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# Content
1 =head1 NAME
2
3 libeio - truly asynchronous POSIX I/O
4
5 =head1 SYNOPSIS
6
7 #include <eio.h>
8
9 =head1 DESCRIPTION
10
11 The newest version of this document is also available as an html-formatted
12 web page you might find easier to navigate when reading it for the first
13 time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.
14
15 Note that this library is a by-product of the C<IO::AIO> perl
16 module, and many of the subtler points regarding requests lifetime
17 and so on are only documented in its documentation at the
18 moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.
19
20 =head2 FEATURES
21
22 This library provides fully asynchronous versions of most POSIX functions
23 dealing with I/O. Unlike most asynchronous libraries, this not only
24 includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
25 similar functions, as well as less rarely ones such as C<mknod>, C<futime>
26 or C<readlink>.
27
28 It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
29 FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
30 emulation elsewhere>).
31
32 The goal is to enable you to write fully non-blocking programs. For
33 example, in a game server, you would not want to freeze for a few seconds
34 just because the server is running a backup and you happen to call
35 C<readdir>.
36
37 =head2 TIME REPRESENTATION
38
39 Libeio represents time as a single floating point number, representing the
40 (fractional) number of seconds since the (POSIX) epoch (somewhere near
41 the beginning of 1970, details are complicated, don't ask). This type is
42 called C<eio_tstamp>, but it is guaranteed to be of type C<double> (or
43 better), so you can freely use C<double> yourself.
44
45 Unlike the name component C<stamp> might indicate, it is also used for
46 time differences throughout libeio.
47
48 =head2 FORK SUPPORT
49
50 Usage of pthreads in a program changes the semantics of fork
51 considerably. Specifically, only async-safe functions can be called after
52 fork. Libeio uses pthreads, so this applies, and makes using fork hard for
53 anything but relatively fork + exec uses.
54
55 This library only works in the process that initialised it: Forking is
56 fully supported, but using libeio in any other process than the one that
57 called C<eio_init> is not.
58
59 You might get around by not I<using> libeio before (or after) forking in
60 the parent, and using it in the child afterwards. You could also try to
61 call the L<eio_init> function again in the child, which will brutally
62 reinitialise all data structures, which isn't POSIX conformant, but
63 typically works.
64
65 Otherwise, the only recommendation you should follow is: treat fork code
66 the same way you treat signal handlers, and only ever call C<eio_init> in
67 the process that uses it, and only once ever.
68
69 =head1 INITIALISATION/INTEGRATION
70
71 Before you can call any eio functions you first have to initialise the
72 library. The library integrates into any event loop, but can also be used
73 without one, including in polling mode.
74
75 You have to provide the necessary glue yourself, however.
76
77 =over 4
78
79 =item int eio_init (void (*want_poll)(void), void (*done_poll)(void))
80
81 This function initialises the library. On success it returns C<0>, on
82 failure it returns C<-1> and sets C<errno> appropriately.
83
84 It accepts two function pointers specifying callbacks as argument, both of
85 which can be C<0>, in which case the callback isn't called.
86
87 There is currently no way to change these callbacks later, or to
88 "uninitialise" the library again.
89
90 =item want_poll callback
91
92 The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
93 it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
94 that is, it will only be called once when eio wants attention, until all
95 pending requests have been handled.
96
97 This callback is called while locks are being held, so I<you must
98 not call any libeio functions inside this callback>. That includes
99 C<eio_poll>. What you should do is notify some other thread, or wake up
100 your event loop, and then call C<eio_poll>.
101
102 =item done_poll callback
103
104 This callback is invoked when libeio detects that all pending requests
105 have been handled. It is "edge-triggered", that is, it will only be
106 called once after C<want_poll>. To put it differently, C<want_poll> and
107 C<done_poll> are invoked in pairs: after C<want_poll> you have to call
108 C<eio_poll ()> until either C<eio_poll> indicates that everything has been
109 handled or C<done_poll> has been called, which signals the same.
110
111 Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
112 have been called again, so watch out for races in your code.
113
114 As with C<want_poll>, this callback is called while locks are being held,
115 so you I<must not call any libeio functions form within this callback>.
116
117 =item int eio_poll ()
118
119 This function has to be called whenever there are pending requests that
120 need finishing. You usually call this after C<want_poll> has indicated
121 that you should do so, but you can also call this function regularly to
122 poll for new results.
