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API documentation for libmpg123, libout123, and libsyn123

Note: This API doc is automatically generated from the current development version that you can get via Subversion or as a daily snapshot from http://mpg123.org/snapshot. There may be differences (additions) compared to the latest stable release. See NEWS.libmpg123, NEWS.libout123, NEWS.libsyn123, and the overall NEWS file on libmpg123 versions and important changes between them.
Let me emphasize that the policy for the lib*123 family is to always stay backwards compatible -- only additions are planned (and it's not yet planned to change the plans;-).
mpg123 position and seeking

Functions

MPG123_EXPORT off_t mpg123_tell (mpg123_handle *mh)
 
MPG123_EXPORT off_t mpg123_tellframe (mpg123_handle *mh)
 
MPG123_EXPORT off_t mpg123_tell_stream (mpg123_handle *mh)
 
MPG123_EXPORT off_t mpg123_seek (mpg123_handle *mh, off_t sampleoff, int whence)
 
MPG123_EXPORT off_t mpg123_feedseek (mpg123_handle *mh, off_t sampleoff, int whence, off_t *input_offset)
 
MPG123_EXPORT off_t mpg123_seek_frame (mpg123_handle *mh, off_t frameoff, int whence)
 
MPG123_EXPORT off_t mpg123_timeframe (mpg123_handle *mh, double sec)
 
MPG123_EXPORT int mpg123_index (mpg123_handle *mh, off_t **offsets, off_t *step, size_t *fill)
 
MPG123_EXPORT int mpg123_set_index (mpg123_handle *mh, off_t *offsets, off_t step, size_t fill)
 
MPG123_EXPORT int mpg123_position (mpg123_handle *mh, off_t frame_offset, off_t buffered_bytes, off_t *current_frame, off_t *frames_left, double *current_seconds, double *seconds_left)
 

Detailed Description

Functions querying and manipulating position in the decoded audio bitstream. The position is measured in decoded audio samples or MPEG frame offset for the specific functions. The term sample refers to a group of samples for multiple channels, normally dubbed PCM frames. The latter term is avoided here because frame means something different in the context of MPEG audio. Since all samples of a PCM frame occur at the same time, there is only very limited ambiguity when talking about playback offset, as counting each channel sample individually does not make sense.

If gapless code is in effect, the positions are adjusted to compensate the skipped padding/delay - meaning, you should not care about that at all and just use the position defined for the samples you get out of the decoder;-) The general usage is modelled after stdlib's ftell() and fseek(). Especially, the whence parameter for the seek functions has the same meaning as the one for fseek() and needs the same constants from stdlib.h:

Note that sample-accurate seek only works when gapless support has been enabled at compile time; seek is frame-accurate otherwise. Also, really sample-accurate seeking (meaning that you get the identical sample value after seeking compared to plain decoding up to the position) is only guaranteed when you do not mess with the position code by using MPG123_UPSPEED, MPG123_DOWNSPEED or MPG123_START_FRAME. The first two mainly should cause trouble with NtoM resampling, but in any case with these options in effect, you have to keep in mind that the sample offset is not the same as counting the samples you get from decoding since mpg123 counts the skipped samples, too (or the samples played twice only once)!

Short: When you care about the sample position, don't mess with those parameters;-)

Streams may be openend in ways that do not support seeking. Also, consider the effect of MPG123_FUZZY.

Function Documentation

◆ mpg123_tell()

MPG123_EXPORT off_t mpg123_tell ( mpg123_handle mh)

Returns the current position in samples. On the next successful read, you'd get audio data with that offset.

Parameters
mhhandle
Returns
sample (PCM frame) offset or MPG123_ERR (null handle)

◆ mpg123_tellframe()

MPG123_EXPORT off_t mpg123_tellframe ( mpg123_handle mh)

Returns the frame number that the next read will give you data from.

Parameters
mhhandle
Returns
frame offset or MPG123_ERR (null handle)

◆ mpg123_tell_stream()

MPG123_EXPORT off_t mpg123_tell_stream ( mpg123_handle mh)

Returns the current byte offset in the input stream.

Parameters
mhhandle
Returns
byte offset or MPG123_ERR (null handle)

◆ mpg123_seek()

MPG123_EXPORT off_t mpg123_seek ( mpg123_handle mh,
off_t  sampleoff,
int  whence 
)

Seek to a desired sample offset. Usage is modelled afer the standard lseek().

Parameters
mhhandle
sampleoffoffset in samples (PCM frames)
whenceone of SEEK_SET, SEEK_CUR or SEEK_END
Returns
The resulting offset >= 0 or error/message code

◆ mpg123_feedseek()

MPG123_EXPORT off_t mpg123_feedseek ( mpg123_handle mh,
off_t  sampleoff,
int  whence,
off_t *  input_offset 
)

Seek to a desired sample offset in data feeding mode. This just prepares things to be right only if you ensure that the next chunk of input data will be from input_offset byte position.

Parameters
mhhandle
sampleoffoffset in samples (PCM frames)
whenceone of SEEK_SET, SEEK_CUR or SEEK_END
input_offsetThe position it expects to be at the next time data is fed to mpg123_decode().
Returns
The resulting offset >= 0 or error/message code

◆ mpg123_seek_frame()

MPG123_EXPORT off_t mpg123_seek_frame ( mpg123_handle mh,
off_t  frameoff,
int  whence 
)

Seek to a desired MPEG frame offset. Usage is modelled afer the standard lseek().

Parameters
mhhandle
frameoffoffset in MPEG frames
whenceone of SEEK_SET, SEEK_CUR or SEEK_END
Returns
The resulting offset >= 0 or error/message code

◆ mpg123_timeframe()

MPG123_EXPORT off_t mpg123_timeframe ( mpg123_handle mh,
double  sec 
)

Return a MPEG frame offset corresponding to an offset in seconds. This assumes that the samples per frame do not change in the file/stream, which is a good assumption for any sane file/stream only.

Returns
frame offset >= 0 or error/message code

◆ mpg123_index()

MPG123_EXPORT int mpg123_index ( mpg123_handle mh,
off_t **  offsets,
off_t *  step,
size_t *  fill 
)

Give access to the frame index table that is managed for seeking. You are asked not to modify the values... Use mpg123_set_index to set the seek index

Parameters
mhhandle
offsetspointer to the index array
stepone index byte offset advances this many MPEG frames
fillnumber of recorded index offsets; size of the array
Returns
MPG123_OK on success

◆ mpg123_set_index()

MPG123_EXPORT int mpg123_set_index ( mpg123_handle mh,
off_t *  offsets,
off_t  step,
size_t  fill 
)

Set the frame index table Setting offsets to NULL and fill > 0 will allocate fill entries. Setting offsets to NULL and fill to 0 will clear the index and free the allocated memory used by the index.

Parameters
mhhandle
offsetspointer to the index array
stepone index byte offset advances this many MPEG frames
fillnumber of recorded index offsets; size of the array
Returns
MPG123_OK on success

◆ mpg123_position()

MPG123_EXPORT int mpg123_position ( mpg123_handle mh,
off_t  frame_offset,
off_t  buffered_bytes,
off_t *  current_frame,
off_t *  frames_left,
double *  current_seconds,
double *  seconds_left 
)

An old crutch to keep old mpg123 binaries happy. WARNING: This function is there only to avoid runtime linking errors with standalone mpg123 before version 1.23.0 (if you strangely update the library but not the end-user program) and actually is broken for various cases (p.ex. 24 bit output). Do never use. It might eventually be purged from the library.

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