xref: /third_party/libsnd/docs/FAQ.md

---
layout: default
title: libsndfile : Frequently Asked Questions.
---

# libsndfile : Frequently Asked Questions

1. [Do you plan to support XYZ codec in libsndfile?](#Q001)
2. [In version 0 the SF\_INFO struct had a pcmbitwidth field but version 1 does not. Why?](#Q002)
3. [Compiling is really slow on MacOS X. Why?](#Q003)
4. [When trying to compile libsndfile on Solaris I get a "bad substitution" error during linking. What can I do to fix this?](#Q004)
5. [Why doesn't libsndfile do interleaving/de-interleaving?](#Q005)
6. [What's the best format for storing temporary files?](#Q006)
7. [On Linux/Unix/MacOS X, what's the best way of detecting the presence of libsndfile?](#Q007)
8. [I have libsndfile installed and now I want to use it. I just want a simple Makefile\! What do I do?](#Q008)
9. [How about adding the ability to write/read sound files to/from memory buffers?](#Q009)
10. [Reading a 16 bit PCM file as normalised floats and then writing them back changes some sample values. Why?](#Q010)
11. [I'm having problems with u-law encoded WAV files generated by libsndfile in Winamp. Why?](#Q011)
12. [I'm looking at sf\_read\*. What are items? What are frames?](#Q012)
13. [Why can't libsndfile open this Sound Designer II (SD2) file?](#Q013)
14. [I'd like to statically link libsndfile to my closed source application. Can I buy a license so that this is possible?](#Q014)
15. [My program is crashing during a call to a function in libsndfile. Is this a bug in libsndfile?](#Q015)
16. [Will you accept a fix for compiling libsndfile with compiler X?](#Q016)
17. [Can libsndfile read/write files from/to UNIX pipes?](#Q017)
18. [Is it possible to build a Universal Binary on Mac OS X?](#Q018)
19. [I have project files for Visual Studio / XCode / Whatever. Why don't you distribute them with libsndfile?](#Q019)
20. [Why doesn't libsndfile support MP3?](#Q020)
21. [How do I use libsndfile in a closed source or commercial program and comply with the license?](#Q021)
22. [What versions of windows does libsndfile work on?](#Q022)
23. [I'm cross compiling libsndfile for another platform. How can I run the test suite?](#Q023)

-----

## Q1 : Do you plan to support XYZ codec in libsndfile? {#Q001}

If source code for XYZ codec is available under a suitable license (LGPL, BSD,
MIT etc) then yes, I'd like to add it.

If suitable documentation is available on how to decode and encode the format
then maybe, depending on how much work is involved.

If XYZ is some proprietary codec where no source code or documentation is
available then no.

So if you want support for XYZ codec, first find existing source code or
documentation. If you can't find either then the answer is no.

## Q2 : In version 0 the SF\_INFO struct had a pcmbitwidth field but version 1 does not. Why? {#Q002}

This was dropped for a number of reasons:

- pcmbitwidth makes little sense on compressed or floating point formats
- with the new API you really don't need to know it

As documented [here](api.md#note-1) there is now a well defined behaviour which
ensures that no matter what the bit width of the source file, the scaling always
does something sensible. This makes it safe to read 8, 16, 24 and 32 bit PCM
files using `sf_read_short()` and always have the optimal behaviour.

## Q3 : Compiling is really slow on MacOS X. Why? {#Q003}

When you configure and compile libsndfile, it uses the /bin/sh shell for a
number of tasks (ie configure script and libtool). Older versions of OS X
(10.2?) shipped a really crappy Bourne shell as /bin/sh which resulted in
**really** slow compiles. Newer version of OS X ship GNU Bash as /bin/sh and
this answer doesn't apply in that case.

To fix this I suggest that you install the GNU Bash shell, rename /bin/sh to
/bin/sh.old and make a symlink from /bin/sh to the bash shell. Bash is designed
to behave as a Bourne shell when it is called as /bin/sh.

When I did this on my iBook running MacOS X, compile times dropped from 13
minutes to 3 minutes.

## Q4 : When trying to compile libsndfile on Solaris I get a "bad substitution" error on linking. Why? {#Q004}

It seems that the Solaris Bourne shell disagrees with GNU libtool.

To fix this I suggest that you install the GNU Bash shell, rename /bin/sh to
/bin/sh.old and make a symlink from /bin/sh to the bash shell. Bash is designed
to behave as a Bourne shell when it is called as /bin/sh.