123
124 If any request invocation returns a non-zero value, then C<eio_poll ()>
125 immediately returns with that value as return value.
126
127 Otherwise, if all requests could be handled, it returns C<0>. If for some
128 reason not all requests have been handled, i.e. some are still pending, it
129 returns C<-1>.
130
131 =back
132
133 For libev, you would typically use an C<ev_async> watcher: the
134 C<want_poll> callback would invoke C<ev_async_send> to wake up the event
135 loop. Inside the callback set for the watcher, one would call C<eio_poll
136 ()>.
137
138 If C<eio_poll ()> is configured to not handle all results in one go
139 (i.e. it returns C<-1>) then you should start an idle watcher that calls
140 C<eio_poll> until it returns something C<!= -1>.
141
142 A full-featured connector between libeio and libev would look as follows
143 (if C<eio_poll> is handling all requests, it can of course be simplified a
144 lot by removing the idle watcher logic):
145
146 static struct ev_loop *loop;
147 static ev_idle repeat_watcher;
148 static ev_async ready_watcher;
149
150 /* idle watcher callback, only used when eio_poll */
151 /* didn't handle all results in one call */
152 static void
153 repeat (EV_P_ ev_idle *w, int revents)
154 {
155 if (eio_poll () != -1)
156 ev_idle_stop (EV_A_ w);
157 }
158
159 /* eio has some results, process them */
160 static void
161 ready (EV_P_ ev_async *w, int revents)
162 {
163 if (eio_poll () == -1)
164 ev_idle_start (EV_A_ &repeat_watcher);
165 }
166
167 /* wake up the event loop */
168 static void
169 want_poll (void)
170 {
171 ev_async_send (loop, &ready_watcher)
172 }
173
174 void
175 my_init_eio ()
176 {
177 loop = EV_DEFAULT;
178
179 ev_idle_init (&repeat_watcher, repeat);
180 ev_async_init (&ready_watcher, ready);
181 ev_async_start (loop &watcher);
182
183 eio_init (want_poll, 0);
184 }
185
186 For most other event loops, you would typically use a pipe - the event
187 loop should be told to wait for read readiness on the read end. In
188 C<want_poll> you would write a single byte, in C<done_poll> you would try
189 to read that byte, and in the callback for the read end, you would call
190 C<eio_poll>.
191
192 You don't have to take special care in the case C<eio_poll> doesn't handle
193 all requests, as the done callback will not be invoked, so the event loop
194 will still signal readiness for the pipe until I<all> results have been
195 processed.
196
197
198 =head1 HIGH LEVEL REQUEST API
199
200 Libeio has both a high-level API, which consists of calling a request
201 function with a callback to be called on completion, and a low-level API
202 where you fill out request structures and submit them.
203
204 This section describes the high-level API.
205
206 =head2 REQUEST SUBMISSION AND RESULT PROCESSING
207
208 You submit a request by calling the relevant C<eio_TYPE> function with the
209 required parameters, a callback of type C<int (*eio_cb)(eio_req *req)>
210 (called C<eio_cb> below) and a freely usable C<void *data> argument.
211
212 The return value will either be 0, in case something went really wrong
213 (which can basically only happen on very fatal errors, such as C<malloc>
214 returning 0, which is rather unlikely), or a pointer to the newly-created
215 and submitted C<eio_req *>.
216
217 The callback will be called with an C<eio_req *> which contains the
218 results of the request. The members you can access inside that structure
219 vary from request to request, except for:
220
221 =over 4
222
223 =item C<ssize_t result>
224
225 This contains the result value from the call (usually the same as the
226 syscall of the same name).
227
228 =item C<int errorno>
229
230 This contains the value of C<errno> after the call.
231
232 =item C<void *data>
233
234 The C<void *data> member simply stores the value of the C<data> argument.
235
236 =back
237
238 The return value of the callback is normally C<0>, which tells libeio to
239 continue normally. If a callback returns a nonzero value, libeio will
240 stop processing results (in C<eio_poll>) and will return the value to its
241 caller.
242
243 Memory areas passed to libeio must stay valid as long as a request
244 executes, with the exception of paths, which are being copied
245 internally. Any memory libeio itself allocates will be freed after the
246 finish callback has been called. If you want to manage all memory passed
247 to libeio yourself you can use the low-level API.
