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

Comparing libeio/eio.pod (file contents):
Revision 1.6 by sf-exg, Tue May 31 10:09:38 2011 UTC vs.
Revision 1.36 by root, Sun Jan 24 16:36:20 2016 UTC

…		…
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 enable 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>.
…		…
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",
…		…
92	This callback is invoked when libeio detects that all pending requests	104	This callback is invoked when libeio detects that all pending requests
93	have been handled. It is "edge-triggered", that is, it will only be	105	have been handled. It is "edge-triggered", that is, it will only be
94	called once after C<want_poll>. To put it differently, C<want_poll> and	106	called once after C<want_poll>. To put it differently, C<want_poll> and
95	C<done_poll> are invoked in pairs: after C<want_poll> you have to call	107	C<done_poll> are invoked in pairs: after C<want_poll> you have to call
96	C<eio_poll ()> until either C<eio_poll> indicates that everything has been	108	C<eio_poll ()> until either C<eio_poll> indicates that everything has been
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 - only one
		110	method is needed.
98		111
99	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>	112	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.	113	have been called again, so watch out for races in your code.
101		114
		115	It is quite common to have an empty C<done_call> callback and only use
		116	the return value from C<eio_poll>, or, when C<eio_poll> is configured to
		117	handle all outstanding replies, it's enough to call C<eio_poll> once.
		118
102	As with C<want_poll>, this callback is called while locks are being held,	119	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>.	120	so you I<must not call any libeio functions from within this callback>.
104		121
105	=item int eio_poll ()	122	=item int eio_poll ()
106		123
107	This function has to be called whenever there are pending requests that	124	This function has to be called whenever there are pending requests that
108	need finishing. You usually call this after C<want_poll> has indicated	125	need finishing. You usually call this after C<want_poll> has indicated
…		…
119	=back	136	=back
120		137
121	For libev, you would typically use an C<ev_async> watcher: the	138	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	139	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	140	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	141	()>.
125	all requests have been handled yet). The race is taken care of because	142
126	libev resets/rearms the async watcher before calling your callback,	143	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)	144	(i.e. it returns C<-1>) then you should start an idle watcher that calls
128	spurious wake-ups, but is generally harmless.	145	C<eio_poll> until it returns something C<!= -1>.
		146
		147	A full-featured connector between libeio and libev would look as follows
		148	(if C<eio_poll> is handling all requests, it can of course be simplified a
		149	lot by removing the idle watcher logic):
		150
		151	static struct ev_loop *loop;
		152	static ev_idle repeat_watcher;
		153	static ev_async ready_watcher;
		154
		155	/* idle watcher callback, only used when eio_poll */
		156	/* didn't handle all results in one call */
		157	static void
		158	repeat (EV_P_ ev_idle *w, int revents)
		159	{
		160	if (eio_poll () != -1)
		161	ev_idle_stop (EV_A_ w);
		162	}
		163
		164	/* eio has some results, process them */
		165	static void
		166	ready (EV_P_ ev_async *w, int revents)
		167	{
		168	if (eio_poll () == -1)
		169	ev_idle_start (EV_A_ &repeat_watcher);
		170	}
		171
		172	/* wake up the event loop */
		173	static void
		174	want_poll (void)
		175	{
		176	ev_async_send (loop, &ready_watcher)
		177	}
		178
		179	void
		180	my_init_eio ()
		181	{
		182	loop = EV_DEFAULT;
		183
		184	ev_idle_init (&repeat_watcher, repeat);
		185	ev_async_init (&ready_watcher, ready);
		186	ev_async_start (loop, &watcher);
		187
		188	eio_init (want_poll, 0);
		189	}
129		190
130	For most other event loops, you would typically use a pipe - the event	191	For most other event loops, you would typically use a pipe - the event
131	loop should be told to wait for read readiness on the read end. In	192	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	193	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	194	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	195	C<eio_poll>.
135	your callback again and again until the byte has been read (as the pipe	196
136	read callback does not read it, only C<done_poll>).	197	You don't have to take special care in the case C<eio_poll> doesn't handle
		198	all requests, as the done callback will not be invoked, so the event loop
		199	will still signal readiness for the pipe until I<all> results have been
		200	processed.
