Address family not supported by protocol

The following code is a socket programming sample for a TCP client.

But when I run this, connect() is returned as Address family not supported by protocol.

I have heard, this problem will happen if the platform does not support ipv6.

But AF_INET I wrote is ipv4.

Also my server, that is CentOS6.4, is configured within an inet6 addr .

Does anyone know why?

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
main(){ struct sockaddr_in server; int sock; char buf[32]; int n; sock = socket(AF_INET,SOCK_STREAM,0); perror("socket"); server.sin_family = AF_INET; server.sin_port = htons(12345); inet_pton(AF_INET,"127.0.0.1",&server,sizeof(server)); connect(sock,(struct sockaddr *)&server,sizeof(server)); perror("connect"); memset(buf,0,sizeof(buf)); n = read(sock,buf,sizeof(buf)); perror("read"); printf("%d,%s\n",n,buf); close(sock); return 0;
}

3 Answers

The code passes the wrong destination address and wrong number of arguments to inet_pton(). (For the latter the compiler should have warned you about, btw)

This line

 inet_pton(AF_INET, "127.0.0.1", &server, sizeof(server));

should be

 inet_pton(AF_INET, "127.0.0.1", &server.sin_addr);

Verbatim from man inet_pton:

int inet_pton(int af, const char *src, void *dst);

AF_INET

[...] The address is converted to a struct in_addr and copied to dst, which must be sizeof(struct in_addr) (4) bytes (32 bits) long.

Not related to the problem, but also an issue, is that read() returns ssize_t not int.

The following lines shall be adjusted:

int n;
[...]
printf("%d, %s\n", n, buf);

to become:

ssize_t n;
[...]
printf("%zd, %s\n", n, buf);

Set the server address like this;

addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(host);
addr.sin_port = htons(port);

I seen this error during bind. Cause was of using localhost instead of IP:

./myprogram localhost:7777
*** exception! `bind' failed for `localhost:7777' (97, Address family not supported by protocol)
./myprogram 127.0.0.1:7777
OK! Listening...

In addition: this error happens on one Linux host and does not appear on another. I check and compare network settings on this machines (lo device, /etc/hosts, /etc/host.conf, etc) and not found essential difference