248
249 For example, to open a file, you could do this:
250
251 static int
252 file_open_done (eio_req *req)
253 {
254 if (req->result < 0)
255 {
256 /* open() returned -1 */
257 errno = req->errorno;
258 perror ("open");
259 }
260 else
261 {
262 int fd = req->result;
263 /* now we have the new fd in fd */
264 }
265
266 return 0;
267 }
268
269 /* the first three arguments are passed to open(2) */
270 /* the remaining are priority, callback and data */
271 if (!eio_open ("/etc/passwd", O_RDONLY, 0, 0, file_open_done, 0))
272 abort (); /* something went wrong, we will all die!!! */
273
274 Note that you additionally need to call C<eio_poll> when the C<want_cb>
275 indicates that requests are ready to be processed.
276
277 =head2 CANCELLING REQUESTS
278
279 Sometimes the need for a request goes away before the request is
280 finished. In that case, one can cancel the request by a call to
281 C<eio_cancel>:
282
283 =over 4
284
285 =item eio_cancel (eio_req *req)
286
287 Cancel the request (and all its subrequests). If the request is currently
288 executing it might still continue to execute, and in other cases it might
289 still take a while till the request is cancelled.
290
291 Even if cancelled, the finish callback will still be invoked - the
292 callbacks of all cancellable requests need to check whether the request
293 has been cancelled by calling C<EIO_CANCELLED (req)>:
294
295 static int
296 my_eio_cb (eio_req *req)
297 {
298 if (EIO_CANCELLED (req))
299 return 0;
300 }
301
302 In addition, cancelled requests will I<either> have C<< req->result >>
303 set to C<-1> and C<errno> to C<ECANCELED>, or I<otherwise> they were
304 successfully executed, despite being cancelled (e.g. when they have
305 already been executed at the time they were cancelled).
306
307 C<EIO_CANCELLED> is still true for requests that have successfully
308 executed, as long as C<eio_cancel> was called on them at some point.
309
310 =back
311
312 =head2 AVAILABLE REQUESTS
313
314 The following request functions are available. I<All> of them return the
315 C<eio_req *> on success and C<0> on failure, and I<all> of them have the
316 same three trailing arguments: C<pri>, C<cb> and C<data>. The C<cb> is
317 mandatory, but in most cases, you pass in C<0> as C<pri> and C<0> or some
318 custom data value as C<data>.
319
320 =head3 POSIX API WRAPPERS
321
322 These requests simply wrap the POSIX call of the same name, with the same
323 arguments. If a function is not implemented by the OS and cannot be emulated
324 in some way, then all of these return C<-1> and set C<errorno> to C<ENOSYS>.
325
326 =over 4
327
328 =item eio_open (const char *path, int flags, mode_t mode, int pri, eio_cb cb, void *data)
329
330 =item eio_truncate (const char *path, off_t offset, int pri, eio_cb cb, void *data)
331
332 =item eio_chown (const char *path, uid_t uid, gid_t gid, int pri, eio_cb cb, void *data)
333
334 =item eio_chmod (const char *path, mode_t mode, int pri, eio_cb cb, void *data)
335
336 =item eio_mkdir (const char *path, mode_t mode, int pri, eio_cb cb, void *data)
337
338 =item eio_rmdir (const char *path, int pri, eio_cb cb, void *data)
339
340 =item eio_unlink (const char *path, int pri, eio_cb cb, void *data)
341
342 =item eio_utime (const char *path, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void *data)
343
344 =item eio_mknod (const char *path, mode_t mode, dev_t dev, int pri, eio_cb cb, void *data)
345
346 =item eio_link (const char *path, const char *new_path, int pri, eio_cb cb, void *data)
347
348 =item eio_symlink (const char *path, const char *new_path, int pri, eio_cb cb, void *data)
349
350 =item eio_rename (const char *path, const char *new_path, int pri, eio_cb cb, void *data)
351
352 =item eio_mlock (void *addr, size_t length, int pri, eio_cb cb, void *data)
353
354 =item eio_close (int fd, int pri, eio_cb cb, void *data)
355
356 =item eio_sync (int pri, eio_cb cb, void *data)
357
358 =item eio_fsync (int fd, int pri, eio_cb cb, void *data)
359
360 =item eio_fdatasync (int fd, int pri, eio_cb cb, void *data)
361
362 =item eio_futime (int fd, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void *data)
363
364 =item eio_ftruncate (int fd, off_t offset, int pri, eio_cb cb, void *data)
365
366 =item eio_fchmod (int fd, mode_t mode, int pri, eio_cb cb, void *data)
367
368 =item eio_fchown (int fd, uid_t uid, gid_t gid, int pri, eio_cb cb, void *data)
369
370 =item eio_dup2 (int fd, int fd2, int pri, eio_cb cb, void *data)
371
372 These have the same semantics as the syscall of the same name, their
373 return value is available as C<< req->result >> later.
