execl() question? 1

[cpjust]

I've never really used any of the exec() functions before, but I decided to try it out and found something strange.
The MSDN has the syntax of execl() like this:

int _execl( const char *cmdname, const char *arg0, ... const char *argn, NULL );

However, if I want to use execl() I need to call it like this:

int main()
{
	string prog( "c:/temp/hello.bat" );
	string arg1( "Chris" );
	_execl( prog.c_str(), prog.c_str(), arg1.c_str(), NULL );
	cout << "Should never reach here!" << endl;

	return 0;
}

Why do I need to pass the name of the program that I'm executing twice when the syntax only lists it as the first parameter? If I run:

_execl( prog.c_str(), arg1.c_str(), NULL );

It will tell me that "Chris" is not a recognized command...

 

[ArkM]

It's a common convention (or tradition) in C and C++ to pass exe name via argv[0] pointer to main(). You may follow it or may not - it's not an error to pass any string after 1st arg of execl. In your case you call .bat file, not a C or C++ program, so no matter what's your 2nd parameter value.

However, see your cmd file (hello.bat). It's source of the error diagnostic message.

 

[cpjust]

Thanks! There must be some strange dependency in .bat files that requires arg[0] to always be the same as the .bat name. When I ran the same code using an exe that just spits out all the argv values, I can pass anything as the second parameter.

 

[Salem]

> There must be some strange dependency in .bat files that requires arg[0] to always be the same as the .bat name.
BAT files cannot be run as such, they can only be run by a script interpreter (command.com or cmd.exe).

What in effect gets executed is
[tt]cmd.exe /c c:/temp/hello.bat Chris[/tt]
Where the blue bit is simply a blind copy of the original argv. In the case where it doesn't work, you just end up with
[tt]cmd.exe /c Chris[/tt]
which isn't what you wanted, and thus the "command not found" becomes apparent.

I wonder if execl() just looks at the suffix first, and sees .bat before trying anything else?
For example, does this even work (speculate mode)

_execl( "foo.bat", prog.c_str(), arg1.c_str(), NULL );

It just recognises the .bat, then immediately goes to find the command interpreter and pass argv[] to it, without bothering to see if "foo.bat" actually exists anywhere?

--

 

[cpjust]

I don't know, but I found yet another strange thing when I ran:

execl( "hello.bat", "PrintArgs.exe", "abcd", NULL );

where "PrintArgs.exe" is just:

int main( int argc, char* argv[] )
{
   for ( int i = 0; i < argc; ++i )
   {
      cout << "Arg[" << i << "]: " << argv[i] << endl;
   }
}

It acts as though it skipped right over the .bat file and just executes the "PrintArgs.exe".