On 14.10.2017 23:36, kdevel wrote:
On Saturday, 14 October 2017 at 09:32:32 UTC, Timon Gehr wrote:
Also, UB can and does sometimes mean that the program can execute
arbitrary code. It's called "arbitrary code execution":
https://en.wikipedia.org/wiki/Arbitrary_code_execution
This confuses different levels of reasoning.
It's a correct statement about the semantics of programs produced from
sources with UB by standard-compliant compilers.
In C/C++ "undefined
behavior" is a property of the SOURCE code with respect to the
specification. It states: The spec does not not apply, it does not
define the semantic.
...
I.e., the semantics of a program produced by a conforming compiler can
be arbitrary.
This issue is totally different from the question what a given program
containing undefined behavior actually does after is compiles and the
after the linker produces an executable. This is reasoning about
generated MACHINE code.
...
Sure. This is very much intentional. The current subthread is about what
kind of programs the compiler might produce (in practice) if the
provided source code contains UB. The claim I was refuting was that the
produced program cannot have branching and other behaviors not specified
in the source.
A result of this confusion has been that some clever people tried to
"detect" certain kinds of undefined behavior "after" they "happended".
E.g. <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30475> This is the
danger of undefined behavior: The MACHINE code may also work as the
programmer expected. At least for some time.
I'm not confused about this at all.
