Using the main function in a program is allowed

[llvmbot]

Bugzilla Link	18541
Resolution	FIXED
Resolved on	Jan 22, 2014 18:46
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@dwblaikie,@DougGregor,@zygoloid

Extended Description

This causes an infinite loop:

int main() {auto* x = &main; x();}

[basic.start.main]/3 states:

"The function main shall not be used within a program."

I can even do things like

int x = 0; int main() {auto* f = &main; if (++x == 0) f();}

which run correctly.

gcc rejects taking the address of main.

[dwblaikie]

Could you provide an example of GCC rejecting this? I can't seem to reproduce any diagnostic/error/warning from the code you provided from GCC 4.8.1 (or ToT from several months ago).

This is undefined behavior, no diagnostic required, so Clang isn't wrong here, though it could be better - but I doubt there's a high bug incidence in this area, so it might not be worth the work/added complexity/compile-time to check for this.

[llvmbot]

gcc requires -pedantic, in which case it diagnoses taking address of main. The test run I did also had -Werror, so getting gcc to reject the code requires some coaxing, sorry about that omission.

Also, I don't think violating a "shall" requirement is undefined behavior, I thought it should render the code ill-formed. If that's the case, I have an ill-formed program, and clang doesn't issue a single diagnostic, even if I pass -pedantic to it.

[zygoloid]

Fixed in r199782. For compatibility with GCC and IOCCC entries, we still silently accept this outside -pedantic mode. With -pedantic-errors, we produce:

:1:25: error: ISO C++ does not allow 'main' to be used by a program [-Werror,-Wmain]
int main() { auto *x = &main; x(); }
^