## Q5 : Why doesn't libsndfile do interleaving/de-interleaving? {#Q005}

This problem is bigger than it may seem at first.

For a stereo file, it is a pretty safe bet that a simple interleaving/
de-interleaving could satisfy most users. However, for files with more than 2
channels this is unlikely to be the case. If the user has a 4 channel file and
want to play that file on a stereo output sound card they either want the first
2 channels or they want some mixed combination of the 4 channels.

When you add more channels, the combinations grow exponentially and it becomes
increasingly difficult to cover even a sensible subset of the possible
combinations. On top of that, coding any one style of interleaver/de-interleaver
is trivial, while coding one that can cover all combinations is far from
trivial. This means that this feature will not be added any time soon.

## Q6 : What's the best format for storing temporary files? {#Q006}

When you want to store temporary data there are a number of requirements:

- A simple, easy to parse header.
- The format must provide the fastest possible read and write rates (ie avoid
  conversions and encoding/decoding).
- The file format must be reasonably common and playable by most players.
- Able to store data in either endian-ness.

The format which best meets these requirements is AU, which allows data to be
stored in any one of short, int, float and double (among others) formats.

For instance, if an application uses float data internally, its temporary files
should use a format of (SF_ENDIAN_CPU | SF_FORMAT_AU | SF_FORMAT_FLOAT) which
will store big endian float data in big endian CPUs and little endian float data
on little endian CPUs. Reading and writing this format will not require any
conversions or byte swapping regardless of the host CPU.

## Q7 : On Linux/Unix/MaxOS X, what's the best way of detecting the presence of libsndfile using autoconf? {#Q007}

libsndfile uses the pkg-config (man pkg-config) method of registering itself
with the host system. The best way of detecting its presence is using something
like this in configure.ac (or configure.in):

    PKG_CHECK_MODULES(SNDFILE, sndfile >= 1.0.2, ac_cv_sndfile=1, ac_cv_sndfile=0)

    AC_DEFINE_UNQUOTED([HAVE_SNDFILE],${ac_cv_sndfile},
        [Set to 1 if you have libsndfile.])

    AC_SUBST(SNDFILE_CFLAGS)
    AC_SUBST(SNDFILE_LIBS)

This will automatically set the **SNDFILE_CFLAGS** and **SNDFILE_LIBS**
variables which can be used in Makefile.am like this:

    SNDFILE_CFLAGS = @SNDFILE_CFLAGS@
    SNDFILE_LIBS = @SNDFILE_LIBS@

If you install libsndfile from source, you will probably need to set the
**PKG_CONFIG_PATH** environment variable as suggested at the end of the
libsndfile configure process. For instance on my system I get this:

    -=-=-=-=-=-=-=-=-=-= Configuration Complete =-=-=-=-=-=-=-=-=-=-

        Configuration summary :

        Version : ..................... 1.0.5
        Experimental code : ........... no

        Tools :

        Compiler is GCC : ............. yes
        GCC major version : ........... 3

        Installation directories :

        Library directory : ........... /usr/local/lib
        Program directory : ........... /usr/local/bin
        Pkgconfig directory : ......... /usr/local/lib/pkgconfig

    Compiling some other packages against libsndfile may require
    the addition of "/usr/local/lib/pkgconfig" to the
    PKG_CONFIG_PATH environment variable.

## Q8 : I have libsndfile installed and now I want to use it. I just want a simple Makefile\! What do I do? {#Q008}

The **pkg-config** program makes finding the correct compiler flag values and
library location far easier. During the installation of libsndfile, a file named
**sndfile.pc** is installed in the directory **${libdir}/pkgconfig** (ie if
libsndfile is installed in **/usr/local/lib**, **sndfile.pc** will be installed
in **/usr/local/lib/pkgconfig/**).

In order for pkg-config to find sndfile.pc it may be necessary to point the
environment variable **PKG_CONFIG_PATH** in the right direction.

    export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig

Then, to compile a C file into an object file, the command would be:

    gcc `pkg-config --cflags sndfile` -c somefile.c

and to link a number of objects into an executable that links against
libsndfile, the command would be:

    gcc `pkg-config --libs sndfile` obj1.o obj2.o -o program

## Q9 : How about adding the ability to write/read sound files to/from memory buffers? {#Q009}

This has been [added](api.md#open_virtual) for version 1.0.12.