		201
		202
		203	=head1 HIGH LEVEL REQUEST API
		204
		205	Libeio has both a high-level API, which consists of calling a request
		206	function with a callback to be called on completion, and a low-level API
		207	where you fill out request structures and submit them.
		208
		209	This section describes the high-level API.
		210
		211	=head2 REQUEST SUBMISSION AND RESULT PROCESSING
		212
		213	You submit a request by calling the relevant C<eio_TYPE> function with the
		214	required parameters, a callback of type C<int (eio_cb)(eio_req req)>
		215	(called C<eio_cb> below) and a freely usable C<void *data> argument.
		216
		217	The return value will either be 0, in case something went really wrong
		218	(which can basically only happen on very fatal errors, such as C<malloc>
		219	returning 0, which is rather unlikely), or a pointer to the newly-created
		220	and submitted C<eio_req *>.
		221
		222	The callback will be called with an C<eio_req *> which contains the
		223	results of the request. The members you can access inside that structure
		224	vary from request to request, except for:
		225
		226	=over 4
		227
		228	=item C<ssize_t result>
		229
		230	This contains the result value from the call (usually the same as the
		231	syscall of the same name).
		232
		233	=item C<int errorno>
		234
		235	This contains the value of C<errno> after the call.
		236
		237	=item C<void *data>
		238
		239	The C<void *data> member simply stores the value of the C<data> argument.
		240
		241	=back
		242
		243	Members not explicitly described as accessible must not be
		244	accessed. Specifically, there is no guarantee that any members will still
		245	have the value they had when the request was submitted.
		246
		247	The return value of the callback is normally C<0>, which tells libeio to
		248	continue normally. If a callback returns a nonzero value, libeio will
		249	stop processing results (in C<eio_poll>) and will return the value to its
		250	caller.
		251
		252	Memory areas passed to libeio wrappers must stay valid as long as a
		253	request executes, with the exception of paths, which are being copied
		254	internally. Any memory libeio itself allocates will be freed after the
		255	finish callback has been called. If you want to manage all memory passed
		256	to libeio yourself you can use the low-level API.
		257
		258	For example, to open a file, you could do this:
		259
		260	static int
		261	file_open_done (eio_req *req)
		262	{
		263	if (req->result < 0)
		264	{
		265	/* open() returned -1 */
		266	errno = req->errorno;
		267	perror ("open");
		268	}
		269	else
		270	{
		271	int fd = req->result;
		272	/* now we have the new fd in fd */
		273	}
		274
		275	return 0;
		276	}
		277
		278	/* the first three arguments are passed to open(2) */
		279	/* the remaining are priority, callback and data */
		280	if (!eio_open ("/etc/passwd", O_RDONLY, 0, 0, file_open_done, 0))
		281	abort (); /* something went wrong, we will all die!!! */
		282
		283	Note that you additionally need to call C<eio_poll> when the C<want_cb>
		284	indicates that requests are ready to be processed.
		285
		286	=head2 CANCELLING REQUESTS
		287
		288	Sometimes the need for a request goes away before the request is
		289	finished. In that case, one can cancel the request by a call to
		290	C<eio_cancel>:
		291
		292	=over 4
		293
		294	=item eio_cancel (eio_req *req)
		295
		296	Cancel the request (and all its subrequests). If the request is currently
		297	executing it might still continue to execute, and in other cases it might
		298	still take a while till the request is cancelled.
		299
		300	When cancelled, the finish callback will not be invoked.
		301
		302	C<EIO_CANCELLED> is still true for requests that have successfully
		303	executed, as long as C<eio_cancel> was called on them at some point.
		304
		305	=back
		306
		307	=head2 AVAILABLE REQUESTS
		308
		309	The following request functions are available. I<All> of them return the
		310	C<eio_req *> on success and C<0> on failure, and I<all> of them have the
		311	same three trailing arguments: C<pri>, C<cb> and C<data>. The C<cb> is
		312	mandatory, but in most cases, you pass in C<0> as C<pri> and C<0> or some
		313	custom data value as C<data>.
