getaddrinfo_a(3) Library Functions Manual getaddrinfo_a(3)
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 immedi-
ately, 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 ig-
nored. Each request is described by a gaicb structure, defined as fol-
lows:
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 getad-
drinfo(3). Thus, ar_name corresponds to the node argument and ar_ser-
vice to the service argument, identifying an Internet host and a ser-
vice. The ar_request element corresponds to the hints argument, speci-
fying 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 re-
quests 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(3type). 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(3type) 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(3type) 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 speci-
fies a timeout in seconds plus nanoseconds (see nanosleep(2) for de-
tails 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 re-
quests 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 interrup-
tion might have been caused by signal notification of some com-
pleted 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 explic-
itly 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(), │ Thread safety │ MT-Safe │
│ gai_error(), gai_cancel() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
GNU.
HISTORY
glibc 2.2.3.
The interface of getaddrinfo_a() was modeled after the lio_listio(3) in-
terface.
EXAMPLES
Two examples are provided: a simple example that resolves several re-
quests 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 getad-
drinfo(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 <err.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MALLOC(n, type) ((type *) reallocarray(NULL, n, sizeof(type)))
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(1, struct gaicb);
if (reqs[i] == NULL)
err(EXIT_FAILURE, "malloc");
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 <assert.h>
#include <err.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CALLOC(n, type) ((type *) calloc(n, sizeof(type)))
#define REALLOCF(ptr, n, type) \
({ \
static_assert(__builtin_types_compatible_p(typeof(ptr), type *)); \
\
(type *) reallocarrayf(ptr, n, sizeof(type)); \
})
static struct gaicb **reqs = NULL;
static size_t nreqs = 0;
static inline void *
reallocarrayf(void *p, size_t nmemb, size_t size)
{
void *q;
q = reallocarray(p, nmemb, size);
if (q == NULL && nmemb != 0 && size != 0)
free(p);
return q;
}
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 = REALLOCF(reqs, nreqs, struct gaicb *);
if (reqs == NULL)
err(EXIT_FAILURE, "reallocf");
reqs[nreqs - 1] = CALLOC(1, struct gaicb);
if (reqs[nreqs - 1] == NULL)
err(EXIT_FAILURE, "calloc");
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;
wait_reqs = CALLOC(nreqs, const struct gaicb *);
if (wait_reqs == NULL)
err(EXIT_FAILURE, "calloc");
/* 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(3type)
Linux man-pages 6.9.1 2024-06-15 getaddrinfo_a(3)
Generated by dwww version 1.16 on Tue Dec 16 04:03:13 CET 2025.