374
375 =item eio_read (int fd, void *buf, size_t length, off_t offset, int pri, eio_cb cb, void *data)
376
377 =item eio_write (int fd, void *buf, size_t length, off_t offset, int pri, eio_cb cb, void *data)
378
379 These two requests are called C<read> and C<write>, but actually wrap
380 C<pread> and C<pwrite>. On systems that lack these calls (such as cygwin),
381 libeio uses lseek/read_or_write/lseek and a mutex to serialise the
382 requests, so all these requests run serially and do not disturb each
383 other. However, they still disturb the file offset while they run, so it's
384 not safe to call these functions concurrently with non-libeio functions on
385 the same fd on these systems.
386
387 Not surprisingly, pread and pwrite are not thread-safe on Darwin (OS/X),
388 so it is advised not to submit multiple requests on the same fd on this
389 horrible pile of garbage.
390
391 =item eio_mlockall (int flags, int pri, eio_cb cb, void *data)
392
393 Like C<mlockall>, but the flag value constants are called
394 C<EIO_MCL_CURRENT> and C<EIO_MCL_FUTURE>.
395
396 =item eio_msync (void *addr, size_t length, int flags, int pri, eio_cb cb, void *data)
397
398 Just like msync, except that the flag values are called C<EIO_MS_ASYNC>,
399 C<EIO_MS_INVALIDATE> and C<EIO_MS_SYNC>.
400
401 =item eio_readlink (const char *path, int pri, eio_cb cb, void *data)
402
403 If successful, the path read by C<readlink(2)> can be accessed via C<<
404 req->ptr2 >> and is I<NOT> null-terminated, with the length specified as
405 C<< req->result >>.
406
407 if (req->result >= 0)
408 {
409 char *target = strndup ((char *)req->ptr2, req->result);
410
411 free (target);
412 }
413
414 =item eio_realpath (const char *path, int pri, eio_cb cb, void *data)
415
416 Similar to the realpath libc function, but unlike that one, C<<
417 req->result >> is C<-1> on failure. On success, the result is the length
418 of the returned path in C<ptr2> (which is I<NOT> 0-terminated) - this is
419 similar to readlink.
420
421 =item eio_stat (const char *path, int pri, eio_cb cb, void *data)
422
423 =item eio_lstat (const char *path, int pri, eio_cb cb, void *data)
424
425 =item eio_fstat (int fd, int pri, eio_cb cb, void *data)
426
427 Stats a file - if C<< req->result >> indicates success, then you can
428 access the C<struct stat>-like structure via C<< req->ptr2 >>:
429
430 EIO_STRUCT_STAT *statdata = (EIO_STRUCT_STAT *)req->ptr2;
431
432 =item eio_statvfs (const char *path, int pri, eio_cb cb, void *data)
433
434 =item eio_fstatvfs (int fd, int pri, eio_cb cb, void *data)
435
436 Stats a filesystem - if C<< req->result >> indicates success, then you can
437 access the C<struct statvfs>-like structure via C<< req->ptr2 >>:
438
439 EIO_STRUCT_STATVFS *statdata = (EIO_STRUCT_STATVFS *)req->ptr2;
440
441 =back
442
443 =head3 READING DIRECTORIES
444
445 Reading directories sounds simple, but can be rather demanding, especially
446 if you want to do stuff such as traversing a directory hierarchy or
447 processing all files in a directory. Libeio can assist these complex tasks
448 with it's C<eio_readdir> call.
449
450 =over 4
451
452 =item eio_readdir (const char *path, int flags, int pri, eio_cb cb, void *data)
453
454 This is a very complex call. It basically reads through a whole directory
455 (via the C<opendir>, C<readdir> and C<closedir> calls) and returns either
456 the names or an array of C<struct eio_dirent>, depending on the C<flags>
457 argument.
458
459 The C<< req->result >> indicates either the number of files found, or
460 C<-1> on error. On success, null-terminated names can be found as C<< req->ptr2 >>,
461 and C<struct eio_dirents>, if requested by C<flags>, can be found via C<<
462 req->ptr1 >>.
