mmioDescend function

Articolo
06/06/2016

The mmioDescend function descends into a chunk of a RIFF file that was opened by using the mmioOpen function. It can also search for a given chunk.

Syntax

MMRESULT mmioDescend(
   HMMIO      hmmio,
   LPMMCKINFO lpck,
   LPMMCKINFO lpckParent,
   UINT       wFlags
);

Parameters

hmmio
File handle of an open RIFF file.
lpck
Pointer to a buffer that receives an MMCKINFO structure.
lpckParent
Pointer to an optional application-defined MMCKINFO structure identifying the parent of the chunk being searched for. If this parameter is not NULL, mmioDescend assumes the MMCKINFO structure it refers to was filled when mmioDescend was called to descend into the parent chunk, and mmioDescend searches for a chunk within the parent chunk. Set this parameter to NULL if no parent chunk is being specified.

wFlags
Search flags. If no flags are specified, mmioDescend descends into the chunk beginning at the current file position. The following values are defined.

Value	Meaning
MMIO_FINDCHUNK	Searches for a chunk with the specified chunk identifier.
MMIO_FINDLIST	Searches for a chunk with the chunk identifier "LIST" and with the specified form type.
MMIO_FINDRIFF	Searches for a chunk with the chunk identifier "RIFF" and with the specified form type.

Return value

Returns MMSYSERR_NOERROR if successful or an error otherwise. Possible error values include the following.

Return code	Description
MMIOERR_CHUNKNOTFOUND	The end of the file (or the end of the parent chunk, if given) was reached before the desired chunk was found.

Remarks

A "RIFF" chunk consists of a four-byte chunk identifier (type FOURCC), followed by a four-byte chunk size (type DWORD), followed by the data portion of the chunk, followed by a null pad byte if the size of the data portion is odd. If the chunk identifier is "RIFF" or "LIST", the first four bytes of the data portion of the chunk are a form type or list type (type FOURCC).

If you use mmioDescend to search for a chunk, make sure the file position is at the beginning of a chunk before calling the function. The search begins at the current file position and continues to the end of the file. If a parent chunk is specified, the file position should be somewhere within the parent chunk before calling mmioDescend. In this case, the search begins at the current file position and continues to the end of the parent chunk.

If mmioDescend is unsuccessful in searching for a chunk, the current file position is undefined. If mmioDescend is successful, the current file position is changed. If the chunk is a "RIFF" or "LIST" chunk, the new file position will be just after the form type or list type (12 bytes from the beginning of the chunk). For other chunks, the new file position will be the start of the data portion of the chunk (8 bytes from the beginning of the chunk).

The mmioDescend function fills the MMCKINFO structure pointed to by the lpck parameter with the following information:

The ckid member is the chunk. If the MMIO_FINDCHUNK, MMIO_FINDRIFF, or MMIO_FINDLIST flag is specified for wFlags, the MMCKINFO structure is also used to pass parameters to mmioDescend. In this case, the ckid member specifies the four-character code of the chunk identifier, form type, or list type to search for.
The cksize member is the size, in bytes, of the data portion of the chunk. The size includes the form type or list type (if any), but does not include the 8-byte chunk header or the pad byte at the end of the data (if any).
The fccType member is the form type if ckid is "RIFF", or the list type if ckid is "LIST". Otherwise, it is NULL.
The dwDataOffset member is the file offset of the beginning of the data portion of the chunk. If the chunk is a "RIFF" chunk or a "LIST" chunk, this member is the offset of the form type or list type.
The dwFlags member contains other information about the chunk. Currently, this information is not used and is set to zero.

Requirements

Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Header	Mmsystem.h (include Windows.h)
Library	Winmm.lib
DLL	Winmm.dll

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