wav: drop redundant zero initialisations

[sox.git] / libsox.3

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.TH SoX 3 "February 19, 2011" "libsox" "Sound eXchange"
.SH NAME
libsox \- SoX, an audio file-format and effect library
.SH SYNOPSIS
.nf
.B #include <sox.h>
.P
.B int sox_format_init(void);
.P
.B void sox_format_quit(void);
.P
.B sox_format_t sox_open_read(const char *\fIpath\fB, const sox_signalinfo_t *\fIinfo\fB, const char *\fIfiletype\fB);
.P
.B sox_format_t sox_open_write(sox_bool (*\fIoverwrite_permitted\fB)(const char *\fIfilename\fB), const char *\fIpath\fB, const sox_signalinfo_t *\fIinfo\fB, const char *\fIfiletype\fB, const char *\fIcomment\fB, sox_size_t \fIlength\fB, const sox_instrinfo_t *\fIinstr\fB, const sox_loopinfo_t *\fIloops\fB);
.P
.B sox_size_t sox_read(sox_format_t \fIft\fB, sox_ssample_t *\fIbuf\fB, sox_size_t \fIlen\fB);
.P
.B sox_size_t sox_write(sox_format_t \fIft\fB, sox_ssample_t *\fIbuf\fB, sox_size_t \fIlen\fB);
.P
.B int sox_close(sox_format_t \fIft\fB);
.P
.B int sox_seek(sox_format_t \fIft\fB, sox_size_t \fIoffset\fB, int \fIwhence\fB);
.P
.B sox_effect_handler_t const *sox_find_effect(char const *\fIname\fB);
.P
.B sox_effect_t *sox_create_effect(sox_effect_handler_t const *\fIeh\fB);
.P
.B int sox_effect_options(sox_effect_t *\fIeffp\fB, int \fIargc\fB, char * const \fIargv[]\fB);
.P
.B sox_effects_chain_t *sox_create_effects_chain(sox_encodinginfo_t const *\fIin_enc\fB, sox_encodinginfo_t const *\fIout_enc\fB);
.P
.B void sox_delete_effects_chain(sox_effects_chain_t *\fIecp\fB);
.P
.B int sox_add_effect(sox_effects_chaint_t *\fIchain\fB, sox_effect_t*\fIeffp\fB, sox_signalinfo_t *\fIin\fB, sox_signalinfo_t const *\fIout\fB);
.P
.B cc \fIfile.c\fB -o \fIfile \fB-lsox
.fi
.SH DESCRIPTION
.I libsox
is a library of sound sample file format readers/writers and sound
effects processors. It is mainly developed for use by SoX but is
useful for any sound application.
.P
\fBsox_format_init\fR function performs some required initialization
related to all file format handlers.  If compiled with dynamic
library support then this will detect and initialize all external
libraries.  This should be called before any other file operations
are performed.
.P
\fBsox_format_quit\fR function performs some required cleanup
related to all file format handlers.
.P
\fBsox_open_read\fR function opens the file for reading whose name is
the string pointed to by \fIpath\fR and associates an sox_format_t with it. If
\fIinfo\fR is non-NULL then it will be used to specify the data format
of the input file. This is normally only needed for headerless audio
files since the information is not stored in the file. If
\fIfiletype\fR is non-NULL then it will be used to specify the file
type. If this is not specified then the file type is attempted to be
derived by looking at the file header and/or the filename extension. A
special name of "-" can be used to read data from stdin.
.P
\fBsox_open_write\fR function opens the file for writing whose name is
the string pointed to by \fIpath\fR and associates an sox_format_t with it. If
\fIinfo\fR is non-NULL then it will be used to specify the data format
of the output file. Since most file formats can write data in
different data formats, this generally has to be specified. The info
structure from the input format handler can be specified to copy data
over in the same format. If \fIcomment\fR is non-NULL, it will be
written in the file header for formats that support comments. If
\fIfiletype\fR is non-NULL then it will be used to specify the file
type. If this is not specified then the file type is attempted to be
derived by looking at the filename extension. A special name of "-"
can be used to write data to stdout.
.P
The function \fBsox_read\fR reads \fIlen\fR samples in to \fIbuf\fR
using the format handler specified by \fIft\fR. All data read is
converted to 32-bit signed samples before being placed in to
\fIbuf\fR. The value of \fIlen\fR is specified in total samples. If
its value is not evenly divisable by the number of channels, undefined
behavior will occur.
.P
The function \fBsox_write\fR writes \fIlen\fR samples from \fIbuf\fR
using the format handler specified by \fIft\fR. Data in \fIbuf\fR must
be 32-bit signed samples and will be converted during the write
process. The value of \fIlen\fR is specified in total samples. If its
value is not evenly divisable by the number of channels, undefined
behavior will occur.