463
464 Here is an example that prints all the names:
465
466 int i;
467 char *names = (char *)req->ptr2;
468
469 for (i = 0; i < req->result; ++i)
470 {
471 printf ("name #%d: %s\n", i, names);
472
473 /* move to next name */
474 names += strlen (names) + 1;
475 }
476
477 Pseudo-entries such as F<.> and F<..> are never returned by C<eio_readdir>.
478
479 C<flags> can be any combination of:
480
481 =over 4
482
483 =item EIO_READDIR_DENTS
484
485 If this flag is specified, then, in addition to the names in C<ptr2>,
486 also an array of C<struct eio_dirent> is returned, in C<ptr1>. A C<struct
487 eio_dirent> looks like this:
488
489 struct eio_dirent
490 {
491 int nameofs; /* offset of null-terminated name string in (char *)req->ptr2 */
492 unsigned short namelen; /* size of filename without trailing 0 */
493 unsigned char type; /* one of EIO_DT_* */
494 signed char score; /* internal use */
495 ino_t inode; /* the inode number, if available, otherwise unspecified */
496 };
497
498 The only members you normally would access are C<nameofs>, which is the
499 byte-offset from C<ptr2> to the start of the name, C<namelen> and C<type>.
500
501 C<type> can be one of:
502
503 C<EIO_DT_UNKNOWN> - if the type is not known (very common) and you have to C<stat>
504 the name yourself if you need to know,
505 one of the "standard" POSIX file types (C<EIO_DT_REG>, C<EIO_DT_DIR>, C<EIO_DT_LNK>,
506 C<EIO_DT_FIFO>, C<EIO_DT_SOCK>, C<EIO_DT_CHR>, C<EIO_DT_BLK>)
507 or some OS-specific type (currently
508 C<EIO_DT_MPC> - multiplexed char device (v7+coherent),
509 C<EIO_DT_NAM> - xenix special named file,
510 C<EIO_DT_MPB> - multiplexed block device (v7+coherent),
511 C<EIO_DT_NWK> - HP-UX network special,
512 C<EIO_DT_CMP> - VxFS compressed,
513 C<EIO_DT_DOOR> - solaris door, or
514 C<EIO_DT_WHT>).
515
516 This example prints all names and their type:
517
518 int i;
519 struct eio_dirent *ents = (struct eio_dirent *)req->ptr1;
520 char *names = (char *)req->ptr2;
521
522 for (i = 0; i < req->result; ++i)
523 {
524 struct eio_dirent *ent = ents + i;
525 char *name = names + ent->nameofs;
526
527 printf ("name #%d: %s (type %d)\n", i, name, ent->type);
528 }
529
530 =item EIO_READDIR_DIRS_FIRST
531
532 When this flag is specified, then the names will be returned in an order
533 where likely directories come first, in optimal C<stat> order. This is
534 useful when you need to quickly find directories, or you want to find all
535 directories while avoiding to stat() each entry.
536
537 If the system returns type information in readdir, then this is used
538 to find directories directly. Otherwise, likely directories are names
539 beginning with ".", or otherwise names with no dots, of which names with
540 short names are tried first.
541
542 =item EIO_READDIR_STAT_ORDER
543
544 When this flag is specified, then the names will be returned in an order
545 suitable for stat()'ing each one. That is, when you plan to stat()
546 all files in the given directory, then the returned order will likely
547 be fastest.
548
549 If both this flag and C<EIO_READDIR_DIRS_FIRST> are specified, then the
550 likely directories come first, resulting in a less optimal stat order.
551
552 =item EIO_READDIR_FOUND_UNKNOWN
553
554 This flag should not be specified when calling C<eio_readdir>. Instead,
555 it is being set by C<eio_readdir> (you can access the C<flags> via C<<
556 req->int1 >>, when any of the C<type>'s found were C<EIO_DT_UNKNOWN>. The
557 absence of this flag therefore indicates that all C<type>'s are known,
558 which can be used to speed up some algorithms.
559
560 A typical use case would be to identify all subdirectories within a
561 directory - you would ask C<eio_readdir> for C<EIO_READDIR_DIRS_FIRST>. If
562 then this flag is I<NOT> set, then all the entries at the beginning of the
563 returned array of type C<EIO_DT_DIR> are the directories. Otherwise, you
564 should start C<stat()>'ing the entries starting at the beginning of the
565 array, stopping as soon as you found all directories (the count can be
566 deduced by the link count of the directory).
