it is not possible... > > Can you use socketpair() instead of pipe()? Yes I can but socket are bidirectionnal and cost more about memory/speed than pipe. Bastien
Re: Portabilty of poll trick
On Tue, Apr 27, 2021 at 3:47 PM Bastien ROUCARIES wrote: > > Le mar. 27 avr. 2021 à 22:40, Bruno Haible a écrit : > > > > Hi Bastien, > > > > > I want to assess the safety and portability of the following trick, > > > > I would want to help you with this, but I can't. You have not stated: > > - What is this code supposed to do? > I want to shutdown (2) a pipe, in a multithread application, in order > to get out a poll(2) wait state > > - Why is it a "trick"? What advantages does it have compared to the code > > a naïve developer would write? > The naive delevopper will: > - close the end of the pipe, but it does not break poll(2), and > moreover in multithread context > it is not safe due to fd reuse > -use timeout but in this case why use poll... > > The goal is also to shutdown an eventfd but without kernel support, I > suppose it is not possible... Can you use socketpair() instead of pipe()?
Re: Portabilty of poll trick
Le mar. 27 avr. 2021 à 22:40, Bruno Haible a écrit : > > Hi Bastien, > > > I want to assess the safety and portability of the following trick, > > I would want to help you with this, but I can't. You have not stated: > - What is this code supposed to do? I want to shutdown (2) a pipe, in a multithread application, in order to get out a poll(2) wait state > - Why is it a "trick"? What advantages does it have compared to the code > a naïve developer would write? The naive delevopper will: - close the end of the pipe, but it does not break poll(2), and moreover in multithread context it is not safe due to fd reuse -use timeout but in this case why use poll... The goal is also to shutdown an eventfd but without kernel support, I suppose it is not possible... Bastien > > > > > for getting outside poll. > > I don't understand what you mean here. > > Bruno >
Re: Portabilty of poll trick
Hi Bastien, > I want to assess the safety and portability of the following trick, I would want to help you with this, but I can't. You have not stated: - What is this code supposed to do? - Why is it a "trick"? What advantages does it have compared to the code a naïve developer would write? > for getting outside poll. I don't understand what you mean here. Bruno
Portabilty of poll trick
Hi,
I want to assess the safety and portability of the following trick, for getting
outside poll. Replacing by using dup2 a read poll object by a writtable one. I
think to use this for stopping polling in multithread program
It work really well, and could for instance allow me to tear down eventfd of
other event like file that does not support shutdown
CC also Michael Kerrisk (mtk.manpa...@gmail.com) for comments on this trick
#include
#include
#include
#include
#include
#include
#include
static void *run_poll (void *arg)
{
struct pollfd fds[1];
int rc;
fds[0].fd = ((int *)arg)[0];
fds[0].events = POLLERR | POLLHUP | POLLNVAL | POLLIN;
//
// Poll indefinitely
//
printf("starting poll\n");
fflush(stdout);
rc = poll((struct pollfd *) , 1, -1);
if ( rc == -1 )
{
printf("POLL returned error %d: %s\n", errno, strerror(errno));
}
else
{
printf("POLL returned %d (revents = 0x%08x): %s %s %s %s %s\n",
rc,
fds[0].revents,
( ( fds[0].revents & POLLIN ) ? "pollin" : "noin" ),
( ( fds[0].revents & POLLOUT ) ? "pollout" : "noout" ),
( ( fds[0].revents & POLLERR ) ? "pollerr" : "noerr" ),
( ( fds[0].revents & POLLHUP ) ? "pollhup" : "nohup" ),
( ( fds[0].revents & POLLNVAL ) ? "pollnval" : "nonval" ));
}
return NULL;
}
int main (int argc, char **argv)
{
pthread_t thread;
int rc;
int fdst[2];
pipe(fdst);
rc = pthread_create(, NULL, run_poll, );
sleep(3);
printf("removing access\n");
dup2(fdst[0],fdst[1]);
sleep(3);
return 0;
}
signature.asc
Description: This is a digitally signed message part.
