'\" t
.\" Copyright (c) 2009 Petr Baudis <pasky@suse.cz>
.\" and clean-ups and additions (C) Copyright 2010 Michael Kerrisk
.\"                                 <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References: http://people.redhat.com/drepper/asynchnl.pdf,
.\"     http://www.imperialviolet.org/2005/06/01/asynchronous-dns-lookups-with-glibc.html
.\"
.TH getaddrinfo_a 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
getaddrinfo_a, gai_suspend, gai_error, gai_cancel \- asynchronous
network address and service translation
.SH LIBRARY
Asynchronous name lookup library
.RI ( libanl ", " \-lanl )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <netdb.h>
.PP
.BI "int getaddrinfo_a(int " mode ", struct gaicb *" list [restrict],
.BI "                  int " nitems ", struct sigevent *restrict " sevp );
.BI "int gai_suspend(const struct gaicb *const " list "[], int " nitems ,
.BI "                  const struct timespec *" timeout );
.PP
.BI "int gai_error(struct gaicb *" req );
.BI "int gai_cancel(struct gaicb *" req );
.fi
.SH DESCRIPTION
The
.BR getaddrinfo_a ()
function performs the same task as
.BR getaddrinfo (3),
but allows multiple name look-ups to be performed asynchronously,
with optional notification on completion of look-up operations.
.PP
The
.I mode
argument has one of the following values:
.TP
.B GAI_WAIT
Perform the look-ups synchronously.
The call blocks until the look-ups have completed.
.TP
.B GAI_NOWAIT
Perform the look-ups asynchronously.
The call returns immediately,
and the requests are resolved in the background.
See the discussion of the
.I sevp
argument below.
.PP
The array
.I list
specifies the look-up requests to process.
The
.I nitems
argument specifies the number of elements in
.IR list .
The requested look-up operations are started in parallel.
NULL elements in
.I list
are ignored.
Each request is described by a
.I gaicb
structure, defined as follows:
.PP
.in +4n
.EX
struct gaicb {
    const char            *ar_name;
    const char            *ar_service;
    const struct addrinfo *ar_request;
    struct addrinfo       *ar_result;
};
.EE
.in
.PP
The elements of this structure correspond to the arguments of
.BR getaddrinfo (3).
Thus,
.I ar_name
corresponds to the
.I node
argument and
.I ar_service
to the
.I service
argument, identifying an Internet host and a service.
The
.I ar_request
element corresponds to the
.I hints
argument, specifying the criteria for selecting
the returned socket address structures.
Finally,
.I ar_result
corresponds to the
.I res
argument; you do not need to initialize this element,
it will be automatically set when the request
is resolved.
The
.I addrinfo
structure referenced by the last two elements is described in
.BR getaddrinfo (3).
.PP
When
.I mode
is specified as
.BR GAI_NOWAIT ,
notifications about resolved requests
can be obtained by employing the
.I sigevent
structure pointed to by the
.I sevp
argument.
For the definition and general details of this structure, see
.BR sigevent (7).
The
.I sevp\->sigev_notify
field can have the following values:
.TP
.B SIGEV_NONE
Don't provide any notification.
.TP
.B SIGEV_SIGNAL
When a look-up completes, generate the signal
.I sigev_signo
for the process.
See
.BR sigevent (7)
for general details.
The
.I si_code
field of the
.I siginfo_t
structure will be set to
.BR SI_ASYNCNL .
.\" si_pid and si_uid are also set, to the values of the calling process,
.\" which doesn't provide useful information, so we'll skip mentioning it.
.TP
.B SIGEV_THREAD
When a look-up completes, invoke
.I sigev_notify_function
as if it were the start function of a new thread.
See
.BR sigevent (7)
for details.
.PP
For
.B SIGEV_SIGNAL
and
.BR SIGEV_THREAD ,
it may be useful to point
.I sevp\->sigev_value.sival_ptr
to
.IR list .
.PP
The
.BR gai_suspend ()
function suspends execution of the calling thread,
waiting for the completion of one or more requests in the array
.IR list .
The
.I nitems
argument specifies the size of the array
.IR list .
The call blocks until one of the following occurs:
.IP \[bu] 3
One or more of the operations in
.I list
completes.
.IP \[bu]
The call is interrupted by a signal that is caught.
.IP \[bu]
The time interval specified in
.I timeout
elapses.
This argument specifies a timeout in seconds plus nanoseconds (see
.BR nanosleep (2)
for details of the
.I timespec
structure).
If
.I timeout
is NULL, then the call blocks indefinitely
(until one of the events above occurs).
.PP
No explicit indication of which request was completed is given;
you must determine which request(s) have completed by iterating with
.BR gai_error ()
over the list of requests.
.PP
The
.BR gai_error ()
function returns the status of the request
.IR req :
either
.B 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.
.PP
The
.BR gai_cancel ()
function cancels the request
.IR req .
If the request has been canceled successfully,
the error status of the request will be set to
.B 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
.BR gai_cancel ()
has never been called.
If
.I req
is NULL, an attempt is made to cancel all outstanding requests
that the process has made.
.SH RETURN VALUE
The
.BR getaddrinfo_a ()
function returns 0 if all of the requests have been enqueued successfully,
or one of the following nonzero error codes:
.TP
.B 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.
.TP
.B EAI_MEMORY
Out of memory.
.TP
.B EAI_SYSTEM
.I mode
is invalid.
.PP
The
.BR 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:
.TP
.B EAI_AGAIN
The given timeout expired before any of the requests could be completed.
.TP
.B EAI_ALLDONE
There were no actual requests given to the function.
.TP
.B 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.
.PP
The
.BR gai_error ()
function can return
.B 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
.BR getaddrinfo (3),
or the error code
.B EAI_CANCELED
if the request has been canceled explicitly before it could be finished.
.PP
The
.BR gai_cancel ()
function can return one of these values:
.TP
.B EAI_CANCELED
The request has been canceled successfully.
.TP
.B EAI_NOTCANCELED
The request has not been canceled.
.TP
.B EAI_ALLDONE
The request has already completed.
.PP
The
.BR gai_strerror (3)
function translates these error codes to a human readable string,
suitable for error reporting.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR getaddrinfo_a (),
.BR gai_suspend (),
.BR gai_error (),
.BR gai_cancel ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
GNU.
.SH HISTORY
glibc 2.2.3.
.PP
The interface of
.BR getaddrinfo_a ()
was modeled after the
.BR lio_listio (3)
interface.
.SH 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.
.SS Synchronous example
The program below simply resolves several hostnames in parallel,
giving a speed-up compared to resolving the hostnames sequentially using
.BR getaddrinfo (3).
The program might be used like this:
.PP
.in +4n
.EX
$ \fB./a.out mirrors.kernel.org enoent.linuxfoundation.org gnu.org\fP
mirrors.kernel.org: 139.178.88.99
enoent.linuxfoundation.org: Name or service not known
gnu.org: 209.51.188.116
.EE
.in
.PP
Here is the program source code
.PP
.\" SRC BEGIN (sync.c)
.EX
#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...\en", 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\en",
                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\en",
                        gai_strerror(ret));
                exit(EXIT_FAILURE);
            }
            puts(host);

