SoX(3)

NAME
libsox - SoX, an audio file-format and effect library

SYNOPSIS
#include <sox.h>

int sox_format_init(void);

ft_t sox_open_read(const char *path, const sox_signalinfo_t *info, const char *filetype);

ft_t sox_open_write(sox_bool (*overwrite_permitted)(const char *filename), const char *path, const sox_signalinfo_t *info, const char *filetype, const char *comment, sox_size_t length, const sox_instrinfo_t *instr, const sox_loopinfo_t *loops);

sox_size_t sox_read(ft_t ft, sox_ssample_t *buf, sox_size_t len);

sox_size_t sox_write(ft_t ft, sox_ssample_t *buf, sox_size_t len);

int sox_close(ft_t ft);

int sox_seek(ft_t ft, sox_size_t offset, int whence);

sox_effect_handler_t const *sox_find_effect(char const *name);

void sox_create_effect(sox_effect_t effp, sox_effect_handler_t const *e);

int sox_get_effect(sox_effect_t effp, const char *name);

int sox_update_effect(eff_t effp, const sox_signalinfo_t *in, const sox_signalinfo_t *out, int effect_mask);

cc file.c -o file lsox

DESCRIPTION
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.

sox_format_init 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.

sox_open_input function opens the file for reading whose name is the string pointed to by path and associates an ft_t with it. If info 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 filetype 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.

sox_open_output function opens the file for writing whose name is the string pointed to by path and associates an ft_t with it. If info 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 comment is non-NULL, it will be written in the file header for formats that support comments. If filetype 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.

The function sox_read reads len samples in to buf using the format handler specified by ft. All data read is converted to 32-bit signed samples before being placed in to buf. The value of len is specified in total samples. If its value is not evenly divisable by the number of channels, undefined behavior will occur.

The function sox_write writes len samples from buf using the format handler specified by ft. Data in buf must be 32-bit signed samples and will be converted during the write process. The value of len is specified in total samples. If its value is not evenly divisable by the number of channels, undefined behavior will occur.

The sox_close function dissociates the named ft_t from its underlying file or set of functions. If the format handler was being used for output, any buffered data is written first.

sox_format_quite function performs some required cleanup related to all file format handlers.

The function sox_find_effect finds effect name, returning a pointer to its sox_effect_handler_t if it exists, and NULL otherwise.

The function sox_create_effect instantiates an effect into a sox_effect_t given a sox_effect_handler_t *. Any missing methods are automatically set to the corresponding nothing method.

The sox_update_effect function copies input and output signal info into effect structures. The effect_mask parameter is the return value of the previous call to this function; for the first call, pass 0. The function returns the updated effect mask.

SoX includes skeleton C files to assist you in writing new formats (skelform.c) and effects (skeleff.c). sox.c itself is a good starting point for new programs. Note that new formats can often just deal with the header and then use raw.c's routines for reading and writing.

RETURN VALUE
Upon successful completion sox_open_input and sox_open_output return an ft_t (which is a pointer). Otherwise, NULL is returned. TODO: Need a way to return reason for failures. Currently, relies on sox_warn to print information.

sox_read and sox_write 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: sox_read does not distiguish between end-of-file and error. Need an feof() and ferror() concept to determine which occured.

Upon successful completion sox_close returns 0. Otherwise, SOX_EOF is returned. In either case, any further access (including another call to sox_close()) to the handler results in undefined behavior. TODO: Need a way to return reason for failures. Currently, relies on sox_warn to print information.

Upon successful completion sox_seek returns 0. Otherwise, SOX_EOF is returned. TODO Need to set a global error and implement sox_tell.

ERRORS
TODO

INTERNALS
SoX's formats and effects operate on an internal buffer format of signed 32-bit longs. 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.

This does cause problems in processing the data. For example:
    *obuf++ = (*ibuf++ + *ibuf++)/2;
would not mix down left and right channels into one monophonic channel, because the resulting samples would overflow 32 bits. Instead, the ``avg'' effects must use:
    *obuf++ = *ibuf++/2 + *ibuf++/2;

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.

FORMATS
A 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 format operates from two data structures: a format structure, and a private structure.

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 format pointer, and the private structure for the format .

The private area is just a preallocated data array for the 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.

A format has 6 routines:

startread
    Set up the format parameters, or read in a data header, or do what needs to be done.

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.

stopread
    Do what needs to be done.

startwrite
    Set up the format parameters, or write out a data header, or do what needs to be done.

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.

stopwrite
    Fix up any file header, or do what needs to be done.

EFFECTS
An effects loop has one input and one output stream. It has 5 routines.

getopts
    is called with a character string argument list for the effect.

start
    is called with the signal parameters for the input and output streams.

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.

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.

stop
    is called when there are no more input samples to process. stop may generate output samples on its own. See echo.c for how to do this, and see that what it does is absolutely bogus.

BUGS
This manual page is both incomplete and out of date.

SEE ALSO
sox(1), soxexam(7)

LICENSE
Copyright 1991 Lance Norskog and Sundry Contributors. Copyright 1998-2007 by Chris Bagwell and SoX Contributors. 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. 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.

AUTHORS
Chris Bagwell (cbagwell@users.sourceforge.net). Other authors and contributors are listed in the AUTHORS file that is distributed with the source code.