Opening
a Device
Before using
a device, you must initialize it by using the open (MCI_OPEN) command. This command
loads the driver into memory (if it isn t already loaded) and retrieves the
device identifier you will use to identify the device in subsequent MCI
commands. You should check the return value of the mciSendString or mciSendCommand function before using a new
device identifier to ensure that the identifier is valid. (You can also
retrieve a device identifier by using the mciGetDeviceID function.)
Like all MCI
command messages, MCI_OPEN has an associated structure. These structures
are sometimes called parameter blocks. The default structure for MCI_OPEN
is MCI_OPEN_PARMS.
Certain devices (such as waveform and overlay) have extended
structures (such as MCI_WAVE_OPEN_PARMS and MCI_OVLY_OPEN_PARMS) to accommodate additional
optional parameters. Unless you need to use these additional parameters, you
can use the MCI_OPEN_PARMS structure with any MCI device.
The number of
devices you can have open is limited only by the amount of available memory.
Using an Alias
When you open
a device, you can use the alias flag
to specify a device identifier for the device. This flag lets you assign
a short device identifier for compound devices with lengthy filenames, and it
lets you open multiple instances of the same file or device.
For example,
the following command assigns the device identifier birdcall to the lengthy
filename C:\NABIRDS\SOUNDS\MOCKMTNG.WAV:
mciSendString(
"open c:\nabirds\sounds\mockmtng.wav
type waveaudio alias birdcall",
lpszReturnString,
lstrlen(lpszReturnString), NULL);
In the
command-message interface, you specify an alias by using the lpstrAlias
member of the MCI_OPEN_PARMS
structure.
Specifying a Device Type
When you open
a device, you can use the type flag to refer to a device type, rather than to
a specific device driver. The following example opens the waveform-audio file
C:\WINDOWS\CHIMES.WAV (using the type flag to specify the waveaudio
device type) and assigns the alias chimes :
mciSendString(
"open c:\windows\chimes.wav type
waveaudio alias chimes",
lpszReturnString,
lstrlen(lpszReturnString), NULL);
In the
command-message interface, the functionality of the type flag is supplied by
the lpstrDeviceType member of the MCI_OPEN_PARMS structure.
Simple and Compound Devices
MCI
classifies device drivers as compound or simple. Drivers for compound
devices require the name of a data file for playback; drivers for simple
devices do not.
Simple
devices include cdaudio and videodisc devices. There are two ways
to open simple devices:
Specify a pointer to a
null-terminated string containing the device name from the registry or the
SYSTEM.INI file.
For example, you can open a videodisc device by using the
following command:
mciSendString("open videodisc",
lpszReturnString,
lstrlen(lpszReturnString), NULL);
In this case, videodisc is the device name from the registry or the
[mci] section of SYSTEM.INI.
Specify the actual name of the
device driver. Opening a device using the device-driver filename, however,
makes the application device-specific and can prevent the application from
running if the system configuration changes. If you use a filename, you do not
need to specify the complete path or the filename extension; MCI assumes
drivers are located in a system directory and have the .DRV filename extension.
Compound
devices include waveaudio and sequencer devices. The data for a
compound device is sometimes called a device element. This document,
however, generally refers to this data as a file, even though in some cases the
data might not be stored as a file.
There are
three ways to open a compound device:
Specify only the device name.
This lets you open a compound device without associating a filename. Most
compound devices process only the capability14PJB2D (MCI_GETDEVCAPS) and close (MCI_CLOSE) commands when they are
opened this way.
Specify only the filename. The
device name is determined from the associations in the registry.
Specify the filename and the
device name. MCI ignores the entries in the registry and opens the specified
device name.
To associate
a data file with a particular device, you can specify the filename and device
name. For example, the following command opens the waveaudio device with
the filename MYVOICE.SND:
mciSendString("open myvoice.snd type
waveaudio", lpszReturnString,
lstrlen(lpszReturnString), NULL);
In the
command-string interface, you can also abbreviate the device name specification
by using the alternative exclamation-point format, as documented with the open command.
Opening a Device Using the Filename Extension
If the open (MCI_OPEN) command specifies only
the filename, MCI uses the filename extension to select the appropriate device
from the list in the registry or the [mci extensions] section of the SYSTEM.INI
file. The entries in the [mci extensions] section use the following form:
filename_extension=device_name
MCI
implicitly uses device_name if the extension is found and if a device
name has not been specified in the open command.
The following
example shows a typical [mci extensions] section:
[mci extensions]
wav=waveaudio
mid=sequencer
rmi=sequencer
Using these
definitions, MCI opens the waveaudio device if the following command is
issued:
mciSendString("open train.wav",
lpszReturnString,
lstrlen(lpszReturnString), NULL);
New Data Files
To create a
new data file, simply specify a blank filename. MCI does not save a new file until
you save it by using the save
(MCI_SAVE)
command. When creating a new file, you must include a device alias with the
open (MCI_OPEN) command.
The following
example opens a new waveaudio file, starts and stops recording, then
saves and closes the file:
mciSendString("open new type waveaudio alias
capture", lpszReturnString,
lstrlen(lpszReturnString), NULL);
mciSendString("record capture", lpszReturnString,
lstrlen(lpszReturnString), NULL);
mciSendString("stop capture", lpszReturnString,
lstrlen(lpszReturnString), NULL);
mciSendString("save capture orca.wav", lpszReturnString,
lstrlen(lpszReturnString), NULL);
mciSendString("close capture", lpszReturnString,
lstrlen(lpszReturnString), NULL);
Shareable Devices
The
shareable (MCI_OPEN_SHAREABLE) flag of the open (MCI_OPEN) command lets multiple
applications access the same device (or file) and device instance
simultaneously. If your application opens a device or file as shareable, other
applications can also access it by opening it as shareable. The shared device
or file gives each application the ability to change the parameters governing
its operating state. Each time a device or file is opened as shareable, MCI
returns a unique device identifier, even though the identifiers refer to the
same instance.
If your
application opens a device or file without specifying that it is shareable, no
other application can access it until your application closes it. Also, if a
device supports only one open instance, the open command will fail if
you specify the shareable flag.
If your
application opens a device and specifies that it is shareable, your application
should not make any assumptions about the state of this device. Your
application might need to compensate for changes made by other applications
accessing the device.
Most compound
files are not shareable; however, you can open multiple files, or you can open
a single file multiple times. If you open a single file multiple times, MCI
creates an independent instance for each, with each instance having a unique
operating status.
If you open
multiple instances of a file, you must assign a unique device identifier to
each. You can use an alias, as described in the following section, to assign a
unique name for each file.