## Q10 : Reading a 16 bit PCM file as normalised floats and then writing them back changes some sample values. Why? {#Q010}

This is caused by the fact that the conversion from 16 bit short to float is
done by dividing by 32768 (0x8000 in hexadecimal) while the conversion from
float to 16 bit short is done by multiplying by 32767 (0x7FFF in hex). So for
instance, a value in a 16 bit PCM file of 20000 gets read as a floating point
number of 0.6103515625 (20000.0 / 0x8000). Converting that back to a 16 bit
short results in a value of 19999.3896484375 (0.6103515625 \* 0x7FFF) which then
gets rounded down to 19999.

You will notice that for this particular case, the error is 1 in 20000 or
0.005%. Interestingly, for values of less than 16369, dividing by 0x8000
followed by multiplying by 0x7FFF and then rounding the result, gives back the
original value. It turns out that as long as the host operating system supplies
the 1999 ISO C Standard functions **lrintf** and **lrint** (or a replacement has
been supplied) then the maximum possible error is 1 in 16369 or about 0.006%.

Regardless of the size of the error, the reason why this is done is rather
subtle.

In a file containing 16 bit PCM samples, the values are restricted to the range
[-32768, 32767] while we want floating point values in the range [-1.0, 1.0].
The only way to do this conversion is to do a floating point division by a value
of 0x8000. Converting the other way, the only way to ensure that floating point
values in the range [-1.0, 1.0] are within the valid range allowed by a 16 bit
short is to multiply by 0x7FFF.

Some people would say that this is a severe short-coming of libsndfile. I would
counter that anybody who is constantly converting back and forth between 16 bit
shorts and normalised floats is going to suffer other losses in audio quality
that they should also be concerned about.

Since this problem only occurs when converting between integer data on disk and
normalized floats in the application, it can be avoided by using something other
than normalized floats in the application. Alternatives to normalized floats are
the **short** and **int** data types (ie using sf_read_short or sf_read_int) or
using un-normalized floats (see
[SFC_SET_NORM_FLOAT](command.html#sfc_set_norm_float)).

Another way to deal with this problem is to consider 16 bit short data as a
final destination format only, not as an intermediate storage format. All
intermediate data (ie which is going to be processed further) should be stored
in floating point format which is supported by all of the most common file
formats. If floating point files are considered too large (2 times the size of a
16 bit PCM file), it would also be possible to use 24 bit PCM as an intermediate
storage format (and which is also supported by most common file types).

## Q11 : I'm having problems with u-law encoded WAV files generated by libsndfile in Winamp. Why? {#Q011}

This is actually a Winamp problem. The official Microsoft spec suggests that the
'fmt ' chunk should be 18 bytes. Unfortunately at least one of Microsoft's own
applications (Sound Recorder on Win98 I believe) did not accept 18 bytes 'fmt '
chunks.

Michael Lee did some experimenting and found that:

> I have checked that Windows Media Player 9, QuickTime Player 6.4, RealOne
> Player 2.0 and GoldWave 5.06 can all play u-law files with 16-byte or 18-byte
> 'fmt ' chunk. Only Winamp (2.91) and foobar2000 are unable to play u-law files
> with 16-byte 'fmt ' chunk.

Even this is a very small sampling of all the players out there. For that reason
it is probably not a good idea to change this now because there is the risk of
breaking something that currently works.

## Q12 : I'm looking at sf_read*. What are items? What are frames? {#Q012}

An `item` is a single sample of the data type you are reading; ie a single
`short` value for `sf_read_short` or a single `float` for `sf_read_float`.

For a sound file with only one channel, a frame is the same as a item (ie a
single sample) while for multi channel sound files, a single frame contains a
single item for each channel.

Here are two simple, correct examples, both of which are assumed to be working
on a stereo file, first using items:

```c
#define CHANNELS 2
short data [CHANNELS * 100] ;
sf_count items_read = sf_read_short (file, data, 200) ;
assert (items_read == 200) ;
```

and now reading the exact same amount of data using frames:

```c
#define CHANNELS 2
short data [CHANNELS * 100] ;
sf_count frames_read = sf_readf_short (file, data, 100) ;
assert (frames_read == 100) ;
```

## Q13 : Why can't libsndfile open this Sound Designer II (SD2) file? {#Q013}

This is somewhat complicated. First some background.