.P
The \fBsox_close\fR function dissociates the named \fIsox_format_t\fR from its
underlying file or set of functions. If the format handler was being
used for output, any buffered data is written first.
.P
The function \fBsox_find_effect\fR finds effect \fIname\fR, returning
a pointer to its \fIsox_effect_handler_t\fR if it exists, and NULL
otherwise.
.P
The function \fBsox_create_effect\fR instantiates an effect into a
\fIsox_effect_t\fR given a \fIsox_effect_handler_t *\fR. Any missing
methods are automatically set to the corresponding \fBnothing\fR
method.
.P
The function \fBsox_effect_options\fR allows passing options into the effect to control its behavior.  It will return SOX_EOF if there were any invalid options passed in.  On success, the \fIeffp->in_signal\fR will optional contain the rate and channel count it requires input data from and \fIeffp->out_signal\fR will optionally contain the rate and channel count it outputs in.  When present, this information should be used to make sure appropriate effects are placed in the effects chain to handle any needed conversions.
.P
Passing in options is currently only supported when they are passed in before the effect is ever started.  The behavior is undefined if its called once the effect is started.
.P
\fBsox_create_effects_chain\fR will instantiate an effects chain that
effects can be added to.  \fIin_enc\fR and \fIout_enc\fR are the 
signal encoding of the input and output of the chain respectively.
The pointers to \fIin_enc\fR and \fIout_enc\fR
are stored internally and so their memory should not be freed.  Also,
it is OK if their values change over time to reflect new input or
output encodings as they are referenced only as effects
start up or are restarted.
.P
\fBsox_delete_effects_chain\fR will release any resources reserved during
the creation of the chain.  This will also call \fBsox_delete_effects\fR
if any effects are still in the chain.
.P
\fBsox_add_effect\fR adds an effect to the chain.  \fIin\fR specifies the input
signal info for this effect.  \fIout\fR is a suggestion
as to what the output signal should be but depending on the effects
given options and on \fIin\fR the effect can choose to do differently.
Whatever output rate and channels the effect does produce are written
back to \fIin\fR.  It is meant that \fIin\fR be stored and passed to each
new call to \fBsox_add_effect\fR so that changes will be propagated to each new effect.
.P
SoX includes skeleton C files to assist you in writing new
formats (skelform.c) and effects (skeleff.c). Note that new formats 
can often just deal with the header and then use raw.c's routines 
for reading and writing.

example0.c and example1.c are a good starting point to see how
to write applications using libsox.  sox.c itself is also a good
reference.

.SH RETURN VALUE
Upon successful completion \fBsox_open_read\fR and
\fBsox_open_write\fR return an \fIsox_format_t\fR (which is a pointer).
Otherwise, NULL is returned. TODO: Need a way to return reason for
failures. Currently, relies on \fBsox_warn\fR to print information.
.P
\fBsox_read\fR and \fBsox_write\fR return the number of samples
successfully read or written. If an error occurs, or the end-of-file
is reached, the return value is a short item count or SOX_EOF. TODO:
\fBsox_read\fR does not distiguish between end-of-file and error. Need
an feof() and ferror() concept to determine which occurred.
.P
Upon successful completion \fBsox_close\fR returns 0. Otherwise, SOX_EOF
is returned. In either case, any further access (including another
call to \fBsox_close\fR()) to the handler results in undefined
behavior. TODO: Need a way to return reason for failures. Currently,
relies on sox_warn to print information.
.P
Upon successful completion \fBsox_seek\fR returns 0. Otherwise, SOX_EOF
is returned. TODO Need to set a global error and implement sox_tell.
.SH ERRORS
TODO
.SH INTERNALS
SoX's formats and effects operate with an internal sample format of
signed 32-bit integer.  The data processing routines are called with
buffers of these samples, and buffer sizes which refer to the number
of samples processed, not the number of bytes.  File readers translate
the input samples to signed 32-bit integers and return the number of
samples read.  For example, data in linear signed byte format is
left-shifted 24 bits.
.P
Representing samples as integers can cause problems when processing the audio.  
For example, if an effect to
mix down left and right channels into one monophonic channel
were to use the line
.EX
   *obuf++ = (*ibuf++ + *ibuf++)/2;
.EE
distortion might occur since
the intermediate addition can overflow 32 bits.
The line
.EX
   *obuf++ = *ibuf++/2 + *ibuf++/2;
.EE
would get round the overflow problem (at the expense of the least significant
bit).
.P
Stereo data is stored with the left and right speaker data in
successive samples.
