[ViewVC] Diff of: cvs/libeio/eio.pod

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Revision 1.4 by root, Wed Apr 22 11:04:49 2009 UTC vs.
Revision 1.26 by root, Mon Jul 18 02:59:58 2011 UTC

…		…
11	The newest version of this document is also available as an html-formatted	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	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>.	13	time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.
14		14
15	Note that this library is a by-product of the C<IO::AIO> perl	15	Note that this library is a by-product of the C<IO::AIO> perl
16	module, and many of the subtler points regarding requets lifetime	16	module, and many of the subtler points regarding requests lifetime
17	and so on are only documented in its documentation at the	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>.	18	moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.
19		19
20	=head2 FEATURES	20	=head2 FEATURES
21		21
22	This library provides fully asynchronous versions of most POSIX functions	22	This library provides fully asynchronous versions of most POSIX functions
23	dealign with I/O. Unlike most asynchronous libraries, this not only	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	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>	25	similar functions, as well as less rarely ones such as C<mknod>, C<futime>
26	or C<readlink>.	26	or C<readlink>.
27		27
28	It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and	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	29	FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
30	emulation elsewhere>).	30	emulation elsewhere>).
31		31
32	The goal is to enbale you to write fully non-blocking programs. For	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	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	34	just because the server is running a backup and you happen to call
35	C<readdir>.	35	C<readdir>.
36		36
37	=head2 TIME REPRESENTATION	37	=head2 TIME REPRESENTATION
38		38
39	Libeio represents time as a single floating point number, representing the	39	Libeio represents time as a single floating point number, representing the
40	(fractional) number of seconds since the (POSIX) epoch (somewhere near	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	41	the beginning of 1970, details are complicated, don't ask). This type is
42	called C<eio_tstamp>, but it is guarenteed to be of type C<double> (or	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.	43	better), so you can freely use C<double> yourself.
44		44
45	Unlike the name component C<stamp> might indicate, it is also used for	45	Unlike the name component C<stamp> might indicate, it is also used for
46	time differences throughout libeio.	46	time differences throughout libeio.
47		47
48	=head2 FORK SUPPORT	48	=head2 FORK SUPPORT
49		49
50	Calling C<fork ()> is fully supported by this module. It is implemented in these steps:	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.
51		54
52	1. wait till all requests in "execute" state have been handled	55	This library only works in the process that initialised it: Forking is
53	(basically requests that are already handed over to the kernel).	56	fully supported, but using libeio in any other process than the one that
54	2. fork	57	called C<eio_init> is not.
55	3. in the parent, continue business as usual, done	58
56	4. in the child, destroy all ready and pending requests and free the	59	You might get around by not I<using> libeio before (or after) forking in
57	memory used by the worker threads. This gives you a fully empty	60	the parent, and using it in the child afterwards. You could also try to
58	libeio queue.	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.
59		68
60	=head1 INITIALISATION/INTEGRATION	69	=head1 INITIALISATION/INTEGRATION
61		70
62	Before you can call any eio functions you first have to initialise the	71	Before you can call any eio functions you first have to initialise the
63	library. The library integrates into any event loop, but can also be used	72	library. The library integrates into any event loop, but can also be used
…		…
72	This function initialises the library. On success it returns C<0>, on	81	This function initialises the library. On success it returns C<0>, on
73	failure it returns C<-1> and sets C<errno> appropriately.	82	failure it returns C<-1> and sets C<errno> appropriately.
74		83
75	It accepts two function pointers specifying callbacks as argument, both of	84	It accepts two function pointers specifying callbacks as argument, both of
76	which can be C<0>, in which case the callback isn't called.	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.
77		89
78	=item want_poll callback	90	=item want_poll callback
79		91
80	The C<want_poll> callback is invoked whenever libeio wants attention (i.e.	92	The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
81	it wants to be polled by calling C<eio_poll>). It is "edge-triggered",	93	it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
…		…
97	handled or C<done_poll> has been called, which signals the same.	109	handled or C<done_poll> has been called, which signals the same.
98		110
99	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>	111	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
100	have been called again, so watch out for races in your code.	112	have been called again, so watch out for races in your code.
101		113
102	As with C<want_poll>, this callback is called while lcoks are being held,	114	As with C<want_poll>, this callback is called while locks are being held,
103	so you I<must not call any libeio functions form within this callback>.	115	so you I<must not call any libeio functions form within this callback>.
104		116
105	=item int eio_poll ()	117	=item int eio_poll ()
106		118
107	This function has to be called whenever there are pending requests that	119	This function has to be called whenever there are pending requests that
…		…
119	=back	131	=back
120		132
121	For libev, you would typically use an C<ev_async> watcher: the	133	For libev, you would typically use an C<ev_async> watcher: the
122	C<want_poll> callback would invoke C<ev_async_send> to wake up the event	134	C<want_poll> callback would invoke C<ev_async_send> to wake up the event