567
568 =back
569
570 =back
571
572 =head3 OS-SPECIFIC CALL WRAPPERS
573
574 These wrap OS-specific calls (usually Linux ones), and might or might not
575 be emulated on other operating systems. Calls that are not emulated will
576 return C<-1> and set C<errno> to C<ENOSYS>.
577
578 =over 4
579
580 =item eio_sendfile (int out_fd, int in_fd, off_t in_offset, size_t length, int pri, eio_cb cb, void *data)
581
582 Wraps the C<sendfile> syscall. The arguments follow the Linux version, but
583 libeio supports and will use similar calls on FreeBSD, HP/UX, Solaris and
584 Darwin.
585
586 If the OS doesn't support some sendfile-like call, or the call fails,
587 indicating support for the given file descriptor type (for example,
588 Linux's sendfile might not support file to file copies), then libeio will
589 emulate the call in userspace, so there are almost no limitations on its
590 use.
591
592 =item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)
593
594 Calls C<readahead(2)>. If the syscall is missing, then the call is
595 emulated by simply reading the data (currently in 64kiB chunks).
596
597 =item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)
598
599 Calls C<sync_file_range>. If the syscall is missing, then this is the same
600 as calling C<fdatasync>.
601
602 Flags can be any combination of C<EIO_SYNC_FILE_RANGE_WAIT_BEFORE>,
603 C<EIO_SYNC_FILE_RANGE_WRITE> and C<EIO_SYNC_FILE_RANGE_WAIT_AFTER>.
604
605 =item eio_fallocate (int fd, int mode, off_t offset, off_t len, int pri, eio_cb cb, void *data)
606
607 Calls C<fallocate> (note: I<NOT> C<posix_fallocate>!). If the syscall is
608 missing, then it returns failure and sets C<errno> to C<ENOSYS>.
609
610 The C<mode> argument can be C<0> (for behaviour similar to
611 C<posix_fallocate>), or C<EIO_FALLOC_FL_KEEP_SIZE>, which keeps the size
612 of the file unchanged (but still preallocates space beyond end of file).
613
614 =back
615
616 =head3 LIBEIO-SPECIFIC REQUESTS
617
618 These requests are specific to libeio and do not correspond to any OS call.
619
620 =over 4
621
622 =item eio_mtouch (void *addr, size_t length, int flags, int pri, eio_cb cb, void *data)
623
624 Reads (C<flags == 0>) or modifies (C<flags == EIO_MT_MODIFY) the given
625 memory area, page-wise, that is, it reads (or reads and writes back) the
626 first octet of every page that spans the memory area.
627
628 This can be used to page in some mmapped file, or dirty some pages. Note
629 that dirtying is an unlocked read-write access, so races can ensue when
630 the some other thread modifies the data stored in that memory area.
631
632 =item eio_custom (void (*)(eio_req *) execute, int pri, eio_cb cb, void *data)
633
634 Executes a custom request, i.e., a user-specified callback.
635
636 The callback gets the C<eio_req *> as parameter and is expected to read
637 and modify any request-specific members. Specifically, it should set C<<
638 req->result >> to the result value, just like other requests.
639
640 Here is an example that simply calls C<open>, like C<eio_open>, but it
641 uses the C<data> member as filename and uses a hardcoded C<O_RDONLY>. If
642 you want to pass more/other parameters, you either need to pass some
643 struct or so via C<data> or provide your own wrapper using the low-level
644 API.
645
646 static int
647 my_open_done (eio_req *req)
648 {
649 int fd = req->result;
650
651 return 0;
652 }
653
654 static void
655 my_open (eio_req *req)
656 {
657 req->result = open (req->data, O_RDONLY);
658 }
659
660 eio_custom (my_open, 0, my_open_done, "/etc/passwd");
661
662 =item eio_busy (eio_tstamp delay, int pri, eio_cb cb, void *data)
663
664 This is a request that takes C<delay> seconds to execute, but otherwise
665 does nothing - it simply puts one of the worker threads to sleep for this
666 long.
667
668 This request can be used to artificially increase load, e.g. for debugging
669 or benchmarking reasons.
670
671 =item eio_nop (int pri, eio_cb cb, void *data)
672
673 This request does nothing, except go through the whole request cycle. This
674 can be used to measure latency or in some cases to simplify code, but is
675 not really of much use.
676
677 =back
678
679 =head3 GROUPING AND LIMITING REQUESTS
680
681 There is one more rather special request, C<eio_grp>. It is a very special
682 aio request: Instead of doing something, it is a container for other eio
683 requests.
