ICaptureGraphBuilder2::AllocCapFile method (strmif.h)

Article
07/27/2023

[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The AllocCapFile method preallocates a capture file to a specified size. For best results, always capture to a defragmented, preallocated capture file that is larger than the size of the capture data.

Syntax

HRESULT AllocCapFile(
  [in] LPCOLESTR lpstr,
  [in] DWORDLONG dwlSize
);

Parameters

[in] lpstr

Pointer to a wide-character string that contains the name of the file to create or resize.

[in] dwlSize

Size of the file to allocate, in bytes.

Return value

If this method succeeds, it returns S_OK. Otherwise, it returns an HRESULT error code.

Remarks

This method fails if the file is read-only.

It is best to allocate as much space as possible—ideally, more than needed. However, this can result in a very large file that contains relatively little data. For example, a 1-gigabyte (GB) capture file might contain a few megabytes of captured video. Use the ICaptureGraphBuilder2::CopyCaptureFile method to copy the data into a new file. That method copies only the data and ignores the empty portion of the original file.

If you use this method to preallocate the file, call IFileSinkFilter2::SetMode on the file-writer filter with the value zero. If the filter is set to AM_FILE_OVERWRITE, it will delete the preallocated file. Note that some file-writer filters do not support mode 0.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	strmif.h (include Dshow.h)
Library	Strmiids.lib