Holy cow, I wrote a book!
Back in 16-bit Windows,
MS-DOS cast a long and dark shadow.
The really ugly low-level munging was very much in the MS-DOS spirit.
You opened files by setting up registers and issuing an int 21h,
just like in MS-DOS.
Although the interrupt went to Windows instead,
Windows maintained the MS-DOS calling convention.
Process startup followed the same
"real men write in assembly language" philosophy.
All the parameters to a 16-bit program
were passed in registers.
The entry point to a 16-bit process received the following
parameters on Windows 3.1:
Hey, nobody said that 16-bit Windows was designed for portability.
The first thing a 16-bit program did was call the
This function receives its parameters in registers,
precisely in the format that they are received by the program
The InitTask function initializes the stack,
the data segment, the heap,
retrieves and prepares the command line,
recovers the nCmdShow parameter that was passed
to WinExec, all the normal startup stuff.
It even edits the stack of the caller so that real-mode stack
walking works (critical for memory management in real-mode).
When InitTask is all finished,
it returns with the registers set for the next phase:
Once InitTask returns, the stack, heap, and data segment
are "ready to run," and if you have no other preparations to do,
you can head right for the application's WinMain function.
Minimal startup code therefore would go like this:
call far InitTask
test ax, ax
push di ; hInstance
push si ; hPrevInstance
push es ; lpszCmdLine selector
push bx ; lpszCmdLine offset
push dx ; nCmdShow
... some lines of code that aren't important to the discussion ...
call far WinMain ; call the application's WinMain function
; return value from WinMain is in the AL register,
; conveniently positioned for the exit process coming up next
mov ah, 4Ch ; exit process function code
int 21h ; do it
Why wasn't the application entry point called main?
Well, for one thing, the name main was already taken,
and Windows didn't have the authority to reserve an alternate definition.
There was no C language standardization committee back then;
C was what
Dennis said it was,
and it was hardly guaranteed that Dennis would take any special steps to
preserve Windows source code
compatibility in any future version of the C language.
Since K&R didn't specify that implementations could extend the
acceptable forms of the main function,
it was entirely possible that there was a legal C compiler that
rejected programs that declared main incorrectly.
The current C language standard explicitly permits implementation-specific
alternate definitions for main, but
requiring all compilers to support
this new Windows-specific version
in order to compile Windows programs
would gratuitously restrict the set of compilers you could use for
writing Windows programs.
If you managed to overcome that obstacle,
you'd have the problem that the Windows version of main
would have to be something like this:
int main(int argc, char *argv, HINSTANCE hinst,
HINSTANCE hinstPrev, int nCmdShow);
Due to the way C linkage was performed,
all variations of a function had to agree on the parameters
they had in common.
This means that the Windows version would have to add its parameters
onto the end of the longest existing version of main,
and then you'd have to cross your fingers and hope that the C language
never added another alternate version of main.
If you went this route, your crossed fingers failed you, because
it turns out that
a third parameter was added to main
some time later,
and it conflicted with your Windows-friendly version.
Suppose you managed to convince Dennis not to allow that three-parameter
version of main.
You still have to come up with those first two parameters,
which means that every program's startup code needs to contain
a command line parser.
Back in the 16-bit days, people scrimped to save every byte.
Telling them, "Oh, and all your programs are going to be 2KB bigger"
probably wouldn't make you a lot of friends.
I mean, that's four sectors of I/O off a floppy disk!
But probably the reason why the Windows entry point was given a
is to emphasize that it's a different execution environment.
If it were called main,
people would take C programs designed for a console environment,
throw them into their Windows compiler, and then run them,
with disastrous results.