1AIO(7) Linux Programmer's Manual AIO(7)
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6 aio - POSIX asynchronous I/O overview
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9 The POSIX asynchronous I/O (AIO) interface allows applications to ini‐
10 tiate one or more I/O operations that are performed asynchronously
11 (i.e., in the background). The application can elect to be notified of
12 completion of the I/O operation in a variety of ways: by delivery of a
13 signal, by instantiation of a thread, or no notification at all.
14 The POSIX AIO interface consists of the following functions:
16 aio_read(3)
18 Enqueue a read request. This is the asynchronous analog of
19 read(2).
20 aio_write(3)
22 Enqueue a write request. This is the asynchronous analog of
23 write(2).
24 aio_fsync(3)
26 Enqueue a sync request for the I/O operations on a file descrip‐
27 tor. This is the asynchronous analog of fsync(2) and fdata‐
28 sync(2).
29 aio_error(3)
31 Obtain the error status of an enqueued I/O request.
32 aio_return(3)
34 Obtain the return status of a completed I/O request.
35 aio_suspend(3)
37 Suspend the caller until one or more of a specified set of I/O
38 requests completes.
39 aio_cancel(3)
41 Attempt to cancel outstanding I/O requests on a specified file
42 descriptor.
43 lio_listio(3)
45 Enqueue multiple I/O requests using a single function call.
46 The aiocb ("asynchronous I/O control block") structure defines parame‐
48 ters that control an I/O operation. An argument of this type is em‐
49 ployed with all of the functions listed above. This structure has the
50 following form:
51 #include <aiocb.h>
53 struct aiocb {
55 /* The order of these fields is implementation-dependent */
56 int aio_fildes; /* File descriptor */
58 off_t aio_offset; /* File offset */
59 volatile void *aio_buf; /* Location of buffer */
60 size_t aio_nbytes; /* Length of transfer */
61 int aio_reqprio; /* Request priority */
62 struct sigevent aio_sigevent; /* Notification method */
63 int aio_lio_opcode; /* Operation to be performed;
64 lio_listio() only */
65 /* Various implementation-internal fields not shown */
67 };
68 /* Operation codes for 'aio_lio_opcode': */
70 enum { LIO_READ, LIO_WRITE, LIO_NOP };
72 The fields of this structure are as follows:
74 aio_fildes
76 The file descriptor on which the I/O operation is to be per‐
77 formed.
78 aio_offset
80 This is the file offset at which the I/O operation is to be per‐
81 formed.
82 aio_buf
84 This is the buffer used to transfer data for a read or write op‐
85 eration.
86 aio_nbytes
88 This is the size of the buffer pointed to by aio_buf.
89 aio_reqprio
91 This field specifies a value that is subtracted from the calling
92 thread's real-time priority in order to determine the priority
93 for execution of this I/O request (see pthread_setsched‐
94 param(3)). The specified value must be between 0 and the value
95 returned by sysconf(_SC_AIO_PRIO_DELTA_MAX). This field is ig‐
96 nored for file synchronization operations.
97 aio_sigevent
99 This field is a structure that specifies how the caller is to be
100 notified when the asynchronous I/O operation completes. Possi‐
101 ble values for aio_sigevent.sigev_notify are SIGEV_NONE,
102 SIGEV_SIGNAL, and SIGEV_THREAD. See sigevent(7) for further de‐
103 tails.
104 aio_lio_opcode
106 The type of operation to be performed; used only for lio_lis‐
107 tio(3).
108 In addition to the standard functions listed above, the GNU C library
110 provides the following extension to the POSIX AIO API:
111 aio_init(3)
113 Set parameters for tuning the behavior of the glibc POSIX AIO
114 implementation.
115
117 EINVAL The aio_reqprio field of the aiocb structure was less than 0, or
118 was greater than the limit returned by the call
119 sysconf(_SC_AIO_PRIO_DELTA_MAX).
120
122 The POSIX AIO interfaces are provided by glibc since version 2.1.
123
125 POSIX.1-2001, POSIX.1-2008.
126
128 It is a good idea to zero out the control block buffer before use (see
129 memset(3)). The control block buffer and the buffer pointed to by
130 aio_buf must not be changed while the I/O operation is in progress.
131 These buffers must remain valid until the I/O operation completes.
132 Simultaneous asynchronous read or write operations using the same aiocb
134 structure yield undefined results.
