acmFormatEnumW function (msacm.h)

Article
02/09/2023

The acmFormatEnum function enumerates waveform-audio formats available for a given format tag from an ACM driver. This function continues enumerating until there are no more suitable formats for the format tag or the callback function returns FALSE.

Syntax

MMRESULT ACMAPI acmFormatEnumW(
  HACMDRIVER          had,
  LPACMFORMATDETAILSW pafd,
  ACMFORMATENUMCBW    fnCallback,
  DWORD_PTR           dwInstance,
  DWORD               fdwEnum
);

Parameters

had

Handle to the ACM driver to query for waveform-audio format details. If this parameter is NULL, the ACM uses the details from the first suitable ACM driver.

pafd

Pointer to an ACMFORMATDETAILS structure to contain the format details passed to the fnCallback function. This structure must have the cbStruct, pwfx, and cbwfx members of the ACMFORMATDETAILS structure initialized. The dwFormatTag member must also be initialized to either WAVE_FORMAT_UNKNOWN or a valid format tag.

The fdwSupport member of the structure must be initialized to zero.

To find the required size of the pwfx buffer, call acmMetrics with the ACM_METRIC_MAX_SIZE_FORMAT flag.

fnCallback

Address of an application-defined callback function. See acmFormatEnumCallback. This parameter cannot be NULL.

dwInstance

A 64-bit (DWORD_PTR) or 32-bit (DWORD) application-defined value that is passed to the callback function along with ACM format details.

fdwEnum

Flags for enumerating the formats for a given format tag. The following values are defined.

[ACMFORMATDETAILS](./nf-msacm-acmformatdetails.md) structure is valid. The enumerator will only enumerate destination formats that can be converted from the given pwfx format.If this flag is used, the wFormatTag member of the WAVEFORMATEX structure cannot be WAVE_FORMAT_UNKNOWN. [ACMFORMATDETAILS](./nf-msacm-acmformatdetails.md) structure is valid. The enumerator will enumerate only a format that conforms to this attribute. [ACMFORMATDETAILS](./nf-msacm-acmformatdetails.md) structure is valid. The enumerator will enumerate all suggested destination formats for the given pwfx format. This mechanism can be used instead of the acmFormatSuggest function to allow an application to choose the best suggested format for conversion. The dwFormatIndex member will always be set to zero on return.If this flag is used, the wFormatTag member of the WAVEFORMATEX structure cannot be WAVE_FORMAT_UNKNOWN.

Value	Meaning
ACM_FORMATENUMF_CONVERT
ACM_FORMATENUMF_HARDWARE	The enumerator should only enumerate formats that are supported as native input or output formats on one or more of the installed waveform-audio devices. This flag provides a way for an application to choose only formats native to an installed waveform-audio device. This flag must be used with one or both of the ACM_FORMATENUMF_INPUT and ACM_FORMATENUMF_OUTPUT flags. Specifying both ACM_FORMATENUMF_INPUT and ACM_FORMATENUMF_OUTPUT will enumerate only formats that can be opened for input or output. This is true regardless of whether this flag is specified.
ACM_FORMATENUMF_INPUT	Enumerator should enumerate only formats that are supported for input (recording).
ACM_FORMATENUMF_NCHANNELS
ACM_FORMATENUMF_NSAMPLESPERSEC	The nSamplesPerSec member of the WAVEFORMATEX structure pointed to by the pwfx member of the ACMFORMATDETAILS structure is valid. The enumerator will enumerate only a format that conforms to this attribute.
ACM_FORMATENUMF_OUTPUT	Enumerator should enumerate only formats that are supported for output (playback).
ACM_FORMATENUMF_SUGGEST
ACM_FORMATENUMF_WBITSPERSAMPLE	The wBitsPerSample member of the WAVEFORMATEX structure pointed to by the pwfx member of the ACMFORMATDETAILS structure is valid. The enumerator will enumerate only a format that conforms to this attribute.
ACM_FORMATENUMF_WFORMATTAG	The wFormatTag member of the WAVEFORMATEX structure pointed to by the pwfx member of the ACMFORMATDETAILS structure is valid. The enumerator will enumerate only a format that conforms to this attribute. The dwFormatTag member of the ACMFORMATDETAILS structure must be equal to the wFormatTag member.The value of wFormatTag cannot be WAVE_FORMAT_UNKNOWN in this case.