		314
		315	=head3 POSIX API WRAPPERS
		316
		317	These requests simply wrap the POSIX call of the same name, with the same
		318	arguments. If a function is not implemented by the OS and cannot be emulated
		319	in some way, then all of these return C<-1> and set C<errorno> to C<ENOSYS>.
		320
		321	=over 4
		322
		323	=item eio_open (const char path, int flags, mode_t mode, int pri, eio_cb cb, void data)
		324
		325	=item eio_truncate (const char path, off_t offset, int pri, eio_cb cb, void data)
		326
		327	=item eio_chown (const char path, uid_t uid, gid_t gid, int pri, eio_cb cb, void data)
		328
		329	=item eio_chmod (const char path, mode_t mode, int pri, eio_cb cb, void data)
		330
		331	=item eio_mkdir (const char path, mode_t mode, int pri, eio_cb cb, void data)
		332
		333	=item eio_rmdir (const char path, int pri, eio_cb cb, void data)
		334
		335	=item eio_unlink (const char path, int pri, eio_cb cb, void data)
		336
		337	=item eio_utime (const char path, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void data)
		338
		339	=item eio_mknod (const char path, mode_t mode, dev_t dev, int pri, eio_cb cb, void data)
		340
		341	=item eio_link (const char path, const char new_path, int pri, eio_cb cb, void *data)
		342
		343	=item eio_symlink (const char path, const char new_path, int pri, eio_cb cb, void *data)
		344
		345	=item eio_rename (const char path, const char new_path, int pri, eio_cb cb, void *data)
		346
		347	=item eio_mlock (void addr, size_t length, int pri, eio_cb cb, void data)
		348
		349	=item eio_close (int fd, int pri, eio_cb cb, void *data)
		350
		351	=item eio_sync (int pri, eio_cb cb, void *data)
		352
		353	=item eio_fsync (int fd, int pri, eio_cb cb, void *data)
		354
		355	=item eio_fdatasync (int fd, int pri, eio_cb cb, void *data)
		356
		357	=item eio_futime (int fd, eio_tstamp atime, eio_tstamp mtime, int pri, eio_cb cb, void *data)
		358
		359	=item eio_ftruncate (int fd, off_t offset, int pri, eio_cb cb, void *data)
		360
		361	=item eio_fchmod (int fd, mode_t mode, int pri, eio_cb cb, void *data)
		362
		363	=item eio_fchown (int fd, uid_t uid, gid_t gid, int pri, eio_cb cb, void *data)
		364
		365	=item eio_dup2 (int fd, int fd2, int pri, eio_cb cb, void *data)
		366
		367	These have the same semantics as the syscall of the same name, their
		368	return value is available as C<< req->result >> later.
		369
		370	=item eio_read (int fd, void buf, size_t length, off_t offset, int pri, eio_cb cb, void data)
		371
		372	=item eio_write (int fd, void buf, size_t length, off_t offset, int pri, eio_cb cb, void data)
		373
		374	These two requests are called C<read> and C<write>, but actually wrap
		375	C<pread> and C<pwrite>. On systems that lack these calls (such as cygwin),
		376	libeio uses lseek/read_or_write/lseek and a mutex to serialise the
		377	requests, so all these requests run serially and do not disturb each
		378	other. However, they still disturb the file offset while they run, so it's
		379	not safe to call these functions concurrently with non-libeio functions on
		380	the same fd on these systems.
		381
		382	Not surprisingly, pread and pwrite are not thread-safe on Darwin (OS/X),
		383	so it is advised not to submit multiple requests on the same fd on this
		384	horrible pile of garbage.
		385
		386	=item eio_mlockall (int flags, int pri, eio_cb cb, void *data)
		387
		388	Like C<mlockall>, but the flag value constants are called
		389	C<EIO_MCL_CURRENT> and C<EIO_MCL_FUTURE>.
		390
		391	=item eio_msync (void addr, size_t length, int flags, int pri, eio_cb cb, void data)
