Is main() a callback function?

[k5054]

raddevus wrote:

* Cannot have its address taken. * Cannot be called.

Lies. You can call main, and you can take its address. This compiles and runs in VS2019:

#include int main(int argc, char **argv)
{
if(argc <= 1) {
auto main_ptr{main};
std::cout << "pointer to main = " << main_ptr << std::endl;
std::cout << "exiting ..." << std::endl;
return 0;
}
std::cout << "argc = " << argc << std::endl;
main(--argc, argv);
}

Interestingly, in linux you auto main_ptr{main} is 1, but for windows it looks like an address: 0008151E Update: I should also point out that the instances where you might need to call main from within you program are vanishingly small. In general, if you think you need to, you're almost certainly wrong.

[OriginalGriff]

Difficult to tell, it was yelling a lot and slurring it's words.

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony AntiTwitter: @DalekDave is now a follower!

[Rick York]

Actually things are a little different. In the Visual Studio source, VC/Tools/MSVC/**version**/crt/src/vcruntime/vcstartup_internal.h has the prototypes and the calls are in exe_common.inl. WinMain and main are two different calls along with their wide character versions.

"They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

[Rick York]

That has always been my thought.

"They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

[TheGreatAndPowerfulOz]

Real programmers use galaxy-sized gravitic lenses.

#SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

[PIEBALDconsult]

Hold my Jolt Cola...

[honey the codewitch]

Focused using butterflies

Real programmers use butterflies

[TheGreatAndPowerfulOz]

Did you give it your number, baby? ;P

#SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

[TheGreatAndPowerfulOz]

honey the codewitch wrote:

Focused using butterflies

that are manipulated using gravitic lenses

#SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

[honey the codewitch]

created from the silk of butterfly cocoons :-D

Real programmers use butterflies

[DRHuff]

So it was yaccing with a lisp?

I, for one, like Roman Numerals.

[Rick York]

If you have Visual Studio you can see what the C run-time library does by looking at its source. I was checking that out just today.

"They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

[Lost User]

I suspected this might be the case, but I have not had access to the source for a number of years.

[Stefan_Lang]

Only if used as such, e. g. in this code:

#include

using namespace std;

static int call_countdown = 3;
typedef int (*MyCallbackFun)();
int foo(MyCallbackFun cb, int count)
{
static int foo_counter = 0;
++foo_counter;
cout << "enter foo[" << foo_counter <<"]: " << count << endl;
int result = cb();
cout << "exit foo[" << foo_counter << "]: " << result << endl;
return result;
}
int bar()
{
static int bar_counter = 0;
++bar_counter;
cout << "bar[" << bar_counter << "]" << endl;
return -1;
}
int main()
{
static int call_counter = 0;
++call_counter;
cout << "enter main[" << call_counter << "]" << endl;

int result = 0;
if (call\_counter < 5)
{
    result = (call\_counter>2)
        ? foo(bar, call\_counter)
        : foo(main, call\_counter);
}
cout << "exit main\[" << call\_counter << "\]: " << result << endl;

return call\_counter;

}

You can test it here: https://www.onlinegdb.com/online_c++_compiler[^] or trust me that the output is:

enter main[1]
enter foo[1]: 1
enter main[2]
enter foo[2]: 2
enter main[3]
enter foo[3]: 3
bar[1]
exit foo[3]: -1
exit main[3]: -1
exit foo[3]: 3
exit main[3]: 3
exit foo[3]: 3
exit main[3]: 3

The tricky bit about this is that by using main() as a callback function, you're also using it recursively, which complicates matters considerably: it's easy to mess up the code and get an endless recursion. (that's why I added counters and output in every function) It's doable, but definitiely not a good idea.

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

[TheGreatAndPowerfulOz]

lol, you got me there.

#SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

[Rick York]

There were only minor changes. For the most part, what you wrote is correct.

"They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

[Slow Eddie]

No. It's a state in the Northeast. :laugh:

Oh Well.....