        } else {
            puts(gai_strerror(ret));
        }
    }
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SS Asynchronous example
This example shows a simple interactive
.BR getaddrinfo_a ()
front-end.
The notification facility is not demonstrated.
.PP
An example session might look like this:
.PP
.in +4n
.EX
$ \fB./a.out\fP
> 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
.EE
.in
.PP
The program source is as follows:
.PP
.\" SRC BEGIN (async.c)
.EX
#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] == \[aq]\en\[aq])
        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\en",
                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\en", id);
            return;
        }

        wait_reqs[n] = reqs[n];
    }

    ret = gai_suspend(wait_reqs, nreqs, NULL);
    if (ret) {
        printf("gai_suspend(): %s\en", 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\en", 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\en", id);
            return;
        }

        ret = gai_cancel(reqs[n]);
        printf("[%s] %s: %s\en", 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\en",
                        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 \[aq]a\[aq]:
                add_requests();
                break;
            case \[aq]w\[aq]:
                wait_requests();
                break;
            case \[aq]c\[aq]:
                cancel_requests();
                break;
            case \[aq]l\[aq]:
                list_requests();
                break;
            default:
                fprintf(stderr, "Bad command: %c\en", cmd[0]);
                break;
            }
        }
    }
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR getaddrinfo (3),
.BR inet (3),
.BR lio_listio (3),
.BR hostname (7),
.BR ip (7),
.BR sigevent (7)