Return value

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

Return code	Description
ACMERR_NOTPOSSIBLE	The details for the format cannot be returned.
MMSYSERR_INVALFLAG	At least one flag is invalid.
MMSYSERR_INVALHANDLE	The specified handle is invalid.
MMSYSERR_INVALPARAM	At least one parameter is invalid.

Remarks

This function will return MMSYSERR_NOERROR (zero) if no suitable ACM drivers are installed. Moreover, the callback function will not be called.

Examples

The following example shows how to enumerate formats that have the WAVE_FORMAT_MPEGLAYER3 format tag.


MMRESULT EnumerateMP3Codecs()
{
    DWORD cbMaxSize = 0;
    MMRESULT result = MMSYSERR_NOERROR;
    ACMFORMATDETAILS acmFormatDetails;

    // Buffer to hold the format information.
    BYTE *pFormat = NULL;   // Caller allocated.

    // Find the largest format buffer needed.
    result = acmMetrics(NULL, ACM_METRIC_MAX_SIZE_FORMAT, &cbMaxSize);
    if (result != MMSYSERR_NOERROR)
    {
        return result;
    }

    // Allocate the format buffer.
    pFormat = new BYTE[cbMaxSize];
    if (pFormat == NULL)
    {
        return MMSYSERR_NOMEM;
    }

    ZeroMemory(pFormat, cbMaxSize);

    // Ask for WAVE_FORMAT_MPEGLAYER3 formats.
    WAVEFORMATEX* pWaveFormat = (WAVEFORMATEX*)pFormat;
    pWaveFormat->wFormatTag = WAVE_FORMAT_MPEGLAYER3;

    // Set up the acmFormatDetails structure.
    ZeroMemory(&acmFormatDetails, sizeof(acmFormatDetails));
    acmFormatDetails.cbStruct = sizeof(ACMFORMATDETAILS);
    acmFormatDetails.pwfx = pWaveFormat; 
    acmFormatDetails.cbwfx = cbMaxSize;

    // For the ACM_FORMATENUMF_WFORMATTAG request, the format
    // tag in acmFormatDetails must match the format tag in
    // the pFormat buffer.
    acmFormatDetails.dwFormatTag = WAVE_FORMAT_MPEGLAYER3;

    result = acmFormatEnum(NULL, &acmFormatDetails, acmFormatEnumCallback,
        0, ACM_FORMATENUMF_WFORMATTAG);

    delete [] pFormat;

    return result;
}

The next example shows the callback function for the previous example. The callback function is called once for each matching format or until the callback returns FALSE.


BOOL CALLBACK acmFormatEnumCallback(
  HACMDRIVERID       hadid,      
  LPACMFORMATDETAILS pafd,       
  DWORD_PTR          dwInstance, 
  DWORD              fdwSupport  
)
{
    BOOL bContinue = TRUE;
    MPEGLAYER3WAVEFORMAT *pMP3WaveFormat = NULL;

    if (pafd->pwfx->wFormatTag == WAVE_FORMAT_MPEGLAYER3)
    {
        pMP3WaveFormat = (MPEGLAYER3WAVEFORMAT*)pafd->pwfx;

        // TODO: Examine the format. 

        // To halt the enumeration, set bContinue to FALSE.
    }


    return bContinue;
}

Note

The msacm.h header defines acmFormatEnum as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

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	msacm.h
Library	Msacm32.lib
DLL	Msacm32.dll