SD2 files are native to the Apple Macintosh platform and use features of the Mac
filesystem (file resource forks) to store the file's sample rate, number of
channels, sample width and more. When you look at a file and its resource fork
on Mac OS X it looks like this:

    -rw-r--r--  1 erikd erikd   46512 Oct 18 22:57 file.sd2
    -rw-r--r--  1 erikd erikd     538 Oct 18 22:57 file.sd2/rsrc

Notice how the file itself looks like a directory containing a single file named
**rsrc**. When libsndfile is compiled for MacOS X, it should open (for write and
read) SD2 file with resource forks like this without any problems. It will also
handle files with the resource fork in a separate file as described below.

When SD2 files are moved to other platforms, the resource fork of the file can
sometimes be dropped altogether. All that remains is the raw audio data and no
information about the number of channels, sample rate or bit width which makes
it a little difficult for libsndfile to open the file.

However, it is possible to safely move an SD2 file to a Linux or Windows
machine. For instance, when an SD2 file is copied from inside MacOS X to a
windows shared directory or a Samba share (ie Linux), MacOS X is clever enough
to store the resource fork of the file in a separate hidden file in the same
directory like this:

    -rw-r--r--  1 erikd erikd     538 Oct 18 22:57 ._file.sd2
    -rw-r--r--  1 erikd erikd   46512 Oct 18 22:57 file.sd2

Regardless of what platform it is running on, when libsndfile is asked to open a
file named **"foo"** and it can't recognize the file type from the data in the
file, it will attempt to open the resource fork and if that fails, it then tries
to open a file named **"._foo"** to see if the file has a valid resource fork.
This is the same regardless of whether the file is being opened for read or
write.

In short, libsndfile should open SD2 files with a valid resource fork on all of
the platforms that libsndfile supports. If a file has lost its resource fork,
the only option is the open the file using the SF_FORMAT_RAW option and guessing
its sample rate, channel count and bit width.

Occasionally, when SD2 files are moved to other systems, the file is
[BinHexed](http://www.macdisk.com/binhexen.php3) which wraps the resource fork
and the data fork together. For these files, it would be possible to write a
BinHex parser but there is not a lot to gain considering how rare these BinHexed
SD2 files are.

## Q14 : I'd like to statically link libsndfile to my closed source application. Can I buy a license so that this is possible? {#Q014}

Unfortunately no. libsndfile contains code written by other people who have
agreed that their code be used under the GNU LGPL but no more. Even if they were
to agree, there would be significant difficulties in dividing up the payments
fairly.

The **only** way you can legally use libsndfile as a statically linked library
is if your application is released under the GNU GPL or LGPL.

## Q15 : My program is crashing during a call to a function in libsndfile. Is this a bug in libsndfile? {#Q015}

libsndfile is being used by large numbers of people all over the world without
any problems like this. That means that it is much more likely that your code
has a bug than libsndfile. However, it is still possible that there is a bug in
libsndfile.

To figure out whether it is your code or libsndfile you should do the following:

- Make sure you are compiling your code with warnings switched on and that you
  fix as many warnings as possible. With the GNU compiler (gcc) I would
  recommend at least **-W -Wall -Werror** which will force you to fix all
  warnings before you can run the code.
- Try using a memory debugger. [Valgrind](http://valgrind.kde.org/) on x86 Linux
  is excellent. [Purify](http://www.ibm.com/software/awdtools/purify/) also has
  a good reputation.
- If the code is clean after the above two steps and you still get a crash in
  libsndfile, then send me a small snippet of code (no more than 30-40 lines)
  which includes the call to sf_open() and also shows how all variables passed
  to/returned from sf_open() are defined.

## Q16 : Will you accept a fix for compiling libsndfile with compiler X? {#Q016}

If compiler X is a C++ compiler then no. C and C++ are different enough to make
writing code that compiles as valid C and valid C++ too difficult. I would
rather spend my time fixing bugs and adding features.

If compiler X is a C compiler then I will do what I can as long as that does not
hamper the correctness, portability and maintainability of the existing code. It
should be noted however that libsndfile uses features specified by the 1999 ISO
C Standard. This can make compiling libsndfile with some older compilers
difficult.

## Q17 : Can libsndfile read/write files from/to UNIX pipes? {#Q017}

Yes, libsndfile can read files from pipes. Unfortunately, the write case is much
more complicated.

File formats like AIFF and WAV have information at the start of the file (the
file header) which states the length of the file, the number of sample frames
etc. This information must be filled in correctly when the file header is
written, but this information is not reliably known until the file is closed.
This means that libsndfile cannot write AIFF, WAV and many other file types to a
pipe.

