Problem with socketpair , AF_UNIX and select call - can anybody through any light on this!

[Stephen Croll]

Howard Wilkinson wrote:
> Can you suggest such a list - I can only find lists for POSIX threads!
comp.unix.programmer
> Code goes as follows
>
>     int fd[2] = { -1, -1 };
>     fd_set rfd;
>     int rv;
>     char buf[1024];
>     struct timeval timeout = { 10, 0 };
>
>     rv = socketpair(PF_UNIX, SOCK_DGRAM, 0, fd);
>     if (rv < 0) Err("socketpair");
>
>     fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL, 0) | O_NONBLOCK);
>
>     FD_ZERO(rfd);
>     FD_SET(fd[0], rfd);
>
>     rv = select(fd[0]+1, &rfd, NULL, NULL, &timeout);
>
>     if (FD_ISSET(fd[0], rfd)) {
>     	printf("Socket is ready to read\n");
>     	rv = recv(fd[0], buf, sizeof(buf), MSG_DONTWAIT);
>     	if (rv < 0) Err("recv - %s(%d)", strerror(errno), errno);
>     	printf("Socket returned %d bytes\n", rv);
>     } else {
>     	printf("Socket is not ready to read\n");
>     }
>     exit (0);
>
> I contend that this should (if I understand the semantics correctly) 
> print out "Socket not ready to read" instead I get "recv - Resource 
> temporarily unavailable(11)"
I haven't really looked at the code, except for making mods to get it to 
compile, but it works for me:

pika.localdomain:~/tmp 0:12> uname -a
Linux pika.localdomain 2.6.22.9-91.fc7 #1 SMP Thu Sep 27 20:47:39 EDT 
2007 x86_64 x86_64 x86_64 GNU/Linux

pika.localdomain:~/tmp 0:13> cat socketpair.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

int main()
{
    int fd[2] = { -1, -1 };
    fd_set rfd;
    int rv;
    char buf[1024];
    struct timeval timeout = { 10, 0 };
   
    rv = socketpair(PF_UNIX, SOCK_DGRAM, 0, fd);
    if (rv < 0) printf("socketpair");
   
    fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL, 0) | O_NONBLOCK);
   
    FD_ZERO(&rfd);
    FD_SET(fd[0], &rfd);
   
    rv = select(fd[0]+1, &rfd, NULL, NULL, &timeout);
   
    if (FD_ISSET(fd[0], &rfd)) {
        printf("Socket is ready to read\n");
        rv = recv(fd[0], buf, sizeof(buf), MSG_DONTWAIT);
        if (rv < 0) printf("recv - %s(%d)", strerror(errno), errno);
        printf("Socket returned %d bytes\n", rv);
    } else {
        printf("Socket is not ready to read\n");
    }

    exit (0);
}

pika.localdomain:~/tmp 0:14> gcc -Wall socketpair.c

pika.localdomain:~/tmp 0:15> time ./a.out
Socket is not ready to read

real    0m10.000s
user    0m0.001s
sys     0m0.000s

pika.localdomain:~/tmp 0:16> ./a.out
Socket is not ready to read

--
Steve Croll