Retrieving command line parameters from WinMain in win32

Remember the main function from our standard C programs or the win32 console programs. It used to have two arguments argc and argv as follows

int main(int argc, char * argv[]) ...

These parameters helped us send command line parameters to our programs. ‘argc’ would hold the number of arguments passed and argv would be an array of   the parameters passed. But when we are doing a GUI based win32 application in visual studio, we would use the WinMain function as our entry point. This function doesnt have an argc or an argv argument. So how do we pass command line parameters to Windows programs, and how do we access them in our programs.

lpCmdLine argument

One solution is the WinMain function itself. Let us look at the typical declaration of WinMain

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)..

As we can see in the declaration, we have an argument called lpCmdLine of type LPSTR (char *). This variable stores the entire command line minus the name of the program. For eg if we had a program called ‘test.exe’ and if we called it as

test.exe Some arguments here

The variable lpCmdLine would hold the value “Some arguments here”. This is one way of accessing command line parameters,  although not as helpful as the classic argc and argv. We would have to write the code to parse the lpCmdLine variable ourselves, thus increasing the complexity of our program. Also, as we can see lpCmdLine is actually an ANSI string and thus would not work with Unicode characters being passed at commandline.

GetCommandLine()

Another method is to use GetCommandLine() api. This function returns the entire command line, i.e. it includes the program name (which could include the absolute path) and also all the parameters in a single string. This is similar to directly accessing lpCmdLine, but the advantage is that it is mapped to either the ANSI GetCommandLineA() function or the unicode GetCommandLineW() function depending on your projects settings. This solves the problem of unicode input thru command line, but it still doesnt provide a count of arguments neither the arguments as seperate variables like argv.

CommandLineToArgvW()

The last method that I am going to discuss is CommandLineToArgvW This function only works with the Unicode/Wide character set and there seem to be no CommandLineToArgvA ANSI equivalent function. The prototype is as follows

LPWSTR *CommandLineToArgvW(LPCWSTR lpCmdLine, int *pNumArgs)

This function comes closest to the simplicity of argc and argv, or rather it gives us access to our argc and argv in the windows program. As we can see the function accepts two arguments, one is a unicode commandline string which needs to be parsed, the second is a pointer to an int variable. The function stores the number of arguments in this variable when it returns.

The function returns an array of strings, which is similar to argv. Lets look at an example.

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE prevInstance, LPSTR lpCmdLine, int nShowCmd)
{
    LPWSTR *szArgList;
    int argCount;

     szArgList = CommandLineToArgvW(GetCommandLine(), &argCount);
     if (szArgList == NULL)
     {
         MessageBox(NULL, L"Unable to parse command line", L"Error", MB_OK);
         return 10;
     }

     for(int i = 0; i < argCount; i++)
     {
         MessageBox(NULL, szArgList[i], L"Arglist contents", MB_OK);
     }

     LocalFree(szArgList);
     return 0;
}

As we can see above by using this function, we get the commandline as the number of arguments(argc) and a list of strings (argv). The only thing to remember here is that, the function allocates a block of memory for the argument list. We have to manually delete this memory whenever we are done with it, or we will have a leaking application.

EDIT: Variables __argc and __argv

As pointed below by anonymous, Microsoft compiler provides us with the variables __argc and __argv. These variables are automatically populated at runtime by windows. The variables are available in two flavors, the ASCII version and the Unicode version. __argv is the ascii version, whereas __wargv is the unicode version. To use them we have to include stdlib.h, which is automatically included whenever you include windows.h. To let the project setting decide which version of the variable to use, we can access __targv, by including TCHAR.h.

This method is the simplest of them all, but the caveat is that we have to link to the VC++ runtime library i.e. msvcrt.dll (which we would for almost 99% of windows programs).

I hope this helps everyone, would love to hear everyone’s comments regarding the same.