Visual Studio C++ 2010 how can __argv be a null pointer?

int _tmain(int argc, _TCHAR* argv[])
{

    if (argc != 3) {
        printf("Format is straightline.exe <EO records file> <output file>");
        return 1;
    }
    string eoPath = string(__argv[1]);
    //...other stuff ...
}

If __argc == 3, how can __argv be a null pointer?

My debugger is telling me that __argv is pointing to 0x00000000 after the program crashed when I was trying to reference __argv[1] (and I have verified that __argc == 3). This is a minimal program and it happened in the beginning before I did any sort of processing.

If this is complied as unicode, then __argv will be null, while __wargv will contain what you want. I believe that there is a __targv that should contain the command line arguments regardless of unicode or ascii.

But why use any of these if you can just use argv passed in as a parameter to _tmain?

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The pointer to __argv can (and will) be null in Unicode configurations.

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__argc, __argv/__wargv and __envp/__wenvp are special globals used by the CRT init, you shouldn't ever touch these, rather just stick to the variables passed to your main/wmain/_tmain function, these are derived from the aforementioned globals, and they are guaranteed to be correct, and thanks to Microsofts macro's for the _tmain variant, they will also use the correct character encoding (UNICODE vs MBCS/ASCII).

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Comments
  • I'm sure I just don't know this, but where is __argv (not argv) coming from?
  • Who ever made any guarantees about the behaviour of variables starting with double-underscores?
  • Hey thanks for pointing that out guys. In fact I was just perusing my code when I noticed that. I used to have to use those underscores as the standard way; I forget exactly why. Something to do with weird Windows/Microsoft/VisualStudio weirdness. Anyway, I have switched to the non-underscore and it is working, thankfully
  • Wow, so after this, I was having trouble converting TCHAR* to char*. But amazingly I changed _tmain to simply main, and _TCHAR* to simply char*, and it still runs as normal and now the char* is simply a char*, so everything works now
  • Because of [w]WinMain.
  • and the opposite is true .. in non-unicode apps __wargv is null.
  • The __argc and __argv variables exist so that programs using WinMain() as the entry point can access argv and argc parameters, since WinMain() doesn't pass them to you. The main() function of course doesn't have this problem since it provides them in the function signature.
  • @Insilico: that article is for Windows 3.1, and any WinMain based application that needs the command-line should use GetCommandLine to retrieve it (even though it does get passed to you, hence the lpCmdline parameter of WinMain)
  • Yes, but it shows that it is in fact documented. While you can use GetCommandLine(), it gives you the entire string and not an array of arguments. You can convert it via CommandLineToArgvW() but that's only available for Unicode builds. Besides, __argv and __argc are what's passed to your main() function, so it's exactly equivalent.