Programmatic Radio Control for Smartphones and Pocket PCs

Lengthy title, yes. I’m working on an application for my Windows Mobile 6.0 Phone, and I needed to toggle the radio (phone receiver) on and off. It’s not very difficult to disable and enable the radio programmatically, but it did take quite a bit of research since I’m new to this platform. I couldn’t find any good, well-written examples on this using Google, so I decided to discuss my solution.

I used Visual Studio’s “Windows Mobile 5.0 Smartphone SDK (ARMV4I)” emulator and architecture for testing. The code is written in straight C. The process is pretty simple:

  1. Initialize with the TAPI DLL.
  2. Enumerate all the line devices. Find the one called “Cellular Line”.
  3. Open the line, change its equipment state, close the line.
  4. Shutdown from the TAPI DLL.

For initializing and shutting down access to TAPI, I made a few helper functions.

Select All Code:
#include <tapi.h>
#include <extapi.h>
#include <string.h>
 
HLINEAPP hLineApp = NULL;
DWORD g_NumDevices = 0;
 
int InitializeLineUtils(LINECALLBACK callback)
{
	int ret;
 
	/* Init TAPI, get device count */
	if ((ret = lineInitialize(&hLineApp,
			g_hInstance,  /* From WinMain */
			callback,
			NULL,
			&g_NumDevices))
		!= 0)
	{
		return ret;
	}
 
	return 0;
}
 
void ShutdownLineUtils()
{
	if (hLineApp != NULL)
	{
		lineShutdown(hLineApp);
		hLineApp = NULL;
	}
}

The next step is to find a device matching a specific name. Device names can be either in Unicode or ASCII, so my helper function takes both to eliminate any conversion. First the device API version is negotiated, then the device capabilities are queried. Note that the structure to do this is pretty low-level — you have to reallocate memory until the device decides it’s big enough to squirrel all its data past the end of the struct definition.

This function finds a matching device and returns its negotiated API version:

Select All Code:
DWORD FindLineDeviceByName(LPCWSTR wstr, LPCSTR cstr, DWORD *pAPIVersion)
{
	LONG ret;
	DWORD api_version;
	LINEDEVCAPS *dev_caps;
	LINEEXTENSIONID ext_id;
	DWORD device_id, dev_cap_size;
 
	dev_cap_size = sizeof(LINEDEVCAPS);
	dev_caps = (LINEDEVCAPS *)malloc(dev_cap_size);
	dev_caps->dwTotalSize = dev_cap_size;
 
	/* For each device... */
	for (device_id = 0; device_id < g_NumDevices; device_id++)
	{
		/* Negotiate an API version (0x00020000 is my CURRENT) */
		if ((ret = lineNegotiateAPIVersion(hLineApp,
				device_id,
				0x00020000,
				TAPI_CURRENT_VERSION,
				&api_version,
				&ext_id))
			!= 0)
		{
			continue;
		}
 
		while (TRUE)
		{
			if ((ret = lineGetDevCaps(hLineApp, 
					device_id,
					api_version,
					0,
					dev_caps)) == 0
				&& dev_caps->dwNeededSize <= dev_caps->dwTotalSize)
			{
				break;
			}
 
			if (ret == 0 || ret == LINEERR_STRUCTURETOOSMALL)
			{
				dev_cap_size = dev_caps->dwNeededSize;
				dev_caps = (LINEDEVCAPS *)realloc(dev_caps, dev_cap_size);
				dev_caps->dwTotalSize = dev_cap_size;
			}
			else
			{
				break;
			}
		}
 
		if (ret != 0)
		{
			continue;
		}
 
		if (dev_caps->dwStringFormat == STRINGFORMAT_UNICODE)
		{
			const wchar_t *ptr = (wchar_t *)((BYTE *)dev_caps 
				+ dev_caps->dwLineNameOffset);
			if (wcscmp(ptr, wstr) == 0)
			{
				*pAPIVersion = api_version;
				return device_id;
			}
		}
		else if (dev_caps->dwStringFormat == STRINGFORMAT_ASCII)
		{
			const char *ptr = (char *)dev_caps + dev_caps->dwLineNameOffset;
			if (strcmp(ptr, cstr) == 0)
			{
				*pAPIVersion = api_version;
				return device_id;
			}
		}
	}
 
	return -1;
}

Lastly, helper functions for opening and closing lines:

Select All Code:
LONG OpenLine(DWORD device_id, 
	      DWORD api_version, 
	      HLINE *pLine, 
	      DWORD inst,
	      DWORD privs,
	      DWORD media)
{
	return lineOpen(hLineApp,
		device_id,
		pLine,
		api_version,
		0,
		inst,
		privs,
		media,
		NULL);
}
 
void CloseLine(HLINE line)
{
	lineClose(line);
}

Now we can write a simple program to toggle the radio:

Select All Code:
VOID FAR PASCAL lineCallbackFunc(DWORD hDevice, 
				 DWORD dwMsg, 
				 DWORD dwCallbackInstance, 
				 DWORD dwParam1, 
				 DWORD dwParam2, 
				 DWORD dwParam3)
{
}
 
int WINAPI WinMain(HINSTANCE hInstance,
		   HINSTANCE hPrevInstance,
		   LPTSTR lpCmdLine,
		   int nCmdShow)
{
	HLINE hLine;
	DWORD state, radio_support;
	DWORD cell_api, cell_device;
 
	if (InitializeLineUtils(lineCallbackFunc) != 0)
	{
		return 1;
	}
 
	if ((cell_device = FindLineDeviceByName(
			L"Cellular Line",
			"Cellular Line",
			&cell_api))
		== -1)
	{
		ShutdownLineUtils();
		return 1;
	}
 
	if (OpenLine(cell_device, cell_api, &hLine, 0, LINECALLPRIVILEGE_NONE, 0) != 0)
	{
		ShutdownLineUtils();
		return 1;
	}
 
	if (lineGetEquipmentState(hLine, &state, &radio_support) == 0)
	{
		if (state != LINEEQUIPSTATE_FULL)
		{
			state = LINEEQUIPSTATE_FULL;
		}
		else
		{
			state = LINEEQUIPSTATE_MINIMUM;
		}
 
		lineSetEquipmentState(hLine, state);
	}
 
	CloseLine(hLine);
	ShutdownLineUtils();
 
	return 0;
}

Side note: LINEEQUIPSTATE_NOTXRX seemed to have no effect for me. My device turned the radio off on “MINIMUM” but refused the other option as unsupported.

As excellent and generally seamless as Visual Studio’s emulator was, it did not respond to lineSetEquipmentState at all. I guess since it’s not an actual device, maybe it doesn’t bother implementing the “equipment.”

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