Messages in this thread | | | Date | Tue, 13 Mar 2001 17:59:21 -0800 (PST) | From | Jeffrey Butler <> | Subject | poll() behaves differently in Linux 2.4.1 vs. Linux 2.2.14 (POLLHUP) |
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Hi, I've noticed that poll() calls on IPv4 sockets do not behave the same under linux 2.4 vs. linux 2.2.14. Linux 2.4 will return POLLHUP for a socket that is not connected (and has never been connected) while Linux 2.2 will not. The following example program demonstrates the problem when it's run under linux 2.4:
--- #include <stdlib.h> #include <sys/types.h> #include <sys/socket.h> #include <sys/poll.h>
int main( int argc, char **argv) { int s; int rv; struct pollfd pfd; s = socket(PF_INET,SOCK_STREAM,0); pfd.fd = s; pfd.events = 0; rv = poll(&pfd,1,1); printf("rv = %d ",rv); if(pfd.revents&POLLIN) {printf("pollin ");} if(pfd.revents&POLLPRI){printf("pollpri ");} if(pfd.revents&POLLOUT){printf("pollout ");} if(pfd.revents&POLLERR){printf("pollerr ");} if(pfd.revents&POLLHUP){printf("pollhup ");} close(s); }
(copy text to file, run 'gcc -o example example.c', to compile)
Under Linux 2.4 (specifically 2.4.1) this program will output: rv = 0 pollhup
Under Linux 2.2.14 it outputs: rv = 0
Other operating systems (not that they are necessarily correct) also output the same as Linux 2.2.14. I tried this on FreeBSD 4.4.1, Solaris 5.7 (on a SPARC), and Windows 2000 with Cygwin 1.1.7.
I'm not sure what POSIX says about this behavior...
This can be traced back to the poll implementation in the Linux kernel:
In 2.2.14: net/ipv4/tcp.c[line 580]: if (sk->shutdown & RCV_SHUTDOWN) mask |= POLLHUP;
vs. In 2.4.1: net/ipv4/tcp.c[line 586]: * NOTE. Check for TCP_CLOSE is added. The goal is to prevent * blocking on fresh not-connected or disconnected socket. --ANK */ if (sk->shutdown == SHUTDOWN_MASK || sk->state == TCP_CLOSE) mask |= POLLHUP;
The behavior observed at user-level matches his comment as POLLHUP is returned for a fresh not-connected socket.
What's the reasoning for this change in behavior? I know of ways of getting around it, but I wanted to bring this to everyone's attention.
thanks, -jeff
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