FLAC: FLAC/callback.h: I/O callback structures

FLAC 1.5.0 Free Lossless Audio Codec

Classes | Typedefs | Variables
FLAC/callback.h: I/O callback structures
FLAC C API

## Classes
struct | FLAC__IOCallbacks

## Typedefs
typedef void * | FLAC__IOHandle
typedef size_t(* | FLAC__IOCallback_Read) (void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)
typedef size_t(* | FLAC__IOCallback_Write) (const void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)
typedef int(* | FLAC__IOCallback_Seek) (FLAC__IOHandle handle, FLAC__int64 offset, int whence)
typedef FLAC__int64(* | FLAC__IOCallback_Tell) (FLAC__IOHandle handle)
typedef int(* | FLAC__IOCallback_Eof) (FLAC__IOHandle handle)
typedef int(* | FLAC__IOCallback_Close) (FLAC__IOHandle handle)

## Variables
FLAC__IOCallback_Read | FLAC__IOCallbacks::read
FLAC__IOCallback_Write | FLAC__IOCallbacks::write
FLAC__IOCallback_Seek | FLAC__IOCallbacks::seek
FLAC__IOCallback_Tell | FLAC__IOCallbacks::tell
FLAC__IOCallback_Eof | FLAC__IOCallbacks::eof
FLAC__IOCallback_Close | FLAC__IOCallbacks::close

## Detailed Description

This module defines the structures for describing I/O callbacks to the other FLAC interfaces.

The purpose of the I/O callback functions is to create a common way for the metadata interfaces to handle I/O.

Originally the metadata interfaces required filenames as the way of specifying FLAC files to operate on. This is problematic in some environments so there is an additional option to specify a set of callbacks for doing I/O on the FLAC file, instead of the filename.

In addition to the callbacks, a FLAC__IOHandle type is defined as an opaque structure for a data source.

The callback function prototypes are similar (but not identical) to the stdio functions fread, fwrite, fseek, ftell, feof, and fclose. If you use stdio streams to implement the callbacks, you can pass fread, fwrite, and fclose anywhere a FLAC__IOCallback_Read, FLAC__IOCallback_Write, or FLAC__IOCallback_Close is required, and a FILE* anywhere a FLAC__IOHandle is required.

Warning
  You generally CANNOT directly use fseek or ftell for FLAC__IOCallback_Seek or FLAC__IOCallback_Tell since on most systems these use 32-bit offsets and FLAC requires 64-bit offsets to deal with large files. You will have to find an equivalent function (e.g. ftello), or write a wrapper. The same is true for feof() since this is usually implemented as a macro, not as a function whose address can be taken.

## Typedef Documentation

## ◆ FLAC__IOHandle

typedef void* FLAC__IOHandle

This is the opaque handle type used by the callbacks. Typically this is a `FILE*` or address of a file descriptor.

## ◆ FLAC__IOCallback_Read

typedef size_t(* FLAC__IOCallback_Read) (void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)

Signature for the read callback. The signature and semantics match POSIX fread() implementations and can generally be used interchangeably. Note that the global variable errno from errno.h is read by some libFLAC functions to detect read errors.

Parameters

  ptr | The address of the read buffer.
  size | The size of the records to be read.
  nmemb | The number of records to be read.
  handle | The handle to the data source.

Return values

  size_t | The number of records read.

## ◆ FLAC__IOCallback_Write

typedef size_t(* FLAC__IOCallback_Write) (const void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)

Signature for the write callback. The signature and semantics match POSIX fwrite() implementations and can generally be used interchangeably.

Parameters

  ptr | The address of the write buffer.
  size | The size of the records to be written.
  nmemb | The number of records to be written.
  handle | The handle to the data source.

Return values

  size_t | The number of records written.

## ◆ FLAC__IOCallback_Seek

typedef int(* FLAC__IOCallback_Seek) (FLAC__IOHandle handle, FLAC__int64 offset, int whence)

Signature for the seek callback. The signature and semantics mostly match POSIX fseek() WITH ONE IMPORTANT EXCEPTION: the offset is a 64-bit type whereas fseek() is generally 'long' and 32-bits wide.

Parameters

  handle | The handle to the data source.
  offset | The new position, relative to whence
  whence | `SEEK_SET`, `SEEK_CUR`, or `SEEK_END`

Return values

  int | `0` on success, `-1` on error.

## ◆ FLAC__IOCallback_Tell

typedef FLAC__int64(* FLAC__IOCallback_Tell) (FLAC__IOHandle handle)

Signature for the tell callback. The signature and semantics mostly match POSIX ftell() WITH ONE IMPORTANT EXCEPTION: the offset is a 64-bit type whereas ftell() is generally 'long' and 32-bits wide.

Parameters

  handle | The handle to the data source.

Return values

  FLAC__int64 | The current position on success, `-1` on error.

## ◆ FLAC__IOCallback_Eof

typedef int(* FLAC__IOCallback_Eof) (FLAC__IOHandle handle)

Signature for the EOF callback. The signature and semantics mostly match POSIX feof() but WATCHOUT: on many systems, feof() is a macro, so in this case a wrapper function must be provided instead.

Parameters

  handle | The handle to the data source.

Return values

  int | `0` if not at end of file, nonzero if at end of file.

## ◆ FLAC__IOCallback_Close

typedef int(* FLAC__IOCallback_Close) (FLAC__IOHandle handle)

Signature for the close callback. The signature and semantics match POSIX fclose() implementations and can generally be used interchangeably.

Parameters

  handle | The handle to the data source.

Return values

  int | `0` on success, `EOF` on error.

## Variable Documentation

## ◆ read

FLAC__IOCallback_Read FLAC__IOCallbacks::read

See FLAC__IOCallbacks

## ◆ write

FLAC__IOCallback_Write FLAC__IOCallbacks::write

See FLAC__IOCallbacks

## ◆ seek

FLAC__IOCallback_Seek FLAC__IOCallbacks::seek

See FLAC__IOCallbacks

## ◆ tell

FLAC__IOCallback_Tell FLAC__IOCallbacks::tell

See FLAC__IOCallbacks

## ◆ eof

FLAC__IOCallback_Eof FLAC__IOCallbacks::eof

See FLAC__IOCallbacks

## ◆ close

FLAC__IOCallback_Close FLAC__IOCallbacks::close

See FLAC__IOCallbacks

Copyright (c) 2000-2009 Josh Coalson Copyright (c) 2011-2025 Xiph.Org Foundation