Quadraphonic data is stored in this order: 
left front, right front, left rear, right rear.
.SH FORMATS
A 
.I format 
is responsible for translating between sound sample files
and an internal buffer.  The internal buffer is store in signed longs
with a fixed sampling rate.  The 
.I format
operates from two data structures:
a format structure, and a private structure.
.P
The format structure contains a list of control parameters for
the sample: sampling rate, data size (8, 16, or 32 bits),
encoding (unsigned, signed, floating point, etc.), number of sound channels.
It also contains other state information: whether the sample file
needs to be byte-swapped, whether sox_seek() will work, its suffix,
its file stream pointer, its 
.I format
pointer, and the 
.I private
structure for the 
.I format .
.P
The 
.I private 
area is just a preallocated data array for the 
.I format
to use however it wishes.  
It should have a defined data structure
and cast the array to that structure.  
See voc.c for the use of a private data area.  
Voc.c has to track the number of samples it 
writes and when finishing, seek back to the beginning of the file
and write it out.
The private area is not very large.
The ``echo'' effect has to malloc() a much larger area for its
delay line buffers.
.P
A 
.I format
has 6 routines:
.TP 20
startread
Set up the format parameters, or read in
a data header, or do what needs to be done.
.TP 20
read
Given a buffer and a length: 
read up to that many samples, 
transform them into signed long integers,
and copy them into the buffer.
Return the number of samples actually read.
.TP 20
stopread
Do what needs to be done.
.TP 20
startwrite
Set up the format parameters, or write out 
a data header, or do what needs to be done.
.TP 20
write
Given a buffer and a length: 
copy that many samples out of the buffer,
convert them from signed longs to the appropriate
data, and write them to the file.
If it can't write out all the samples,
fail.
.TP 20
stopwrite
Fix up any file header, or do what needs to be done.
.SH EFFECTS
Each effect runs with one input and one output stream.
An effect's implementation comprises six functions that may be called
to the follow flow diagram:
.EX
LOOP (invocations with different parameters)
  getopts
  LOOP (invocations with the same parameters)
    LOOP (channels)
      start
    LOOP (whilst there is input audio to process)
      LOOP (channels)
        flow
    LOOP (whilst there is output audio to generate)
      LOOP (channels)
        drain
    LOOP (channels)
      stop
  kill
.EE
Notes: For some effects, some of the functions may not be needed and can
be NULL.  An effect that is marked `MCHAN' does not use the LOOP
(channels) lines and must therefore perform multiple channel processing
inside the affected functions.  Multiple effect instances may be
processed (according to the above flow diagram) in parallel.
.TP 20
getopts
is called with a character string argument list for the effect.
.TP 20
start
is called with the signal parameters for the input and output
streams.
.TP 20 
flow
is called with input and output data buffers,
and (by reference) the input and output data buffer sizes.
It processes the input buffer into the output buffer,
and sets the size variables to the numbers of samples
actually processed.
It is under no obligation to read from the input buffer or
write to the output buffer during the same call.  If the
call returns SOX_EOF then this should be used as an indication
that this effect will no longer read any data and can be used
to switch to drain mode sooner.
.TP 20 
drain
is called after there are no more input data samples.
If the effect wishes to generate more data samples
it copies the generated data into a given buffer
and returns the number of samples generated.
If it fills the buffer, it will be called again, etc.
The echo effect uses this to fade away.
.TP 20
stop
is called when there are no more input samples and no more output
samples to process.
It is typically used to release or close resources (e.g. allocated
memory or temporary files) that were set-up in
.IR start .
See echo.c for an example.
.TP 20
kill
is called to allow resources allocated by
.I getopts
to be released.
See pad.c for an example.
.SH LINKING
The method of linking against libsox depends on how SoX was
built on your system. For a static build, just link against the
libraries as normal. For a dynamic build, you should use libtool to
link with the correct linker flags. See the libtool manual for
details; basically, you use it as:
.EX
   libtool \-\-mode=link gcc \-o prog /path/to/libsox.la
.EE
.SH BUGS
This manual page is both incomplete and out of date.
.SH SEE ALSO
.BR sox (1),
.BR soxformat (7)
.SP
example*.c in the SoX source distribution.
.SH LICENSE
Copyright 1998\-2011 by Chris Bagwell and SoX Contributors.
.br
Copyright 1991 Lance Norskog and Sundry Contributors.
.SP
This library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1, or (at your option)
any later version.
.SP
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.
.SH AUTHORS
Chris Bagwell (cbagwell@users.sourceforge.net).
Other authors and contributors are listed in the ChangeLog file that
is distributed with the source code.