In message <CAC38B59-1F54-4788-87A2-A1A8BE453500@delong.com>, Owen DeLong write s:
=20 It's deeper than just that, though. The whole paradigm is messy, from the point of view of someone who just wants to get stuff done. The examples are (almost?) all fatally flawed. The code that actually = gets at least some of it right ends up being too complex and too hard for people to understand why things are done the way they are. =20 Even in the "old days", before IPv6, geez, look at this: =20 bcopy(host->h_addr_list[n], (char *)&addr->sin_addr.s_addr, = sizeof(addr->sin_addr.s_addr)); =20 That's real comprehensible - and it's essentially the data interface=20=
between the resolver library and the system's addressing structures for syscalls. =20 On one hand, it's "great" that they wanted to abstract the dirty = details of DNS away from users, but I'd say they failed pretty much even at = that. =20 ... JG --=20 Joe Greco - sol.net Network Services - Milwaukee, WI - = http://www.sol.net "We call it the 'one bite at the apple' rule. Give me one chance [and] = then I won't contact you again." - Direct Marketing Ass'n position on e-mail = spam(CNN) With 24 million small businesses in the US alone, that's way too many = apples.
I think that the modern set of getaddrinfo and connect is actually not = that complicated:
/* Hints for getaddrinfo() (tell it what we want) */ memset(&addrinfo, 0, sizeof(addrinfo)); /* Zero out the buffer = */ addrinfo.ai_family=3DPF_UNSPEC; /* Any and all = address families */ addrinfo.ai_socktype=3DSOCK_STREAM; /* Stream Socket */ addrinfo.ai_protocol=3DIPPROTO_TCP; /* TCP */ /* Ask the resolver library for the information. Exit on failure. */ /* argv[1] is the hostname passed in by the user. "demo" is the = service name */ if (rval =3D getaddrinfo(argv[1], "demo", &addrinfo, &res) !=3D 0) { fprintf(stderr, "%s: Failed to resolve address information.\n", = argv[0]); exit(2); }
/* Iterate through the results */ for (r=3Dres; r; r =3D r->ai_next) { /* Create a socket configured for the next candidate */ sockfd6 =3D socket(r->ai_family, r->ai_socktype, r->ai_protocol); /* Try to connect */ if (connect(sockfd6, r->ai_addr, r->ai_addrlen) < 0) { /* Failed to connect */ e_save =3D errno; /* Destroy socket */ (void) close(sockfd6); /* Recover the error information */ errno =3D e_save; /* Tell the user that this attempt failed */ fprintf(stderr, "%s: Failed attempt to %s.\n", argv[0],=20 get_ip_str((struct sockaddr *)r->ai_addr, buf, BUFLEN)); /* Give error details */ perror("Socket error"); } else { /* Success! */ /* Inform the user */ snprintf(s, BUFLEN, "%s: Succeeded to %s.", argv[0], get_ip_str((struct sockaddr *)r->ai_addr, buf, BUFLEN)); debug(5, argv[0], s); /* Flag our success */ success++; /* Stop iterating */ break; } } /* Out of the loop. Either we succeeded or ran out of possibilities */ if (success =3D=3D 0) /* If we ran out of possibilities... */ { /* Inform the user, free up the resources, and exit */ fprintf(stderr, "%s: Failed to connect to %s.\n", argv[0], argv[1]); freeaddrinfo(res); exit(5); } /* Succeeded. Inform the user and continue with the application */ printf("%s: Successfully connected to %s at %s on FD %d.\n", argv[0], = argv[1], get_ip_str((struct sockaddr *)r->ai_addr, buf, BUFLEN), sockfd6); /* Free up the memory held by the resolver results */ freeaddrinfo(res);
It's really hard to make a case that this is all that complex.
I put a lot of extra comments in there to make it clear what's happening = for people who may not be used to coding in C. It also contains a whole = lot of extra user notification and debugging instrumentation because it = is designed as an example people can use to learn with.=20
Yes, this was a lot messier and a lot stranger and harder to get right = with get*by{name,addr}, but, those days are long gone and anyone still = coding with those needs to move forward.
Owen
These days you want something more complicated as everyone is or will be soon multi-homed. The basic loop above has very bad error characteristics if the first machines are not reachable. I've got working select, poll and thread based examples here: http://www.isc.org/community/blog/201101/how-to-connect-to-a-multi-homed-ser....
/* * Copyright (C) 2011 Internet Systems Consortium, Inc. ("ISC") * * Permission to use, copy, modify, and/or distribute this software for any * purpose with or without fee is hereby granted, provided that the above * copyright notice and this permission notice appear in all copies. * * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY * AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR * PERFORMANCE OF THIS SOFTWARE. */ #define TIMEOUT 500 /* ms */ int connect_to_host(struct addrinfo *res0) { struct addrinfo *res; int fd = -1, n, i, j, flags, count, max = -1, *fds; struct timeval *timeout, timeout0 = { 0, TIMEOUT * 1000}; fd_set fdset, wrset; /* * Work out how many possible descriptors we could use. */ for (res = res0, count = 0; res; res = res->ai_next) count++; fds = calloc(count, sizeof(*fds)); if (fds == NULL) { perror("calloc"); goto cleanup; } FD_ZERO(&fdset); for (res = res0, i = 0, count = 0; res; res = res->ai_next) { fd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (fd == -1) { /* * If AI_ADDRCONFIG is not supported we will get * EAFNOSUPPORT returned. Behave as if the address * was not there. */ if (errno != EAFNOSUPPORT) perror("socket"); else if (res->ai_next != NULL) continue; } else if (fd >= FD_SETSIZE) { close(fd); } else if ((flags = fcntl(fd, F_GETFL)) == -1) { perror("fcntl"); close(fd); } else if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1) { perror("fcntl"); close(fd); } else if (connect(fd, res->ai_addr, res->ai_addrlen) == -1) { if (errno != EINPROGRESS) { perror("connect"); close(fd); } else { /* * Record the information for this descriptor. */ fds[i] = fd; FD_SET(fd, &fdset); if (max == -1 || fd > max) max = fd; count++; i++; } } else { /* * We connected without blocking. */ goto done; } if (count == 0) continue; assert(max != -1); do { if (res->ai_next != NULL) timeout = &timeout0; else timeout = NULL; /* The write bit is set on both success and failure. */ wrset = fdset; n = select(max + 1, NULL, &wrset, NULL, timeout); if (n == 0) { timeout0.tv_usec >>= 1; break; } if (n < 0) { if (errno == EAGAIN || errno == EINTR) continue; perror("select"); fd = -1; goto done; } for (fd = 0; fd <= max; fd++) { if (FD_ISSET(fd, &wrset)) { socklen_t len; int err; for (j = 0; j < i; j++) if (fds[j] == fd) break; assert(j < i); /* * Test to see if the connect * succeeded. */ len = sizeof(err); n = getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len); if (n != 0 || err != 0) { close(fd); FD_CLR(fd, &fdset); fds[j] = -1; count--; continue; } /* Connect succeeded. */ goto done; } } } while (timeout == NULL && count != 0); } /* We failed to connect. */ fd = -1; done: /* Close all other descriptors we have created. */ for (j = 0; j < i; j++) if (fds[j] != fd && fds[j] != -1) { close(fds[j]); } if (fd != -1) { /* Restore default blocking behaviour. */ if ((flags = fcntl(fd, F_GETFL)) != -1) { flags &= ~O_NONBLOCK; if (fcntl(fd, F_SETFL, flags) == -1) perror("fcntl"); } else perror("fcntl"); } cleanup: /* Free everything. */ if (fds) free(fds); return (fd); } -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org