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Subjectselect() on a unix dgram socket is buggy in 2.2.
Hi there!

Create a Unix dgram socketpair, and send many small datagrams to it. When
this number reaches `cat /proc/sys/net/unix/max_dgram_qlen` (10 by
default), then a select() call incorrectly tells that the socket is
writeable now, though a send() hangs.

Here's a short strace output:
select(4, NULL, [3], NULL, NULL) = 1 (out [3])
send(3, "", 0, 0
and it blocks here.

The source that produced this output is included at the end of this

This incorrect behaviour occurs in any recent 2.2. kernels (including
2.2.17 and 2.2.18pre14), and 2.4 kernels up to test6.

2.4.0-test7 and above are correct, however, in these kernels far more than
`cat /proc/sys/net/unix/max_dgram_qlen` short datagrams can be sent to the
socket. Therefore I'm not sure this sysctl is used at all. Is it?

Egmont Koblinger

PS. here's the C code:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>

fd_set wfds;
int sockfd[2];
char bigbuf[65536];

int main (int argc, char *argv[]) {
socketpair(PF_UNIX, SOCK_DGRAM, 0, sockfd);
while (1) {
FD_SET(sockfd[0], &wfds);
select(sockfd[0]+1, NULL, &wfds, NULL, NULL);
if (FD_ISSET(sockfd[0], &wfds)) {
send(sockfd[0], bigbuf, 0, 0);
return 0;

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