However, there is at least one file format (AU) which is specifically designed
to be written to a pipe. Like AIFF and WAV, AU has a header with a sample frames
field, but it is specifically allowable to set that frames field to 0x7FFFFFFF
if the file length is not known when the header is written. The AU file format
can also hold data in many of the standard formats (ie SF_FORMAT_PCM_16,
SF_FORMAT_PCM_24, SF_FORMAT_FLOAT etc) as well as allowing data in both big and
little endian format.

See also [FAQ Q6](#Q006).

## Q18 : Is it possible to build a Universal Binary on Mac OS X? {#Q018}

Yes, but you must do two separate configure/build/test runs; one on PowerPC and
one on Intel. It is then possible to merge the binaries into a single universal
binary using one of the programs in the Apple tool chain.

It is **not** possible to build a working universal binary via a single
compile/build run on a single CPU.

The problem is that the libsndfile build process detects features of the CPU its
being built for during the configure process and when building a universal
binary, configure is only run once and that data is then used for both CPUs.
That configure data will be wrong for one of those CPUs. You will still be able
to compile libsndfile, and the test suite will pass on the machine you compiled
it on. However, if you take the universal binary test suite programs compiled on
one CPU and run them on the other, the test suite will fail.

Part of the problem is that the CPU endian-ness is detected at configure time.
Yes, I know the Apple compiler defines one of the macros \_\_LITTLE\_ENDIAN\_\_
and \_\_BIG\_ENDIAN\_\_, but those macros are not part of the 1999 ISO C
Standard and they are not portable.

Endian issues are not the only reason why the cross compiled binary will fail.
The configure script also detects other CPU specific idiosyncrasies to provide
more optimized code.

Finally, the real show stopper problem with universal binaries is the problem
with the test suite. libsndfile contains a huge, comprehensive test suite. When
you compile a universal binary and run the test suite, you only test the native
compile. The cross compiled binary (the one with the much higher chance of
having problems) cannot be tested.

Now, if you have read this far you're probably thinking there must be a way to
fix this and there probably is. The problem is that its a hell of a lot of work
and would require significant changes to the configure process, the internal
code and the test suite. In addition, these changes must not break compilation
on any of the platforms libsndfile is currently working on.

## Q19 : I have project files for Visual Studio / XCode / Whatever. Why don't you distribute them with libsndfile? {#Q019}

Use CMake project.

## Q20 : Why doesn't libsndfile support MP3? {#Q020}

In the past, MP3 was not supported because the technology behind MP3 was
patented. Those patents have now expired and there is an
[open ticket](https://github.com/libsndfile/libsndfile/issues/258) to implement
MP3 support.

## Q21 : How do I use libsndfile in a closed source or commercial program and comply with the license? {#Q021}

Here is a checklist of things you need to do to make sure your use of libsndfile
in a closed source or commercial project complies with the license libsndfile is
released under, the GNU Lesser General Public License (LGPL):

- Make sure you are linking to libsndfile as a shared library (Linux and Unix
  systems), Dynamic Link Library (Microsoft Windows) or dynlib (Mac OS X). If
  you are using some other operating system that doesn't allow dynamically
  linked libraries, you will not be able to use libsndfile unless you release
  the source code to your program.
- In the licensing documentation for your program, add a statement that your
  software depends on libsndfile and that libsndfile is released under the GNU
  Lesser General Public License, either
  [version 2.1](http://www.gnu.org/licenses/lgpl-2.1.txt) or optionally
  [version 3](http://www.gnu.org/licenses/lgpl.txt).
- Include the text for both versions of the license, possibly as separate files
  named libsndfile_lgpl_v2_1.txt and libsndfile_lgpl_v3.txt.

## Q22 : What versions of Windows does libsndfile work on? {#Q022}

New versions of libsndfile binary releases require Wiindows Vista. If you need
Windows XP support, you can build DLL from sources, we don't use specific WinXP
features.

## Q23 : I'm cross compiling libsndfile for another platform. How can I run the test suite? {#Q023}

Since version 1.0.21 the top level Makefile has an extra make target,
'test-tarball'. Building this target creates a tarball called called:

    ` libsndfile-testsuite-${host_triplet}-${version}.tar.gz`

in the top level directory. This tarball can then be copied to the target
platform. Once untarred and test script `test_wrapper.sh` can be run from the
top level of the extracted tarball.