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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ATTRIBUTES |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
getaddrinfo_a(3) Library Functions Manualgetaddrinfo_a(3)getaddrinfo_a, gai_suspend, gai_error, gai_cancel - asynchronous network address and service translation
Asynchronous name lookup library (libanl,-lanl)
#define _GNU_SOURCE/* See feature_test_macros(7) */#include <netdb.h>int getaddrinfo_a(intmode, struct gaicb *list[restrictn],intn, struct sigevent *restrictsevp);int gai_suspend(const struct gaicb *constlist[n], intn,const struct timespec *timeout);int gai_error(struct gaicb *req);int gai_cancel(struct gaicb *req);
Thegetaddrinfo_a() function performs the same task asgetaddrinfo(3), but allows multiple name look-ups to be performed asynchronously, with optional notification on completion of look- up operations. Themode 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 thesevp argument below. The arraylist specifies the look-up requests to process. Then argument specifies the number of elements inlist. The requested look-up operations are started in parallel. NULL elements inlist are ignored. Each request is described by agaicb 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 ofgetaddrinfo(3). Thus,ar_name corresponds to thenode argument andar_service to theservice argument, identifying an Internet host and a service. Thear_request element corresponds to thehints argument, specifying the criteria for selecting the returned socket address structures. Finally,ar_result corresponds to theres argument; you do not need to initialize this element, it will be automatically set when the request is resolved. Theaddrinfo structure referenced by the last two elements is described ingetaddrinfo(3). Whenmode is specified asGAI_NOWAIT, notifications about resolved requests can be obtained by employing thesigevent structure pointed to by thesevp argument. For the definition and general details of this structure, seesigevent(3type). Thesevp->sigev_notify field can have the following values:SIGEV_NONE Don't provide any notification.SIGEV_SIGNAL When a look-up completes, generate the signalsigev_signo for the process. Seesigevent(3type) for general details. Thesi_code field of thesiginfo_t structure will be set toSI_ASYNCNL.SIGEV_THREAD When a look-up completes, invokesigev_notify_function as if it were the start function of a new thread. Seesigevent(3type) for details. ForSIGEV_SIGNALandSIGEV_THREAD, it may be useful to pointsevp->sigev_value.sival_ptr tolist. Thegai_suspend() function suspends execution of the calling thread, waiting for the completion of one or more requests in the arraylist. Then argument specifies the size of the arraylist. The call blocks until one of the following occurs: • One or more of the operations inlist completes. • The call is interrupted by a signal that is caught. • The time interval specified intimeout elapses. This argument specifies a timeout in seconds plus nanoseconds (seenanosleep(2) for details of thetimespec structure). Iftimeout 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 withgai_error() over the list of requests. Thegai_error() function returns the status of the requestreq: eitherEAI_INPROGRESSif the request was not completed yet, 0 if it was handled successfully, or an error code if the request could not be resolved. Thegai_cancel() function cancels the requestreq. If the request has been canceled successfully, the error status of the request will be set toEAI_CANCELEDand 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 ifgai_cancel() has never been called. Ifreq is NULL, an attempt is made to cancel all outstanding requests that the process has made.Thegetaddrinfo_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_SYSTEMmode is invalid. Thegai_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. Thegai_error() function can returnEAI_INPROGRESSfor an unfinished look-up request, 0 for a successfully completed look-up (as described above), one of the error codes that could be returned bygetaddrinfo(3), or the error codeEAI_CANCELEDif the request has been canceled explicitly before it could be finished. Thegai_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. Thegai_strerror(3) function translates these error codes to a human readable string, suitable for error reporting.
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │getaddrinfo_a(),gai_suspend(), │ Thread safety │ MT-Safe │ │gai_error(),gai_cancel() │ │ │ └──────────────────────────────────────┴───────────────┴─────────┘
GNU.
glibc 2.2.3. The interface ofgetaddrinfo_a() was modeled after thelio_listio(3) interface.
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 usinggetaddrinfo(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 interactivegetaddrinfo_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 n, size_t size) { void *q; q = reallocarray(p, n, size); if (q == NULL && n != 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); }getaddrinfo(3),inet(3),lio_listio(3),hostname(7),ip(7),sigevent(3type)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-05getaddrinfo_a(3)Pages that refer to this page:getaddrinfo(3), sigevent(3type), strtok(3)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |