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Current libsndfile source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:22:47 +0100
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1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
2 <HTML>
3
4 <HEAD>
5 <TITLE>
6 libsndfile : Frequently Asked Questions.
7 </TITLE>
8 <META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
9 <META NAME="Description" CONTENT="The libsndfile FAQ.">
10 <META NAME="Keywords" CONTENT="WAV AIFF AU libsndfile sound audio dsp Linux">
11 <LINK REL="stylesheet" HREF="libsndfile.css" TYPE="text/css" MEDIA="all">
12 <LINK REL="stylesheet" HREF="print.css" TYPE="text/css" MEDIA="print">
13 </HEAD>
14
15 <BODY>
16
17 <H1><B>libsndfile : Frequently Asked Questions.</B></H1>
18 <P>
19 <A HREF="#Q001">Q1 : Do you plan to support XYZ codec in libsndfile?</A><BR/>
20 <A HREF="#Q002">Q2 : In version 0 the SF_INFO struct had a pcmbitwidth field
21 but version 1 does not. Why?</A><BR/>
22 <A HREF="#Q003">Q3 : Compiling is really slow on MacOS X. Why?</A><BR/>
23 <A HREF="#Q004">Q4 : When trying to compile libsndfile on Solaris I get a "bad
24 substitution" error during linking. What can I do to fix this?</A><BR/>
25 <A HREF="#Q005">Q5 : Why doesn't libsndfile do interleaving/de-interleaving?</A><BR/>
26 <A HREF="#Q006">Q6 : What's the best format for storing temporary files?</A><BR/>
27 <A HREF="#Q007">Q7 : On Linux/Unix/MacOS X, what's the best way of detecting the
28 presence of libsndfile?</A><BR/>
29 <A HREF="#Q008">Q8 : I have libsndfile installed and now I want to use it. I
30 just want a simple Makefile! What do I do?</A><BR/>
31 <A HREF="#Q009">Q9 : How about adding the ability to write/read sound files to/from
32 memory buffers?</A><BR/>
33 <A HREF="#Q010">Q10 : Reading a 16 bit PCM file as normalised floats and then
34 writing them back changes some sample values. Why?</A><BR/>
35 <A HREF="#Q011">Q11 : I'm having problems with u-law encoded WAV files generated by
36 libsndfile in Winamp. Why?</A><BR/>
37 <A HREF="#Q012">Q12 : I'm looking at sf_read*. What are items? What are frames?</A><BR/>
38 <A HREF="#Q013">Q13 : Why can't libsndfile open this Sound Designer II (SD2)
39 file?</A><BR/>
40 <A HREF="#Q014">Q14 : I'd like to statically link libsndfile to my closed source
41 application. Can I buy a license so that this is possible?</A><BR/>
42 <A HREF="#Q015">Q15 : My program is crashing during a call to a function in libsndfile.
43 Is this a bug in libsndfile?</A><BR/>
44 <A HREF="#Q016">Q16 : Will you accept a fix for compiling libsndfile with compiler X?
45 </A><BR/>
46 <A HREF="#Q017">Q17 : Can libsndfile read/write files from/to UNIX pipes?
47 </A><BR/>
48 <A HREF="#Q018">Q18 : Is it possible to build a Universal Binary on Mac OS X?
49 </A><BR/>
50 <A HREF="#Q019">Q19 : I have project files for Visual Studio / XCode / Whatever. Why
51 don't you distribute them with libsndfile?
52 </A><BR/>
53 <A HREF="#Q020">Q20 : Why doesn't libsndfile support MP3? Lots of other Open Source
54 projects support it!
55 </A><BR/>
56 <A HREF="#Q021">Q21 : How do I use libsndfile in a closed source or commercial program
57 and comply with the license?
58 </A><BR/>
59 <A HREF="#Q022">Q22 : What versions of windows does libsndfile work on?
60 </A><BR/>
61 <A HREF="#Q023">Q23 : I'm cross compiling libsndfile for another platform. How can I
62 run the test suite?
63 </A><BR/>
64 <HR>
65
66 <!-- ========================================================================= -->
67 <A NAME="Q001"></A>
68 <H2><BR/><B>Q1 : Do you plan to support XYZ codec in libsndfile?</B></H2>
69 <P>
70 If source code for XYZ codec is available under a suitable license (LGPL, BSD,
71 MIT etc) then yes, I'd like to add it.
72 </P>
73 <P>
74 If suitable documentation is available on how to decode and encode the format
75 then maybe, depending on how much work is involved.
76 </P>
77 <P>
78 If XYZ is some proprietary codec where no source code or documentation is
79 available then no.
80 </P>
81 <P>
82 So if you want support for XYZ codec, first find existing source code or
83 documentation.
84 If you can't find either then the answer is no.
85 </P>
86 <!-- ========================================================================= -->
87 <A NAME="Q002"></A>
88 <H2><BR/><B>Q2 : In version 0 the SF_INFO struct had a pcmbitwidth field
89 but version 1 does not. Why?</B></H2>
90 <P>
91 This was dropped for a number of reasons:
92 </P>
93 <UL>
94 <LI> pcmbitwidth makes little sense on compressed or floating point formats
95 <LI> with the new API you really don't need to know it
96 </UL>
97 <P>
98 As documented
99 <A HREF="api.html#note1">here</A>
100 there is now a well defined behaviour which ensures that no matter what the
101 bit width of the source file, the scaling always does something sensible.
102 This makes it safe to read 8, 16, 24 and 32 bit PCM files using sf_read_short()
103 and always have the optimal behaviour.
104 </P>
105
106 <!-- ========================================================================= -->
107 <A NAME="Q003"></A>
108 <H2><BR/><B>Q3 : Compiling is really slow on MacOS X. Why?</B></H2>
109 <P>
110 When you configure and compile libsndfile, it uses the /bin/sh shell for a number
111 of tasks (ie configure script and libtool).
112 Older versions of OS X (10.2?) shipped a really crappy Bourne shell as /bin/sh
113 which resulted in <b>really</b> slow compiles.
114 Newer version of OS X ship GNU Bash as /bin/sh and this answer doesn't apply in that
115 case.
116 </P>
117 <P>
118 To fix this I suggest that you install the GNU Bash shell, rename /bin/sh to
119 /bin/sh.old and make a symlink from /bin/sh to the bash shell.
120 Bash is designed to behave as a Bourne shell when is is called as /bin/sh.
121 </P>
122 <P>
123 When I did this on my iBook running MacOS X, compile times dropped from 13 minutes
124 to 3 minutes.
125 </P>
126
127 <!-- ========================================================================= -->
128 <A NAME="Q004"></A>
129 <H2><BR/><B>Q4 : When trying to compile libsndfile on Solaris I get a "bad
130 substitution" error on linking. Why?</B></H2>
131 <P>
132 It seems that the Solaris Bourne shell disagrees with GNU libtool.
133 </P>
134 <P>
135 To fix this I suggest that you install the GNU Bash shell, rename /bin/sh to
136 /bin/sh.old and make a symlink from /bin/sh to the bash shell.
137 Bash is designed to behave as a Bourne shell when is is called as /bin/sh.
138 </P>
139
140 <!-- ========================================================================= -->
141 <A NAME="Q005"></A>
142 <H2><BR/><B>Q5 : Why doesn't libsndfile do interleaving/de-interleaving?</B></H2>
143 <P>
144 This problem is bigger than it may seem at first.
145 </P>
146 <P>
147 For a stereo file, it is a pretty safe bet that a simple interleaving/de-interleaving
148 could satisfy most users.
149 However, for files with more than 2 channels this is unlikely to be the case.
150 If the user has a 4 channel file and want to play that file on a stereo output
151 sound card they either want the first 2 channels or they want some mixed combination
152 of the 4 channels.
153 </P>
154 <P>
155 When you add more channels, the combinations grow exponentially and it becomes
156 increasingly difficult to cover even a sensible subset of the possible combinations.
157 On top of that, coding any one style of interleaver/de-interleaver is trivial, while
158 coding one that can cover all combinations is far from trivial.
159 This means that this feature will not be added any time soon.
160 </P>
161
162 <!-- ========================================================================= -->
163 <A NAME="Q006"></A>
164 <H2><BR/><B>Q6 : What's the best format for storing temporary files?</B></H2>
165
166 <P>
167 When you want to store temporary data there are a number of requirements;
168 </P>
169 <UL>
170 <LI> A simple, easy to parse header.
171 <LI> The format must provide the fastest possible read and write rates (ie
172 avoid conversions and encoding/decoding).
173 <LI> The file format must be reasonably common and playable by most players.
174 <LI> Able to store data in either endian-ness.
175 </UL>
176 <P>
177 The format which best meets these requirements is AU, which allows data to be
178 stored in any one of short, int, float and double (among others) formats.
179 </P>
180 <P>
181 For instance, if an application uses float data internally, its temporary files
182 should use a format of (SF_ENDIAN_CPU | SF_FORMAT_AU | SF_FORMAT_FLOAT) which
183 will store big endian float data in big endian CPUs and little endian float data
184 on little endian CPUs.
185 Reading and writing this format will not require any conversions or byte swapping
186 regardless of the host CPU.
187 </P>
188
189 <!-- ========================================================================= -->
190
191 <A NAME="Q007"></A>
192 <H2><BR/><B>Q7 : On Linux/Unix/MaxOS X, what's the best way of detecting the presence
193 of libsndfile using autoconf?</B></H2>
194
195 <P>
196 libsndfile uses the pkg-config (man pkg-config) method of registering itself with the
197 host system.
198 The best way of detecting its presence is using something like this in configure.ac
199 (or configure.in):
200 </P>
201 <PRE>
202 PKG_CHECK_MODULES(SNDFILE, sndfile >= 1.0.2, ac_cv_sndfile=1, ac_cv_sndfile=0)
203
204 AC_DEFINE_UNQUOTED([HAVE_SNDFILE],${ac_cv_sndfile},
205 [Set to 1 if you have libsndfile.])
206
207 AC_SUBST(SNDFILE_CFLAGS)
208 AC_SUBST(SNDFILE_LIBS)
209 </PRE>
210 <P>
211 This will automatically set the <B>SNDFILE_CFLAGS</B> and <B>SNDFILE_LIBS</B>
212 variables which can be used in Makefile.am like this:
213 </P>
214 <PRE>
215 SNDFILE_CFLAGS = @SNDFILE_CFLAGS@
216 SNDFILE_LIBS = @SNDFILE_LIBS@
217 </PRE>
218 <P>
219 If you install libsndfile from source, you will probably need to set the
220 <B>PKG_CONFIG_PATH</B> environment variable as suggested at the end of the
221 libsndfile configure process. For instance on my system I get this:
222 </P>
223 <PRE>
224 -=-=-=-=-=-=-=-=-=-= Configuration Complete =-=-=-=-=-=-=-=-=-=-
225
226 Configuration summary :
227
228 Version : ..................... 1.0.5
229 Experimental code : ........... no
230
231 Tools :
232
233 Compiler is GCC : ............. yes
234 GCC major version : ........... 3
235
236 Installation directories :
237
238 Library directory : ........... /usr/local/lib
239 Program directory : ........... /usr/local/bin
240 Pkgconfig directory : ......... /usr/local/lib/pkgconfig
241
242 Compiling some other packages against libsndfile may require
243 the addition of "/usr/local/lib/pkgconfig" to the
244 PKG_CONFIG_PATH environment variable.
245 </PRE>
246
247 <!-- ========================================================================= -->
248
249 <A NAME="Q008"></A>
250 <H2><BR/><B>Q8 : I have libsndfile installed and now I want to use it. I just want
251 a simple Makefile! What do I do?</B></H2>
252
253 <P>
254 The <B>pkg-config</B> program makes finding the correct compiler flag values and
255 library location far easier.
256 During the installation of libsndfile, a file named <B>sndfile.pc</B> is installed
257 in the directory <B>${libdir}/pkgconfig</B> (ie if libsndfile is installed in
258 <B>/usr/local/lib</B>, <B>sndfile.pc</B> will be installed in
259 <B>/usr/local/lib/pkgconfig/</B>).
260 </P>
261 <P>
262 In order for pkg-config to find sndfile.pc it may be necessary to point the
263 environment variable <B>PKG_CONFIG_PATH</B> in the right direction.
