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NAME
getaddrinfo_a, gai_suspend, gai_error, gai_cancel - asynchronous network address and
service translation
LIBRARY
Asynchronous name lookup library (libanl, -lanl)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <netdb.h>
int getaddrinfo_a(int mode, struct gaicb *list[restrict],
int nitems, struct sigevent *restrict sevp);
int gai_suspend(const struct gaicb *const list[], int nitems,
const struct timespec *timeout);
int gai_error(struct gaicb *req);
int gai_cancel(struct gaicb *req);
DESCRIPTION
The getaddrinfo_a() function performs the same task as getaddrinfo(3), but allows multiple
name look-ups to be performed asynchronously, with optional notification on completion of
look-up operations.
The mode argument has one of the following values:
GAI_WAIT
Perform the look-ups synchronously. The call blocks until the look-ups have
completed.
GAI_NOWAIT
Perform the look-ups asynchronously. The call returns immediately, and the
requests are resolved in the background. See the discussion of the sevp argument
below.
The array list specifies the look-up requests to process. The nitems argument specifies
the number of elements in list. The requested look-up operations are started in parallel.
NULL elements in list are ignored. Each request is described by a gaicb structure,
defined as follows:
struct gaicb {
const char *ar_name;
const char *ar_service;
const struct addrinfo *ar_request;
struct addrinfo *ar_result;
};
The elements of this structure correspond to the arguments of getaddrinfo(3). Thus,
ar_name corresponds to the node argument and ar_service to the service argument,
identifying an Internet host and a service. The ar_request element corresponds to the
hints argument, specifying the criteria for selecting the returned socket address
structures. Finally, ar_result corresponds to the res argument; you do not need to
initialize this element, it will be automatically set when the request is resolved. The
addrinfo structure referenced by the last two elements is described in getaddrinfo(3).
When mode is specified as GAI_NOWAIT, notifications about resolved requests can be
obtained by employing the sigevent structure pointed to by the sevp argument. For the
definition and general details of this structure, see sigevent(7). The sevp->sigev_notify
field can have the following values:
SIGEV_NONE
Don't provide any notification.
SIGEV_SIGNAL
When a look-up completes, generate the signal sigev_signo for the process. See
sigevent(7) for general details. The si_code field of the siginfo_t structure will
be set to SI_ASYNCNL.
SIGEV_THREAD
When a look-up completes, invoke sigev_notify_function as if it were the start
function of a new thread. See sigevent(7) for details.
For SIGEV_SIGNAL and SIGEV_THREAD, it may be useful to point sevp->sigev_value.sival_ptr
to list.
The gai_suspend() function suspends execution of the calling thread, waiting for the
completion of one or more requests in the array list. The nitems argument specifies the
size of the array list. The call blocks until one of the following occurs:
• One or more of the operations in list completes.
• The call is interrupted by a signal that is caught.
• The time interval specified in timeout elapses. This argument specifies a timeout in
seconds plus nanoseconds (see nanosleep(2) for details of the timespec structure). If
timeout is NULL, then the call blocks indefinitely (until one of the events above
occurs).
No explicit indication of which request was completed is given; you must determine which
request(s) have completed by iterating with gai_error() over the list of requests.
The gai_error() function returns the status of the request req: either EAI_INPROGRESS if
the request was not completed yet, 0 if it was handled successfully, or an error code if
the request could not be resolved.
The gai_cancel() function cancels the request req. If the request has been canceled
successfully, the error status of the request will be set to EAI_CANCELED and normal
asynchronous notification will be performed. The request cannot be canceled if it is
currently being processed; in that case, it will be handled as if gai_cancel() has never
been called. If req is NULL, an attempt is made to cancel all outstanding requests that
the process has made.
RETURN VALUE
The getaddrinfo_a() function returns 0 if all of the requests have been enqueued
successfully, or one of the following nonzero error codes:
EAI_AGAIN
The resources necessary to enqueue the look-up requests were not available. The
application may check the error status of each request to determine which ones
failed.
EAI_MEMORY
Out of memory.