135 The current Linux POSIX AIO implementation is provided in user space by
137 glibc. This has a number of limitations, most notably that maintaining
138 multiple threads to perform I/O operations is expensive and scales
139 poorly. Work has been in progress for some time on a kernel state-ma‐
140 chine-based implementation of asynchronous I/O (see io_submit(2),
141 io_setup(2), io_cancel(2), io_destroy(2), io_getevents(2)), but this
142 implementation hasn't yet matured to the point where the POSIX AIO im‐
143 plementation can be completely reimplemented using the kernel system
144 calls.
145
147 The program below opens each of the files named in its command-line ar‐
148 guments and queues a request on the resulting file descriptor using
149 aio_read(3). The program then loops, periodically monitoring each of
150 the I/O operations that is still in progress using aio_error(3). Each
151 of the I/O requests is set up to provide notification by delivery of a
152 signal. After all I/O requests have completed, the program retrieves
153 their status using aio_return(3).
154 The SIGQUIT signal (generated by typing control-\) causes the program
156 to request cancellation of each of the outstanding requests using
157 aio_cancel(3).
158 Here is an example of what we might see when running this program. In
160 this example, the program queues two requests to standard input, and
161 these are satisfied by two lines of input containing "abc" and "x".
162 $ ./a.out /dev/stdin /dev/stdin
164 opened /dev/stdin on descriptor 3
165 opened /dev/stdin on descriptor 4
166 aio_error():
167 for request 0 (descriptor 3): In progress
168 for request 1 (descriptor 4): In progress
169 abc
170 I/O completion signal received
171 aio_error():
172 for request 0 (descriptor 3): I/O succeeded
173 for request 1 (descriptor 4): In progress
174 aio_error():
175 for request 1 (descriptor 4): In progress
176 x
177 I/O completion signal received
178 aio_error():
179 for request 1 (descriptor 4): I/O succeeded
180 All I/O requests completed
181 aio_return():
182 for request 0 (descriptor 3): 4
183 for request 1 (descriptor 4): 2
184 Program source
186 #include <fcntl.h>
188 #include <stdlib.h>
189 #include <unistd.h>
190 #include <stdio.h>
191 #include <errno.h>
192 #include <aio.h>
193 #include <signal.h>
194 #define BUF_SIZE 20 /* Size of buffers for read operations */
196 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
198 struct ioRequest { /* Application-defined structure for tracking
200 I/O requests */
201 int reqNum;
202 int status;
203 struct aiocb *aiocbp;
204 };
205 static volatile sig_atomic_t gotSIGQUIT = 0;
207 /* On delivery of SIGQUIT, we attempt to
208 cancel all outstanding I/O requests */
209 static void /* Handler for SIGQUIT */
211 quitHandler(int sig)
212 {
213 gotSIGQUIT = 1;
214 }
215 #define IO_SIGNAL SIGUSR1 /* Signal used to notify I/O completion */
217 static void /* Handler for I/O completion signal */
219 aioSigHandler(int sig, siginfo_t *si, void *ucontext)
220 {
221 if (si->si_code == SI_ASYNCIO) {
222 write(STDOUT_FILENO, "I/O completion signal received\n", 31);
223 /* The corresponding ioRequest structure would be available as
225 struct ioRequest *ioReq = si->si_value.sival_ptr;
226 and the file descriptor would then be available via
227 ioReq->aiocbp->aio_fildes */
228 }
229 }
230 int
232 main(int argc, char *argv[])
233 {
234 struct sigaction sa;
235 int s;
236 int numReqs; /* Total number of queued I/O requests */
237 int openReqs; /* Number of I/O requests still in progress */
238 if (argc < 2) {
240 fprintf(stderr, "Usage: %s <pathname> <pathname>...\n",
241 argv[0]);
242 exit(EXIT_FAILURE);
243 }
244 numReqs = argc - 1;
246 /* Allocate our arrays. */
248 struct ioRequest *ioList = calloc(numReqs, sizeof(*ioList));
250 if (ioList == NULL)
251 errExit("calloc");
252 struct aiocb *aiocbList = calloc(numReqs, sizeof(*aiocbList));
254 if (aiocbList == NULL)
255 errExit("calloc");