264 </P>
265 <PRE>
266 export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
267 </PRE>
268
269 <P>
270 Then, to compile a C file into an object file, the command would be:
271 </P>
272 <PRE>
273 gcc `pkg-config --cflags sndfile` -c somefile.c
274 </PRE>
275 <P>
276 and to link a number of objects into an executable that links against libsndfile,
277 the command would be:
278 </P>
279 <PRE>
280 gcc `pkg-config --libs sndfile` obj1.o obj2.o -o program
281 </PRE>
282
283 <!-- ========================================================================= -->
284
285 <A NAME="Q009"></A>
286 <H2><BR/><B>Q9 : How about adding the ability to write/read sound files to/from
287 memory buffers?</B></H2>
288
289 <P>
290 This has been added for version 1.0.13.
291 </P>
292
293 <!-- ========================================================================= -->
294
295 <A NAME="Q010"></A>
296 <H2><BR/><B>Q10 : Reading a 16 bit PCM file as normalised floats and then
297 writing them back changes some sample values. Why?</B></H2>
298
299 <P>
300 This is caused by the fact that the conversion from 16 bit short to float is
301 done by dividing by 32768 (0x8000 in hexadecimal) while the conversion from
302 float to 16 bit short is done by multiplying by 32767 (0x7FFF in hex).
303 So for instance, a value in a 16 bit PCM file of 20000 gets read as a floating
304 point number of 0.6103515625 (20000.0 / 0x8000).
305 Converting that back to a 16 bit short results in a value of 19999.3896484375
306 (0.6103515625 * 0x7FFF) which then gets rounded down to 19999.
307 </P>
308 <P>
309 You will notice that for this particular case, the error is 1 in 20000 or
310 0.005%.
311 Interestingly, for values of less than 16369, dividing by 0x8000 followed
312 by multiplying by 0x7FFF and then rounding the result, gives back the
313 original value.
314 It turns out that as long as the host operating system supplies the 1999 ISO
315 C Standard functions <B>lrintf</B> and <B>lrint</B> (or a replacement has
316 been supplied) then the maximum possible error is 1 in 16369 or about 0.006%.
317 </P>
318 <P>
319 Regardless of the size of the error, the reason why this is done is rather
320 subtle.
321 </P>
322 <P>
323 In a file containing 16 bit PCM samples, the values are restricted to the range
324 [-32768, 32767] while we want floating point values in the range [-1.0, 1.0].
325 The only way to do this conversion is to do a floating point division by a value
326 of 0x8000.
327 Converting the other way, the only way to ensure that floating point values in
328 the range [-1.0, 1.0] are within the valid range allowed by a 16 bit short is
329 to multiply by 0x7FFF.
330 </P>
331 <P>
332 Some people would say that this is a severe short-coming of libsndfile.
333 I would counter that anybody who is constantly converting back and forth
334 between 16 bit shorts and normalised floats is going to suffer other losses
335 in audio quality that they should also be concerned about.
336 </P>
337 <P>
338 Since this problem only occurs when converting between integer data on disk and
339 normalized floats in the application, it can be avoided by using something
340 other than normalized floats in the application.
341 Alternatives to normalized floats are the <b>short</b> and <b>int</b> data
342 types (ie using sf_read_short or sf_read_int) or using un-normalized floats
343 (see
344 <a href="command.html#SFC_SET_NORM_FLOAT">
345 SFC_SET_NORM_FLOAT</a>).
346 </P>
347 <P>
348 Another way to deal with this problem is to consider 16 bit short data as a
349 final destination format only, not as an intermediate storage format.
350 All intermediate data (ie which is going to be processed further) should be
351 stored in floating point format which is supported by all of the most common
352 file formats.
353 If floating point files are considered too large (2 times the size of a 16 bit
354 PCM file), it would also be possible to use 24 bit PCM as an intermediate
355 storage format (and which is also supported by most common file types).
356 </P>
357
358 <!-- ========================================================================= -->
359
360 <A NAME="Q011"></A>
361 <H2><BR/><B>Q11 : I'm having problems with u-law encoded WAV files generated by
362 libsndfile in Winamp. Why?
363 </B></H2>
364
365 <P>
366 This is actually a Winamp problem.
367 The official Microsoft spec suggests that the 'fmt ' chunk should be 18 bytes.
368 Unfortunately at least one of Microsoft's own applications (Sound Recorder on
369 Win98 I believe) did not accept 18 bytes 'fmt ' chunks.
370 </P>
371 <P>
372 Michael Lee did some experimenting and found that:
373 </P>
374 <PRE>
375 I have checked that Windows Media Player 9, QuickTime Player 6.4,
376 RealOne Player 2.0 and GoldWave 5.06 can all play u-law files with
377 16-byte or 18-byte 'fmt ' chunk. Only Winamp (2.91) and foobar2000
378 are unable to play u-law files with 16-byte 'fmt ' chunk.
379 </PRE>
380
381 <P>
382 Even this is a very small sampling of all the players out there.
383 For that reason it is probably not a good idea to change this now because there
384 is the risk of breaking something that currently works.