		392
		393	Just like msync, except that the flag values are called C<EIO_MS_ASYNC>,
		394	C<EIO_MS_INVALIDATE> and C<EIO_MS_SYNC>.
		395
		396	=item eio_readlink (const char path, int pri, eio_cb cb, void data)
		397
		398	If successful, the path read by C<readlink(2)> can be accessed via C<<
		399	req->ptr2 >> and is I<NOT> null-terminated, with the length specified as
		400	C<< req->result >>.
		401
		402	if (req->result >= 0)
		403	{
		404	char target = strndup ((char )req->ptr2, req->result);
		405
		406	free (target);
		407	}
		408
		409	=item eio_realpath (const char path, int pri, eio_cb cb, void data)
		410
		411	Similar to the realpath libc function, but unlike that one, C<<
		412	req->result >> is C<-1> on failure. On success, the result is the length
		413	of the returned path in C<ptr2> (which is I<NOT> 0-terminated) - this is
		414	similar to readlink.
		415
		416	=item eio_stat (const char path, int pri, eio_cb cb, void data)
		417
		418	=item eio_lstat (const char path, int pri, eio_cb cb, void data)
		419
		420	=item eio_fstat (int fd, int pri, eio_cb cb, void *data)
		421
		422	Stats a file - if C<< req->result >> indicates success, then you can
		423	access the C<struct stat>-like structure via C<< req->ptr2 >>:
		424
		425	EIO_STRUCT_STAT statdata = (EIO_STRUCT_STAT )req->ptr2;
		426
		427	=item eio_statvfs (const char path, int pri, eio_cb cb, void data)
		428
		429	=item eio_fstatvfs (int fd, int pri, eio_cb cb, void *data)
		430
		431	Stats a filesystem - if C<< req->result >> indicates success, then you can
		432	access the C<struct statvfs>-like structure via C<< req->ptr2 >>:
		433
		434	EIO_STRUCT_STATVFS statdata = (EIO_STRUCT_STATVFS )req->ptr2;
		435
		436	=back
		437
		438	=head3 READING DIRECTORIES
		439
		440	Reading directories sounds simple, but can be rather demanding, especially
		441	if you want to do stuff such as traversing a directory hierarchy or
		442	processing all files in a directory. Libeio can assist these complex tasks
		443	with it's C<eio_readdir> call.
		444
		445	=over 4
		446
		447	=item eio_readdir (const char path, int flags, int pri, eio_cb cb, void data)
		448
		449	This is a very complex call. It basically reads through a whole directory
		450	(via the C<opendir>, C<readdir> and C<closedir> calls) and returns either
		451	the names or an array of C<struct eio_dirent>, depending on the C<flags>
		452	argument.
		453
		454	The C<< req->result >> indicates either the number of files found, or
		455	C<-1> on error. On success, null-terminated names can be found as C<< req->ptr2 >>,
		456	and C<struct eio_dirents>, if requested by C<flags>, can be found via C<<
		457	req->ptr1 >>.
		458
		459	Here is an example that prints all the names:
		460
		461	int i;
		462	char names = (char )req->ptr2;
		463
		464	for (i = 0; i < req->result; ++i)
		465	{
		466	printf ("name #%d: %s\n", i, names);
		467
		468	/* move to next name */
		469	names += strlen (names) + 1;
		470	}
		471
		472	Pseudo-entries such as F<.> and F<..> are never returned by C<eio_readdir>.
		473
		474	C<flags> can be any combination of:
		475
		476	=over 4
		477
		478	=item EIO_READDIR_DENTS
		479
		480	If this flag is specified, then, in addition to the names in C<ptr2>,
		481	also an array of C<struct eio_dirent> is returned, in C<ptr1>. A C<struct
		482	eio_dirent> looks like this:
		483
		484	struct eio_dirent
		485	{
		486	int nameofs; /* offset of null-terminated name string in (char )req->ptr2 /
		487	unsigned short namelen; /* size of filename without trailing 0 */
		488	unsigned char type; /* one of EIO_DT_* */
		489	signed char score; /* internal use */
		490	ino_t inode; /* the inode number, if available, otherwise unspecified */
		491	};
		492
		493	The only members you normally would access are C<nameofs>, which is the
		494	byte-offset from C<ptr2> to the start of the name, C<namelen> and C<type>.