123	loop. Inside the callback set for the watcher, one would call C<eio_poll	135	loop. Inside the callback set for the watcher, one would call C<eio_poll
124	()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not	136	()>.
125	all requests have been handled yet). The race is taken care of because	137
126	libev resets/rearms the async watcher before calling your callback,	138	If C<eio_poll ()> is configured to not handle all results in one go
127	and therefore, before calling C<eio_poll>. This might result in (some)	139	(i.e. it returns C<-1>) then you should start an idle watcher that calls
128	spurious wake-ups, but is generally harmless.	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	}
129		185
130	For most other event loops, you would typically use a pipe - the event	186	For most other event loops, you would typically use a pipe - the event
131	loop should be told to wait for read readyness on the read end. In	187	loop should be told to wait for read readiness on the read end. In
132	C<want_poll> you would write a single byte, in C<done_poll> you would try	188	C<want_poll> you would write a single byte, in C<done_poll> you would try
133	to read that byte, and in the callback for the read end, you would call	189	to read that byte, and in the callback for the read end, you would call
134	C<eio_poll>. The race is avoided here because the event loop should invoke	190	C<eio_poll>.
135	your callback again and again until the byte has been read (as the pipe	191
136	read callback does not read it, only C<done_poll>).	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
137		809
138	=head2 CONFIGURATION	810	=head2 CONFIGURATION
139		811
140	The functions in this section can sometimes be useful, but the default	812	The functions in this section can sometimes be useful, but the default
141	configuration will do in most case, so you should skip this section on	813	configuration will do in most case, so you should skip this section on
…		…
152	for example, in interactive programs, you might want to limit this time to	824	for example, in interactive programs, you might want to limit this time to
153	C<0.01> seconds or so.	825	C<0.01> seconds or so.
154		826
155	Note that:	827	Note that:
156		828
		829	=over 4
		830
157	a) libeio doesn't know how long your request callbacks take, so the time	831	=item a) libeio doesn't know how long your request callbacks take, so the
158	spent in C<eio_poll> is up to one callback invocation longer then this	832	time spent in C<eio_poll> is up to one callback invocation longer then
159	interval.	833	this interval.
160		834
161	b) this is implemented by calling C<gettimeofday> after each request,	835	=item b) this is implemented by calling C<gettimeofday> after each
162	which can be costly.	836	request, which can be costly.
163		837
164	c) at least one request will be handled.	838	=item c) at least one request will be handled.
		839
		840	=back
165		841
166	=item eio_set_max_poll_reqs (unsigned int nreqs)	842	=item eio_set_max_poll_reqs (unsigned int nreqs)
167		843
168	When C<nreqs> is non-zero, then C<eio_poll> will not handle more than	844	When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
169	C<nreqs> requests per invocation. This is a less costly way to limit the	845	C<nreqs> requests per invocation. This is a less costly way to limit the
…		…
185	=item eio_set_max_idle (unsigned int nthreads)	861	=item eio_set_max_idle (unsigned int nthreads)
186		862
187	Libeio uses threads internally to handle most requests, and will start and stop threads on demand.	863	Libeio uses threads internally to handle most requests, and will start and stop threads on demand.
188		864
189	This call can be used to limit the number of idle threads (threads without	865	This call can be used to limit the number of idle threads (threads without
190	work to do): libeio will keep some threads idle in preperation for more	866	work to do): libeio will keep some threads idle in preparation for more
191	requests, but never longer than C<nthreads> threads.	867	requests, but never longer than C<nthreads> threads.
192		868
193	In addition to this, libeio will also stop threads when they are idle for	869	In addition to this, libeio will also stop threads when they are idle for
194	a few seconds, regardless of this setting.	870	a few seconds, regardless of this setting.
195		871
…		…
214	executed and have results, but have not been finished yet by a call to	890	executed and have results, but have not been finished yet by a call to
215	C<eio_poll>).	891	C<eio_poll>).
216		892
217	=back	893	=back
218		894
219
220	=head1 ANATOMY OF AN EIO REQUEST
221
222	#TODO
223
224
225	=head1 HIGH LEVEL REQUEST API
226
227	#TODO
228
229	=back
230
231
232	=head1 LOW LEVEL REQUEST API
233
234	#TODO
235
236	=head1 EMBEDDING	895	=head1 EMBEDDING
237		896
238	Libeio can be embedded directly into programs. This functionality is not	897	Libeio can be embedded directly into programs. This functionality is not
239	documented and not (yet) officially supported.	898	documented and not (yet) officially supported.
240		899
…		…
256	This symbol governs the stack size for each eio thread. Libeio itself	915	This symbol governs the stack size for each eio thread. Libeio itself
257	was written to use very little stackspace, but when using C<EIO_CUSTOM>	916	was written to use very little stackspace, but when using C<EIO_CUSTOM>
258	requests, you might want to increase this.	917	requests, you might want to increase this.
259		918
260	If this symbol is undefined (the default) then libeio will use its default	919	If this symbol is undefined (the default) then libeio will use its default
261	stack size (C<sizeof (long) * 4096> currently). If it is defined, but	920	stack size (C<sizeof (void ) 4096> currently). If it is defined, but
262	C<0>, then the default operating system stack size will be used. In all	921	C<0>, then the default operating system stack size will be used. In all
263	other cases, the value must be an expression that evaluates to the desired	922	other cases, the value must be an expression that evaluates to the desired
264	stack size.	923	stack size.
265		924
266	=back	925	=back

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Comparing libeio/eio.pod (file contents):
Revision 1.4 by root, Wed Apr 22 11:04:49 2009 UTC vs.
Revision 1.26 by root, Mon Jul 18 02:59:58 2011 UTC