385 </P>
386
387 <!-- ========================================================================= -->
388
389 <A NAME="Q012"></A>
390 <H2><BR/><B>Q12 : I'm looking at sf_read*. What are items? What are frames?
391 </B></H2>
392
393 <P>
394 An <tt>item</tt> is a single sample of the data type you are reading; ie a
395 single <tt>short</tt> value for <tt>sf_read_short</tt> or a single <tt>float</tt>
396 for <tt>sf_read_float</tt>.
397 </P>
398
399 <P>
400 For a sound file with only one channel, a frame is the same as a item (ie a
401 single sample) while for multi channel sound files, a single frame contains a
402 single item for each channel.
403 </P>
404
405 <P>
406 Here are two simple, correct examples, both of which are assumed to be working
407 on a stereo file, first using items:
408 </P>
409
410 <PRE>
411 #define CHANNELS 2
412 short data [CHANNELS * 100] ;
413 sf_count items_read = sf_read_short (file, data, 200) ;
414 assert (items_read == 200) ;
415 </PRE>
416
417 <P>
418 and now reading the exact same amount of data using frames:
419 </P>
420
421 <PRE>
422 #define CHANNELS 2
423 short data [CHANNELS * 100] ;
424 sf_count frames_read = sf_readf_short (file, data, 100) ;
425 assert (frames_read == 100) ;
426 </PRE>
427
428 <!-- ========================================================================= -->
429
430 <A NAME="Q013"></A>
431 <H2><BR/><B>Q13 : Why can't libsndfile open this Sound Designer II (SD2) file?
432 </B></H2>
433
434 <P>
435 This is somewhat complicated.
436 First some background.
437 </P>
438
439 <P>
440 SD2 files are native to the Apple Macintosh platform and use features of
441 the Mac filesystem (file resource forks) to store the file's sample rate,
442 number of channels, sample width and more.
443 When you look at a file and its resource fork on Mac OS X it looks like
444 this:
445 </P>
446
447 <PRE>
448 -rw-r--r-- 1 erikd erikd 46512 Oct 18 22:57 file.sd2
449 -rw-r--r-- 1 erikd erikd 538 Oct 18 22:57 file.sd2/rsrc
450 </PRE>
451
452 <P>
453 Notice how the file itself looks like a directory containing a single file
454 named <B>rsrc</B>.
455 When libsndfile is compiled for MacOS X, it should open (for write and read)
456 SD2 file with resource forks like this without any problems.
457 It will also handle files with the resource fork in a separate file as
458 described below.
459 </P>
460
461 <P>
462 When SD2 files are moved to other platforms, the resource fork of the file
463 can sometimes be dropped altogether.
464 All that remains is the raw audio data and no information about the number
465 of channels, sample rate or bit width which makes it a little difficult for
466 libsndfile to open the file.
467 </P>
468
469 <P>
470 However, it is possible to safely move an SD2 file to a Linux or Windows
471 machine.
472 For instance, when an SD2 file is copied from inside MacOS X to a windows
473 shared directory or a Samba share (ie Linux), MacOS X is clever enough to
474 store the resource fork of the file in a separate hidden file in the
475 same directory like this:
476 </P>
477 <PRE>
478 -rw-r--r-- 1 erikd erikd 538 Oct 18 22:57 ._file.sd2
479 -rw-r--r-- 1 erikd erikd 46512 Oct 18 22:57 file.sd2
480 </PRE>
481
482 <P>
483 Regardless of what platform it is running on, when libsndfile is asked to
484 open a file named <B>"foo"</B> and it can't recognize the file type from
485 the data in the file, it will attempt to open the resource fork and if
486 that fails, it then tries to open a file named <B>"._foo"</B> to see if
487 the file has a valid resource fork.
488 This is the same regardless of whether the file is being opened for read
489 or write.
490 </P>
491
492 <P>
493 In short, libsndfile should open SD2 files with a valid resource fork on
494 all of the platforms that libsndfile supports.
495 If a file has lost its resource fork, the only option is the open the file
496 using the SF_FORMAT_RAW option and guessing its sample rate, channel count
497 and bit width.
498 </P>
499
500 <P>
501 Occasionally, when SD2 files are moved to other systems, the file is
502 <A HREF="http://www.macdisk.com/binhexen.php3">BinHexed</A>
503 which wraps the resource fork and the data fork together.
