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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