684
685 There are two primary use cases for this: a) bundle many requests into a
686 single, composite, request with a definite callback and the ability to
687 cancel the whole request with its subrequests and b) limiting the number
688 of "active" requests.
689
690 Further below you will find more discussion of these topics - first
691 follows the reference section detailing the request generator and other
692 methods.
693
694 =over 4
695
696 =item eio_req *grp = eio_grp (eio_cb cb, void *data)
697
698 Creates, submits and returns a group request. Note that it doesn't have a
699 priority, unlike all other requests.
700
701 =item eio_grp_add (eio_req *grp, eio_req *req)
702
703 Adds a request to the request group.
704
705 =item eio_grp_cancel (eio_req *grp)
706
707 Cancels all requests I<in> the group, but I<not> the group request
708 itself. You can cancel the group request I<and> all subrequests via a
709 normal C<eio_cancel> call.
710
711 =back
712
713 =head4 GROUP REQUEST LIFETIME
714
715 Left alone, a group request will instantly move to the pending state and
716 will be finished at the next call of C<eio_poll>.
717
718 The usefulness stems from the fact that, if a subrequest is added to a
719 group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group
720 will not finish until all the subrequests have finished.
721
722 So the usage cycle of a group request is like this: after it is created,
723 you normally instantly add a subrequest. If none is added, the group
724 request will finish on it's own. As long as subrequests are added before
725 the group request is finished it will be kept from finishing, that is the
726 callbacks of any subrequests can, in turn, add more requests to the group,
727 and as long as any requests are active, the group request itself will not
728 finish.
729
730 =head4 CREATING COMPOSITE REQUESTS
731
732 Imagine you wanted to create an C<eio_load> request that opens a file,
733 reads it and closes it. This means it has to execute at least three eio
734 requests, but for various reasons it might be nice if that request looked
735 like any other eio request.
736
737 This can be done with groups:
738
739 =over 4
740
741 =item 1) create the request object
742
743 Create a group that contains all further requests. This is the request you
744 can return as "the load request".
745
746 =item 2) open the file, maybe
747
748 Next, open the file with C<eio_open> and add the request to the group
749 request and you are finished setting up the request.
750
751 If, for some reason, you cannot C<eio_open> (path is a null ptr?) you
752 can set C<< grp->result >> to C<-1> to signal an error and let the group
753 request finish on its own.
754
755 =item 3) open callback adds more requests
756
757 In the open callback, if the open was not successful, copy C<<
758 req->errorno >> to C<< grp->errorno >> and set C<< grp->errorno >> to
759 C<-1> to signal an error.
760
761 Otherwise, malloc some memory or so and issue a read request, adding the
762 read request to the group.
763
764 =item 4) continue issuing requests till finished
765
766 In the real callback, check for errors and possibly continue with
767 C<eio_close> or any other eio request in the same way.
768
769 As soon as no new requests are added the group request will finish. Make
770 sure you I<always> set C<< grp->result >> to some sensible value.
771
772 =back
773
774 =head4 REQUEST LIMITING
775
776
777 #TODO
778
779 void eio_grp_limit (eio_req *grp, int limit);
780
781
782 =back
783
784
785 =head1 LOW LEVEL REQUEST API
786
787 #TODO
788
789
790 =head1 ANATOMY AND LIFETIME OF AN EIO REQUEST
791
792 A request is represented by a structure of type C<eio_req>. To initialise
793 it, clear it to all zero bytes:
794
795 eio_req req;
796
797 memset (&req, 0, sizeof (req));
798
799 A more common way to initialise a new C<eio_req> is to use C<calloc>:
800
801 eio_req *req = calloc (1, sizeof (*req));
802
803 In either case, libeio neither allocates, initialises or frees the
804 C<eio_req> structure for you - it merely uses it.
805
806 zero
807
808 #TODO
809
810 =head2 CONFIGURATION
811
812 The functions in this section can sometimes be useful, but the default
813 configuration will do in most case, so you should skip this section on
814 first reading.
815
816 =over 4
817
818 =item eio_set_max_poll_time (eio_tstamp nseconds)
819
820 This causes C<eio_poll ()> to return after it has detected that it was
821 running for C<nsecond> seconds or longer (this number can be fractional).