504 For these files, it would be possible to write a BinHex parser but
505 there is not a lot to gain considering how rare these BinHexed SD2
506 files are.
507 </P>
508
509 <!-- ========================================================================= -->
510 <A NAME="Q014"></A>
511 <H2><BR/><B>Q14 : I'd like to statically link libsndfile to my closed source
512 application. Can I buy a license so that this is possible?
513 </B></H2>
514
515 <P>
516 Unfortunately no.
517 libsndfile contains code written by other people who have agreed that their
518 code be used under the GNU LGPL but no more.
519 Even if they were to agree, there would be significant difficulties in
520 dividing up the payments fairly.
521 </P>
522
523 <P>
524 The <B>only</B> way you can legally use libsndfile as a statically linked
525 library is if your application is released under the GNU GPL or LGPL.
526 </P>
527
528 <!-- ========================================================================= -->
529 <A NAME="Q015"></A>
530 <H2><BR/><B>Q15 : My program is crashing during a call to a function in libsndfile.
531 Is this a bug in libsndfile?
532 </B></H2>
533
534 <P>
535 libsndfile is being used by large numbers of people all over the world
536 without any problems like this. That means that it is much more likely
537 that your code has a bug than libsndfile. However, it is still possible
538 that there is a bug in libsndfile.
539 </P>
540 <P>
541 To figure out whether it is your code or libsndfile you should do the
542 following:
543 </P>
544 <UL>
545 <LI>Make sure you are compiling your code with warnings switched on and
546 that you fix as many warnings as possible.
547 With the GNU compiler (gcc) I would recommend at least
548 <B>-W -Wall -Werror</B> which will force you to fix all warnings
549 before you can run the code.
550 <LI>Try using a memory debugger.
551 <A HREF="http://valgrind.kde.org/">Valgrind</A> on x86 Linux is excellent.
552 <A HREF="http://www.ibm.com/software/awdtools/purify/">Purify</A> also
553 has a good reputation.
554 <LI>If the code is clean after the above two steps and you still get
555 a crash in libsndfile, then send me a small snippet of code (no
556 more than 30-40 lines) which includes the call to sf_open() and
557 also shows how all variables passed to/returned from sf_open()
558 are defined.
559 </UL>
560
561 <!-- ========================================================================= -->
562 <A NAME="Q016"></A>
563 <H2><BR/><B>Q16 : Will you accept a fix for compiling libsndfile with compiler X?
564 </B></H2>
565
566 <P>
567 If compiler X is a C++ compiler then no.
568 C and C++ are different enough to make writing code that compiles as valid C
569 and valid C++ too difficult.
570 I would rather spend my time fixing bugs and adding features.
571 </P>
572
573 <P>
574 If compiler X is a C compiler then I will do what I can as long as that does
575 not hamper the correctness, portability and maintainability of the existing
576 code.
577 It should be noted however that libsndfile uses features specified by the 1999
578 ISO C Standard.
579 This can make compiling libsndfile with some older compilers difficult.
580 </P>
581
582 <!-- ========================================================================= -->
583 <A NAME="Q017"></A>
584 <H2><BR/><B>Q17 : Can libsndfile read/write files from/to UNIX pipes?
585 </B></H2>
586
587 <P>
588 Yes, libsndfile can read files from pipes.
589 Unfortunately, the write case is much more complicated.
590 </P>
591
592 <P>
593 File formats like AIFF and WAV have information at the start of the file (the
594 file header) which states the length of the file, the number of sample frames
595 etc.
596 This information must be filled in correctly when the file header is written,
597 but this information is not reliably known until the file is closed.
598 This means that libsndfile cannot write AIFF, WAV and many other file types
599 to a pipe.
600 </P>
601
602 <P>
603 However, there is at least one file format (AU) which is specifically designed
604 to be written to a pipe.
605 Like AIFF and WAV, AU has a header with a sample frames field, but it is
606 specifically allowable to set that frames field to 0x7FFFFFFF if the file
607 length is not known when the header is written.
608 The AU file format can also hold data in many of the standard formats (ie
609 SF_FORMAT_PCM_16, SF_FORMAT_PCM_24, SF_FORMAT_FLOAT etc) as well as allowing
610 data in both big and little endian format.