256 /* Establish handlers for SIGQUIT and the I/O completion signal. */
258 sa.sa_flags = SA_RESTART;
260 sigemptyset(&sa.sa_mask);
261 sa.sa_handler = quitHandler;
263 if (sigaction(SIGQUIT, &sa, NULL) == -1)
264 errExit("sigaction");
265 sa.sa_flags = SA_RESTART | SA_SIGINFO;
267 sa.sa_sigaction = aioSigHandler;
268 if (sigaction(IO_SIGNAL, &sa, NULL) == -1)
269 errExit("sigaction");
270 /* Open each file specified on the command line, and queue
272 a read request on the resulting file descriptor. */
273 for (int j = 0; j < numReqs; j++) {
275 ioList[j].reqNum = j;
276 ioList[j].status = EINPROGRESS;
277 ioList[j].aiocbp = &aiocbList[j];
278 ioList[j].aiocbp->aio_fildes = open(argv[j + 1], O_RDONLY);
280 if (ioList[j].aiocbp->aio_fildes == -1)
281 errExit("open");
282 printf("opened %s on descriptor %d\n", argv[j + 1],
283 ioList[j].aiocbp->aio_fildes);
284 ioList[j].aiocbp->aio_buf = malloc(BUF_SIZE);
286 if (ioList[j].aiocbp->aio_buf == NULL)
287 errExit("malloc");
288 ioList[j].aiocbp->aio_nbytes = BUF_SIZE;
290 ioList[j].aiocbp->aio_reqprio = 0;
291 ioList[j].aiocbp->aio_offset = 0;
292 ioList[j].aiocbp->aio_sigevent.sigev_notify = SIGEV_SIGNAL;
293 ioList[j].aiocbp->aio_sigevent.sigev_signo = IO_SIGNAL;
294 ioList[j].aiocbp->aio_sigevent.sigev_value.sival_ptr =
295 &ioList[j];
296 s = aio_read(ioList[j].aiocbp);
298 if (s == -1)
299 errExit("aio_read");
300 }
301 openReqs = numReqs;
303 /* Loop, monitoring status of I/O requests. */
305 while (openReqs > 0) {
307 sleep(3); /* Delay between each monitoring step */
308 if (gotSIGQUIT) {
310 /* On receipt of SIGQUIT, attempt to cancel each of the
312 outstanding I/O requests, and display status returned
313 from the cancellation requests. */
314 printf("got SIGQUIT; canceling I/O requests: \n");
316 for (int j = 0; j < numReqs; j++) {
318 if (ioList[j].status == EINPROGRESS) {
319 printf(" Request %d on descriptor %d:", j,
320 ioList[j].aiocbp->aio_fildes);
321 s = aio_cancel(ioList[j].aiocbp->aio_fildes,
322 ioList[j].aiocbp);
323 if (s == AIO_CANCELED)
324 printf("I/O canceled\n");
325 else if (s == AIO_NOTCANCELED)
326 printf("I/O not canceled\n");
327 else if (s == AIO_ALLDONE)
328 printf("I/O all done\n");
329 else
330 perror("aio_cancel");
331 }
332 }
333 gotSIGQUIT = 0;
335 }
336 /* Check the status of each I/O request that is still
338 in progress. */
339 printf("aio_error():\n");
341 for (int j = 0; j < numReqs; j++) {
342 if (ioList[j].status == EINPROGRESS) {
343 printf(" for request %d (descriptor %d): ",
344 j, ioList[j].aiocbp->aio_fildes);
345 ioList[j].status = aio_error(ioList[j].aiocbp);
346 switch (ioList[j].status) {
348 case 0:
349 printf("I/O succeeded\n");
350 break;
351 case EINPROGRESS:
352 printf("In progress\n");
353 break;
354 case ECANCELED:
355 printf("Canceled\n");
356 break;
357 default:
358 perror("aio_error");
359 break;
360 }
361 if (ioList[j].status != EINPROGRESS)
363 openReqs--;
364 }
365 }
366 }
367 printf("All I/O requests completed\n");
369 /* Check status return of all I/O requests. */
371 printf("aio_return():\n");
373 for (int j = 0; j < numReqs; j++) {
374 ssize_t s;
375 s = aio_return(ioList[j].aiocbp);
377 printf(" for request %d (descriptor %d): %zd\n",
378 j, ioList[j].aiocbp->aio_fildes, s);
379 }
380 exit(EXIT_SUCCESS);
382 }
383
385 io_cancel(2), io_destroy(2), io_getevents(2), io_setup(2),
386 io_submit(2), aio_cancel(3), aio_error(3), aio_init(3), aio_read(3),
387 aio_return(3), aio_write(3), lio_listio(3)
388 "Asynchronous I/O Support in Linux 2.5", Bhattacharya, Pratt,
390 Pulavarty, and Morgan, Proceedings of the Linux Symposium, 2003,
391 ⟨https://www.kernel.org/doc/ols/2003/ols2003-pages-351-366.pdf⟩
392
394 This page is part of release 5.13 of the Linux man-pages project. A
395 description of the project, information about reporting bugs, and the
396 latest version of this page, can be found at
397 https://www.kernel.org/doc/man-pages/.
398Linux 2021-03-22 AIO(7)