		495
		496	C<type> can be one of:
		497
		498	C<EIO_DT_UNKNOWN> - if the type is not known (very common) and you have to C<stat>
		499	the name yourself if you need to know,
		500	one of the "standard" POSIX file types (C<EIO_DT_REG>, C<EIO_DT_DIR>, C<EIO_DT_LNK>,
		501	C<EIO_DT_FIFO>, C<EIO_DT_SOCK>, C<EIO_DT_CHR>, C<EIO_DT_BLK>)
		502	or some OS-specific type (currently
		503	C<EIO_DT_MPC> - multiplexed char device (v7+coherent),
		504	C<EIO_DT_NAM> - xenix special named file,
		505	C<EIO_DT_MPB> - multiplexed block device (v7+coherent),
		506	C<EIO_DT_NWK> - HP-UX network special,
		507	C<EIO_DT_CMP> - VxFS compressed,
		508	C<EIO_DT_DOOR> - solaris door, or
		509	C<EIO_DT_WHT>).
		510
		511	This example prints all names and their type:
		512
		513	int i;
		514	struct eio_dirent ents = (struct eio_dirent )req->ptr1;
		515	char names = (char )req->ptr2;
		516
		517	for (i = 0; i < req->result; ++i)
		518	{
		519	struct eio_dirent *ent = ents + i;
		520	char *name = names + ent->nameofs;
		521
		522	printf ("name #%d: %s (type %d)\n", i, name, ent->type);
		523	}
		524
		525	=item EIO_READDIR_DIRS_FIRST
		526
		527	When this flag is specified, then the names will be returned in an order
		528	where likely directories come first, in optimal C<stat> order. This is
		529	useful when you need to quickly find directories, or you want to find all
		530	directories while avoiding to stat() each entry.
		531
		532	If the system returns type information in readdir, then this is used
		533	to find directories directly. Otherwise, likely directories are names
		534	beginning with ".", or otherwise names with no dots, of which names with
		535	short names are tried first.
		536
		537	=item EIO_READDIR_STAT_ORDER
		538
		539	When this flag is specified, then the names will be returned in an order
		540	suitable for stat()'ing each one. That is, when you plan to stat()
		541	all files in the given directory, then the returned order will likely
		542	be fastest.
		543
		544	If both this flag and C<EIO_READDIR_DIRS_FIRST> are specified, then the
		545	likely directories come first, resulting in a less optimal stat order.
		546
		547	=item EIO_READDIR_FOUND_UNKNOWN
		548
		549	This flag should not be specified when calling C<eio_readdir>. Instead,
		550	it is being set by C<eio_readdir> (you can access the C<flags> via C<<
		551	req->int1 >>, when any of the C<type>'s found were C<EIO_DT_UNKNOWN>. The
		552	absence of this flag therefore indicates that all C<type>'s are known,
		553	which can be used to speed up some algorithms.
		554
		555	A typical use case would be to identify all subdirectories within a
		556	directory - you would ask C<eio_readdir> for C<EIO_READDIR_DIRS_FIRST>. If
		557	then this flag is I<NOT> set, then all the entries at the beginning of the
		558	returned array of type C<EIO_DT_DIR> are the directories. Otherwise, you
		559	should start C<stat()>'ing the entries starting at the beginning of the
		560	array, stopping as soon as you found all directories (the count can be
		561	deduced by the link count of the directory).
		562
		563	=back
		564
		565	=back
		566
		567	=head3 OS-SPECIFIC CALL WRAPPERS
		568
		569	These wrap OS-specific calls (usually Linux ones), and might or might not
		570	be emulated on other operating systems. Calls that are not emulated will
		571	return C<-1> and set C<errno> to C<ENOSYS>.
		572
		573	=over 4
		574
		575	=item eio_sendfile (int out_fd, int in_fd, off_t in_offset, size_t length, int pri, eio_cb cb, void *data)
		576
		577	Wraps the C<sendfile> syscall. The arguments follow the Linux version, but
		578	libeio supports and will use similar calls on FreeBSD, HP/UX, Solaris and
		579	Darwin.
		580
		581	If the OS doesn't support some sendfile-like call, or the call fails,
		582	indicating support for the given file descriptor type (for example,
		583	Linux's sendfile might not support file to file copies), then libeio will
		584	emulate the call in userspace, so there are almost no limitations on its
		585	use.