EAI_SYSTEM
mode is invalid.
The gai_suspend() function returns 0 if at least one of the listed requests has been
completed. Otherwise, it returns one of the following nonzero error codes:
EAI_AGAIN
The given timeout expired before any of the requests could be completed.
EAI_ALLDONE
There were no actual requests given to the function.
EAI_INTR
A signal has interrupted the function. Note that this interruption might have been
caused by signal notification of some completed look-up request.
The gai_error() function can return EAI_INPROGRESS for an unfinished look-up request, 0
for a successfully completed look-up (as described above), one of the error codes that
could be returned by getaddrinfo(3), or the error code EAI_CANCELED if the request has
been canceled explicitly before it could be finished.
The gai_cancel() function can return one of these values:
EAI_CANCELED
The request has been canceled successfully.
EAI_NOTCANCELED
The request has not been canceled.
EAI_ALLDONE
The request has already completed.
The gai_strerror(3) function translates these error codes to a human readable string,
suitable for error reporting.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│getaddrinfo_a(), gai_suspend(), gai_error(), gai_cancel() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
These functions are GNU extensions; they first appeared in glibc 2.2.3.
NOTES
The interface of getaddrinfo_a() was modeled after the lio_listio(3) interface.
EXAMPLES
Two examples are provided: a simple example that resolves several requests in parallel
synchronously, and a complex example showing some of the asynchronous capabilities.
Synchronous example
The program below simply resolves several hostnames in parallel, giving a speed-up
compared to resolving the hostnames sequentially using getaddrinfo(3). The program might
be used like this:
$ ./a.out mirrors.kernel.org enoent.linuxfoundation.org gnu.org
mirrors.kernel.org: 139.178.88.99
enoent.linuxfoundation.org: Name or service not known
gnu.org: 209.51.188.116
Here is the program source code
#define _GNU_SOURCE
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(int argc, char *argv[])
{
int ret;
struct gaicb *reqs[argc - 1];
char host[NI_MAXHOST];
struct addrinfo *res;
if (argc < 2) {
fprintf(stderr, "Usage: %s HOST...\n", argv[0]);
exit(EXIT_FAILURE);
}
for (size_t i = 0; i < argc - 1; i++) {
reqs[i] = malloc(sizeof(*reqs[0]));
if (reqs[i] == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
memset(reqs[i], 0, sizeof(*reqs[0]));
reqs[i]->ar_name = argv[i + 1];
}
ret = getaddrinfo_a(GAI_WAIT, reqs, argc - 1, NULL);
if (ret != 0) {
fprintf(stderr, "getaddrinfo_a() failed: %s\n",
gai_strerror(ret));
exit(EXIT_FAILURE);
}
for (size_t i = 0; i < argc - 1; i++) {
printf("%s: ", reqs[i]->ar_name);
ret = gai_error(reqs[i]);
if (ret == 0) {
res = reqs[i]->ar_result;
ret = getnameinfo(res->ai_addr, res->ai_addrlen,
host, sizeof(host),
NULL, 0, NI_NUMERICHOST);
if (ret != 0) {
fprintf(stderr, "getnameinfo() failed: %s\n",
gai_strerror(ret));
exit(EXIT_FAILURE);
}
puts(host);
} else {
puts(gai_strerror(ret));
}
}
exit(EXIT_SUCCESS);
}
Asynchronous example
This example shows a simple interactive getaddrinfo_a() front-end. The notification
facility is not demonstrated.