822
823 This can be used to limit the amount of time spent handling eio requests,
824 for example, in interactive programs, you might want to limit this time to
825 C<0.01> seconds or so.
826
827 Note that:
828
829 =over 4
830
831 =item a) libeio doesn't know how long your request callbacks take, so the
832 time spent in C<eio_poll> is up to one callback invocation longer then
833 this interval.
834
835 =item b) this is implemented by calling C<gettimeofday> after each
836 request, which can be costly.
837
838 =item c) at least one request will be handled.
839
840 =back
841
842 =item eio_set_max_poll_reqs (unsigned int nreqs)
843
844 When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
845 C<nreqs> requests per invocation. This is a less costly way to limit the
846 amount of work done by C<eio_poll> then setting a time limit.
847
848 If you know your callbacks are generally fast, you could use this to
849 encourage interactiveness in your programs by setting it to C<10>, C<100>
850 or even C<1000>.
851
852 =item eio_set_min_parallel (unsigned int nthreads)
853
854 Make sure libeio can handle at least this many requests in parallel. It
855 might be able handle more.
856
857 =item eio_set_max_parallel (unsigned int nthreads)
858
859 Set the maximum number of threads that libeio will spawn.
860
861 =item eio_set_max_idle (unsigned int nthreads)
862
863 Libeio uses threads internally to handle most requests, and will start and stop threads on demand.
864
865 This call can be used to limit the number of idle threads (threads without
866 work to do): libeio will keep some threads idle in preparation for more
867 requests, but never longer than C<nthreads> threads.
868
869 In addition to this, libeio will also stop threads when they are idle for
870 a few seconds, regardless of this setting.
871
872 =item unsigned int eio_nthreads ()
873
874 Return the number of worker threads currently running.
875
876 =item unsigned int eio_nreqs ()
877
878 Return the number of requests currently handled by libeio. This is the
879 total number of requests that have been submitted to libeio, but not yet
880 destroyed.
881
882 =item unsigned int eio_nready ()
883
884 Returns the number of ready requests, i.e. requests that have been
885 submitted but have not yet entered the execution phase.
886
887 =item unsigned int eio_npending ()
888
889 Returns the number of pending requests, i.e. requests that have been
890 executed and have results, but have not been finished yet by a call to
891 C<eio_poll>).
892
893 =back
894
895 =head1 EMBEDDING
896
897 Libeio can be embedded directly into programs. This functionality is not
898 documented and not (yet) officially supported.
899
900 Note that, when including C<libeio.m4>, you are responsible for defining
901 the compilation environment (C<_LARGEFILE_SOURCE>, C<_GNU_SOURCE> etc.).
902
903 If you need to know how, check the C<IO::AIO> perl module, which does
904 exactly that.
905
906
907 =head1 COMPILETIME CONFIGURATION
908
909 These symbols, if used, must be defined when compiling F<eio.c>.
910
911 =over 4
912
913 =item EIO_STACKSIZE
914
915 This symbol governs the stack size for each eio thread. Libeio itself
916 was written to use very little stackspace, but when using C<EIO_CUSTOM>
917 requests, you might want to increase this.
918
919 If this symbol is undefined (the default) then libeio will use its default
920 stack size (C<sizeof (void *) * 4096> currently). If it is defined, but
921 C<0>, then the default operating system stack size will be used. In all
922 other cases, the value must be an expression that evaluates to the desired
923 stack size.
924
925 =back
926
927
928 =head1 PORTABILITY REQUIREMENTS
929
930 In addition to a working ISO-C implementation, libeio relies on a few
931 additional extensions:
932
933 =over 4
934
935 =item POSIX threads
936
937 To be portable, this module uses threads, specifically, the POSIX threads
938 library must be available (and working, which partially excludes many xBSD
939 systems, where C<fork ()> is buggy).
940
941 =item POSIX-compatible filesystem API
942
943 This is actually a harder portability requirement: The libeio API is quite
944 demanding regarding POSIX API calls (symlinks, user/group management
945 etc.).
946
947 =item C<double> must hold a time value in seconds with enough accuracy
948
949 The type C<double> is used to represent timestamps. It is required to
950 have at least 51 bits of mantissa (and 9 bits of exponent), which is good
951 enough for at least into the year 4000. This requirement is fulfilled by
952 implementations implementing IEEE 754 (basically all existing ones).
953
954 =back
955
956 If you know of other additional requirements drop me a note.
957
958
959 =head1 AUTHOR
960
961 Marc Lehmann <libeio@schmorp.de>.
962