		586
		587	=item eio_readahead (int fd, off_t offset, size_t length, int pri, eio_cb cb, void *data)
		588
		589	Calls C<readahead(2)>. If the syscall is missing, then the call is
		590	emulated by simply reading the data (currently in 64kiB chunks).
		591
		592	=item eio_syncfs (int fd, int pri, eio_cb cb, void *data)
		593
		594	Calls Linux' C<syncfs> syscall, if available. Returns C<-1> and sets
		595	C<errno> to C<ENOSYS> if the call is missing I<but still calls sync()>,
		596	if the C<fd> is C<< >= 0 >>, so you can probe for the availability of the
		597	syscall with a negative C<fd> argument and checking for C<-1/ENOSYS>.
		598
		599	=item eio_sync_file_range (int fd, off_t offset, size_t nbytes, unsigned int flags, int pri, eio_cb cb, void *data)
		600
		601	Calls C<sync_file_range>. If the syscall is missing, then this is the same
		602	as calling C<fdatasync>.
		603
		604	Flags can be any combination of C<EIO_SYNC_FILE_RANGE_WAIT_BEFORE>,
		605	C<EIO_SYNC_FILE_RANGE_WRITE> and C<EIO_SYNC_FILE_RANGE_WAIT_AFTER>.
		606
		607	=item eio_fallocate (int fd, int mode, off_t offset, off_t len, int pri, eio_cb cb, void *data)
		608
		609	Calls C<fallocate> (note: I<NOT> C<posix_fallocate>!). If the syscall is
		610	missing, then it returns failure and sets C<errno> to C<ENOSYS>.
		611
		612	The C<mode> argument can be C<0> (for behaviour similar to
		613	C<posix_fallocate>), or C<EIO_FALLOC_FL_KEEP_SIZE>, which keeps the size
		614	of the file unchanged (but still preallocates space beyond end of file).
		615
		616	=back
		617
		618	=head3 LIBEIO-SPECIFIC REQUESTS
		619
		620	These requests are specific to libeio and do not correspond to any OS call.
		621
		622	=over 4
		623
		624	=item eio_mtouch (void addr, size_t length, int flags, int pri, eio_cb cb, void data)
		625
		626	Reads (C<flags == 0>) or modifies (C<flags == EIO_MT_MODIFY>) the given
		627	memory area, page-wise, that is, it reads (or reads and writes back) the
		628	first octet of every page that spans the memory area.
		629
		630	This can be used to page in some mmapped file, or dirty some pages. Note
		631	that dirtying is an unlocked read-write access, so races can ensue when
		632	the some other thread modifies the data stored in that memory area.
		633
		634	=item eio_custom (void ()(eio_req ) execute, int pri, eio_cb cb, void *data)
		635
		636	Executes a custom request, i.e., a user-specified callback.
		637
		638	The callback gets the C<eio_req *> as parameter and is expected to read
		639	and modify any request-specific members. Specifically, it should set C<<
		640	req->result >> to the result value, just like other requests.
		641
		642	Here is an example that simply calls C<open>, like C<eio_open>, but it
		643	uses the C<data> member as filename and uses a hardcoded C<O_RDONLY>. If
		644	you want to pass more/other parameters, you either need to pass some
		645	struct or so via C<data> or provide your own wrapper using the low-level
		646	API.
		647
		648	static int
		649	my_open_done (eio_req *req)
		650	{
		651	int fd = req->result;
		652
		653	return 0;
		654	}
		655
		656	static void
		657	my_open (eio_req *req)
		658	{
		659	req->result = open (req->data, O_RDONLY);
		660	}
		661
		662	eio_custom (my_open, 0, my_open_done, "/etc/passwd");
		663
		664	=item eio_busy (eio_tstamp delay, int pri, eio_cb cb, void *data)
		665
		666	This is a request that takes C<delay> seconds to execute, but otherwise
		667	does nothing - it simply puts one of the worker threads to sleep for this
		668	long.
		669
		670	This request can be used to artificially increase load, e.g. for debugging
		671	or benchmarking reasons.
		672
		673	=item eio_nop (int pri, eio_cb cb, void *data)
		674
		675	This request does nothing, except go through the whole request cycle. This
		676	can be used to measure latency or in some cases to simplify code, but is
		677	not really of much use.