An example session might look like this:
$ ./a.out
> a mirrors.kernel.org enoent.linuxfoundation.org gnu.org
> c 2
[2] gnu.org: Request not canceled
> w 0 1
[00] mirrors.kernel.org: Finished
> l
[00] mirrors.kernel.org: 139.178.88.99
[01] enoent.linuxfoundation.org: Processing request in progress
[02] gnu.org: 209.51.188.116
> l
[00] mirrors.kernel.org: 139.178.88.99
[01] enoent.linuxfoundation.org: Name or service not known
[02] gnu.org: 209.51.188.116
The program source is as follows:
#define _GNU_SOURCE
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static struct gaicb **reqs = NULL;
static size_t nreqs = 0;
static char *
getcmd(void)
{
static char buf[256];
fputs("> ", stdout); fflush(stdout);
if (fgets(buf, sizeof(buf), stdin) == NULL)
return NULL;
if (buf[strlen(buf) - 1] == '\n')
buf[strlen(buf) - 1] = 0;
return buf;
}
/* Add requests for specified hostnames. */
static void
add_requests(void)
{
size_t nreqs_base = nreqs;
char *host;
int ret;
while ((host = strtok(NULL, " "))) {
nreqs++;
reqs = realloc(reqs, sizeof(reqs[0]) * nreqs);
reqs[nreqs - 1] = calloc(1, sizeof(*reqs[0]));
reqs[nreqs - 1]->ar_name = strdup(host);
}
/* Queue nreqs_base..nreqs requests. */
ret = getaddrinfo_a(GAI_NOWAIT, &reqs[nreqs_base],
nreqs - nreqs_base, NULL);
if (ret) {
fprintf(stderr, "getaddrinfo_a() failed: %s\n",
gai_strerror(ret));
exit(EXIT_FAILURE);
}
}
/* Wait until at least one of specified requests completes. */
static void
wait_requests(void)
{
char *id;
int ret;
size_t n;
struct gaicb const **wait_reqs = calloc(nreqs, sizeof(*wait_reqs));
/* NULL elements are ignored by gai_suspend(). */
while ((id = strtok(NULL, " ")) != NULL) {
n = atoi(id);
if (n >= nreqs) {
printf("Bad request number: %s\n", id);
return;
}
wait_reqs[n] = reqs[n];
}
ret = gai_suspend(wait_reqs, nreqs, NULL);
if (ret) {
printf("gai_suspend(): %s\n", gai_strerror(ret));
return;
}
for (size_t i = 0; i < nreqs; i++) {
if (wait_reqs[i] == NULL)
continue;
ret = gai_error(reqs[i]);
if (ret == EAI_INPROGRESS)
continue;
printf("[%02zu] %s: %s\n", i, reqs[i]->ar_name,
ret == 0 ? "Finished" : gai_strerror(ret));
}
}
/* Cancel specified requests. */
static void
cancel_requests(void)
{
char *id;
int ret;
size_t n;
while ((id = strtok(NULL, " ")) != NULL) {
n = atoi(id);
if (n >= nreqs) {
printf("Bad request number: %s\n", id);
return;
}
ret = gai_cancel(reqs[n]);
printf("[%s] %s: %s\n", id, reqs[atoi(id)]->ar_name,
gai_strerror(ret));
}
}
/* List all requests. */
static void
list_requests(void)
{
int ret;
char host[NI_MAXHOST];
struct addrinfo *res;
for (size_t i = 0; i < nreqs; i++) {
printf("[%02zu] %s: ", i, reqs[i]->ar_name);
ret = gai_error(reqs[i]);
if (!ret) {
res = reqs[i]->ar_result;
ret = getnameinfo(res->ai_addr, res->ai_addrlen,
host, sizeof(host),
NULL, 0, NI_NUMERICHOST);
if (ret) {
fprintf(stderr, "getnameinfo() failed: %s\n",
gai_strerror(ret));
exit(EXIT_FAILURE);
}
puts(host);
} else {
puts(gai_strerror(ret));
}
}
}
int
main(void)
{
char *cmdline;
char *cmd;
while ((cmdline = getcmd()) != NULL) {
cmd = strtok(cmdline, " ");
if (cmd == NULL) {
list_requests();
} else {
switch (cmd[0]) {
case 'a':
add_requests();
break;
case 'w':
wait_requests();
break;
case 'c':
cancel_requests();
break;
case 'l':
list_requests();
break;
default:
fprintf(stderr, "Bad command: %c\n", cmd[0]);
break;
}
}
}
exit(EXIT_SUCCESS);
}
SEE ALSO
getaddrinfo(3), inet(3), lio_listio(3), hostname(7), ip(7), sigevent(7)