611 </P>
612
613 <P>
614 See also <A HREF="#Q006">FAQ Q6</A>.
615 </P>
616
617 <!-- ========================================================================= -->
618 <A NAME="Q018"></A>
619 <H2><BR/><B>Q18 : Is it possible to build a Universal Binary on Mac OS X?
620 </B></H2>
621
622 <P>
623 Yes, but you must do two separate configure/build/test runs; one on PowerPC
624 and one on Intel.
625 It is then possible to merge the binaries into a single universal binary using
626 one of the programs in the Apple tool chain.
627 </P>
628
629 <P>
630 It is <b>not</b> possible to build a working universal binary via a single
631 compile/build run on a single CPU.
632 </P>
633
634 <P>
635 The problem is that the libsndfile build process detects features of the CPU its
636 being built for during the configure process and when building a universal binary,
637 configure is only run once and that data is then used for both CPUs.
638 That configure data will be wrong for one of those CPUs.
639 You will still be able to compile libsndfile, and the test suite will pass on
640 the machine you compiled it on.
641 However, if you take the universal binary test suite programs compiled on one
642 CPU and run them on the other, the test suite will fail.
643 </P>
644
645 <P>
646 Part of the problem is that the CPU endian-ness is detected at configure time.
647 Yes, I know the Apple compiler defines one of the macros __LITTLE_ENDIAN__
648 and __BIG_ENDIAN__, but those macros are not part of the 1999 ISO C Standard
649 and they are not portable.
650 </P>
651
652 <P>
653 Endian issues are not the only reason why the cross compiled binary will fail.
654 The configure script also detects other CPU specific idiosyncrasies to provide
655 more optimized code.
656 </P>
657
658 <P>
659 Finally, the real show stopper problem with universal binaries is the problem
660 with the test suite.
661 libsndfile contains a huge, comprehensive test suite.
662 When you compile a universal binary and run the test suite, you only test the
663 native compile.
664 The cross compiled binary (the one with the much higher chance of having
665 problems) cannot be tested.
666 </P>
667
668 <P>
669 Now, if you have read this far you're probably thinking there must be a way
670 to fix this and there probably is.
671 The problem is that its a hell of a lot of work and would require significant
672 changes to the configure process, the internal code and the test suite.
673 In addition, these changes must not break compilation on any of the platforms
674 libsndfile is currently working on.
675 </p>
676
677
678 <!-- ========================================================================= -->
679 <A NAME="Q019"></A>
680 <H2><BR/><B>Q19 : I have project files for Visual Studio / XCode / Whatever. Why
681 don't you distribute them with libsndfile?
682 </B></H2>
683
684 <P>
685 There's a very good reason for this.
686 I will only distribute things that I actually have an ability to test and
687 maintain.
688 Project files for a bunch of different compilers and Integrated Development
689 Environments are simply too difficult to maintain.
690 </P>
691
692 <P>
693 The problem is that every time I add a new file to libsndfile or rename an
694 existing file I would have to modify all the project files and then test that
695 libsndfile still built with all the different compilers.
696 </P>
697
698 <P>
699 Maintaining these project files is also rather difficult if I don't have access
700 to the required compiler/IDE.
701 If I just edit the project files without testing them I will almost certainly
702 get it wrong.
703 If I release a version of libsndfile with broken project files, I'll get a bunch
704 of emails from people complaining about it not building and have no way of
705 fixing or even testing it.
706 </P>
707
708 <P>
709 I currently release sources that I personally test on Win32, Linux and
710 MacOS X (PowerPC) using the compiler I trust (GNU GCC).
711 Supporting one compiler on three (actually much more because GCC is available
712 almost everywhere) platforms is doable without too much pain.
713 I also release binaries for Win32 with instructions on how to use those
714 binaries with Visual Studio.
715 As a guy who is mainly interested in Linux, I'm not to keen to jump through
716 a bunch of hoops to support compilers and operating systems I don't use.
717 </P>
718
719 <P>
720 So, I hear you want to volunteer to maintain the project files for Some Crappy
721 Compiler 2007?
722 Well sorry, that won't work either.
723 I have had numerous people over the years offer to maintaining the project
724 files for Microsoft's Visual Studio.
725 Every single time that happened, they maintained it for a release or two and
726 then disappeared off the face of the earth.
727 Hence, I'm not willing to enter into an arrangement like that again.
728 </P>
729
730 <!-- ========================================================================= -->
731 <A NAME="Q020"></A>
732 <H2><BR/><B>Q20 : Why doesn't libsndfile support MP3? Lots of other Open Source
733 projects support it!