		678
		679	=back
		680
		681	=head3 GROUPING AND LIMITING REQUESTS
		682
		683	There is one more rather special request, C<eio_grp>. It is a very special
		684	aio request: Instead of doing something, it is a container for other eio
		685	requests.
		686
		687	There are two primary use cases for this: a) bundle many requests into a
		688	single, composite, request with a definite callback and the ability to
		689	cancel the whole request with its subrequests and b) limiting the number
		690	of "active" requests.
		691
		692	Further below you will find more discussion of these topics - first
		693	follows the reference section detailing the request generator and other
		694	methods.
		695
		696	=over 4
		697
		698	=item eio_req grp = eio_grp (eio_cb cb, void data)
		699
		700	Creates, submits and returns a group request. Note that it doesn't have a
		701	priority, unlike all other requests.
		702
		703	=item eio_grp_add (eio_req grp, eio_req req)
		704
		705	Adds a request to the request group.
		706
		707	=item eio_grp_cancel (eio_req *grp)
		708
		709	Cancels all requests I<in> the group, but I<not> the group request
		710	itself. You can cancel the group request I<and> all subrequests via a
		711	normal C<eio_cancel> call.
		712
		713	=back
		714
		715	=head4 GROUP REQUEST LIFETIME
		716
		717	Left alone, a group request will instantly move to the pending state and
		718	will be finished at the next call of C<eio_poll>.
		719
		720	The usefulness stems from the fact that, if a subrequest is added to a
		721	group I<before> a call to C<eio_poll>, via C<eio_grp_add>, then the group
		722	will not finish until all the subrequests have finished.
		723
		724	So the usage cycle of a group request is like this: after it is created,
		725	you normally instantly add a subrequest. If none is added, the group
		726	request will finish on it's own. As long as subrequests are added before
		727	the group request is finished it will be kept from finishing, that is the
		728	callbacks of any subrequests can, in turn, add more requests to the group,
		729	and as long as any requests are active, the group request itself will not
		730	finish.
		731
		732	=head4 CREATING COMPOSITE REQUESTS
		733
		734	Imagine you wanted to create an C<eio_load> request that opens a file,
		735	reads it and closes it. This means it has to execute at least three eio
		736	requests, but for various reasons it might be nice if that request looked
		737	like any other eio request.
		738
		739	This can be done with groups:
		740
		741	=over 4
		742
		743	=item 1) create the request object
		744
		745	Create a group that contains all further requests. This is the request you
		746	can return as "the load request".
		747
		748	=item 2) open the file, maybe
		749
		750	Next, open the file with C<eio_open> and add the request to the group
		751	request and you are finished setting up the request.
		752
		753	If, for some reason, you cannot C<eio_open> (path is a null ptr?) you
		754	can set C<< grp->result >> to C<-1> to signal an error and let the group
		755	request finish on its own.
		756
		757	=item 3) open callback adds more requests
		758
		759	In the open callback, if the open was not successful, copy C<<
		760	req->errorno >> to C<< grp->errorno >> and set C<< grp->result >> to
		761	C<-1> to signal an error.
		762
		763	Otherwise, malloc some memory or so and issue a read request, adding the
		764	read request to the group.
		765
		766	=item 4) continue issuing requests till finished
		767
		768	In the read callback, check for errors and possibly continue with
		769	C<eio_close> or any other eio request in the same way.
		770
		771	As soon as no new requests are added, the group request will finish. Make
		772	sure you I<always> set C<< grp->result >> to some sensible value.
		773
		774	=back
		775
		776	=head4 REQUEST LIMITING
		777
		778
		779	#TODO
		780
		781	void eio_grp_limit (eio_req *grp, int limit);
		782
		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
…		…
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). In all other cases, the
262	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	921	value must be an expression that evaluates to the desired stack size.
264	stack size.
265		922
266	=back	923	=back
267		924
268		925
269	=head1 PORTABILITY REQUIREMENTS	926	=head1 PORTABILITY REQUIREMENTS

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Revision 1.36 by root, Sun Jan 24 16:36:20 2016 UTC