734 </B></H2>
735
736 <P>
737 MP3 is not supported for one very good reason; doing so requires the payment
738 of licensing fees.
739 As can be seen from
740 <a href="http://www.mp3licensing.com/royalty/software.html">
741 mp3licensing.com</a>
742 the required royalty payments are not cheap.
743 </P>
744
745 <p>
746 Yes, I know other libraries ignore the licensing requirements, but their legal
747 status is extremely dubious.
748 At any time, the body selling the licenses could go after the authors of those
749 libraries.
750 Some of those authors may be students and hence wouldn't be worth pursuing.
751 </P>
752
753 <p>
754 However, libsndfile is released under the name of a company, Mega Nerd Pty Ltd;
755 a company which has income from from libsamplerate licensing, libsndfile based
756 consulting income and other unrelated consulting income.
757 Adding MP3 support to libsndfile could place that income under legal threat.
758 </p>
759
760 <p>
761 Fortunately, Ogg Vorbis exists as an alternative to MP3.
762 Support for Ogg Vorbis was added to libsndfile (mostly due to the efforts of
763 John ffitch of the Csound project) in version 1.0.18.
764 </p>
765
766
767 <!-- ========================================================================= -->
768 <A NAME="Q021"></A>
769 <H2><BR/><B>Q21 : How do I use libsndfile in a closed source or commercial program
770 and comply with the license?
771 </B></H2>
772
773 <p>
774 Here is a checklist of things you need to do to make sure your use of libsndfile
775 in a closed source or commercial project complies with the license libsndfile is
776 released under, the GNU Lesser General Public License (LGPL):
777 </p>
778
779 <ul>
780 <li>Make sure you are linking to libsndfile as a shared library (Linux and Unix
781 systems), Dynamic Link Library (Microsoft Windows) or dynlib (Mac OS X).
782 If you are using some other operating system that doesn't allow dynamically
783 linked libraries, you will not be able to use libsndfile unless you release
784 the source code to your program.
785 <li>In the licensing documentation for your program, add a statement that your
786 software depends on libsndfile and that libsndfile is released under the GNU
787 Lesser General Public License, either
788 <a href="http://www.gnu.org/licenses/lgpl-2.1.txt">version 2.1</a>
789 or optionally
790 <a href="http://www.gnu.org/licenses/lgpl.txt">version 3</a>.
791 <li>Include the text for both versions of the license, possibly as separate
792 files named libsndfile_lgpl_v2_1.txt and libsndfile_lgpl_v3.txt.
793 </ul>
794
795 <!-- ========================================================================= -->
796 <A NAME="Q022"></A>
797 <H2><BR/><B>Q22 : What versions of Windows does libsndfile work on?
798 </B></H2>
799
800 <p>
801 Currently the precompiled windows binaries are thoroughly tested on Windows XP.
802 As such, they should also work on Win2k and Windows Vista.
803 They may also work on earlier versions of Windows.
804 </p>
805
806 <p>
807 Since version 0.1.18 I have also been releasing precompiled binaries for Win64,
808 the 64 bit version of Windows.
809 These binaries have received much less testing than the 32 bit versions, but
810 should work as expected.
811 I'd be very interested in receiving feedback on these binaries.
812 </p>
813
814 <!-- ========================================================================= -->
815 <A NAME="Q023"></A>
816 <H2><BR/><B>Q23 : I'm cross compiling libsndfile for another platform. How can I
817 run the test suite?
818 </B></H2>
819
820 <p>
821 </p>
822
823 <p>
824 Since version 1.0.21 the top level Makefile has an extra make target,
825 'test-tarball'.
826 Building this target creates a tarball called called:
827 </p>
828
829 <center><tt>
830 libsndfile-testsuite-${host_triplet}-${version}.tar.gz
831 </tt></center>
832
833 <p>
834 in the top level directory.
835 This tarball can then be copied to the target platform.
836 Once untarred and test script <tt>test_wrapper.sh</tt> can be run from
837 the top level of the extracted tarball.
838 </p>
839
840 <!-- ========================================================================= -->
841 <HR>
842 <P>
843 The libsndfile home page is here :
844 <A HREF="http://www.mega-nerd.com/libsndfile/">
845 http://www.mega-nerd.com/libsndfile/</A>.
846 <BR/>
847 Version : 1.0.27
848 </P>
849
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