Is a goto in alloca's function scope valid?
alloca, c, c99, undefined-behavior
Solution
Actually, the rule 6.8.6.1 states:
A goto statement is not allowed to jump past any declarations of objects
with variably modified types.
In your code, there does not exist an object with variably modified types. `alloca` does not declare an object (that the compiler has to care of). Thus, there is nothing like a scope for `alloca`, and no reason for undefined behavior in the sense of rule 6.8.6.1.
EDIT
To elaborate the answer a bit: the "undefinedness" of the behavior in case of VLA is due to the promise of a declaration that an object is "known" within its scope (at language level). In general, a declaration sets a context for code execution. There is no need that it is executed at runtime. However, this is not true in case of VLA: here this promise is implemented partly at runtime, breaking C's static declaration approach. To avoid further conflicts that would lead to a dynamic typing system, rule 6.8.6.1 avoids such conflicts.
In contrast, at language level `alloca` is simply a function; its call does not constitute any scope. It makes only a promise about its run-time behavior in case it is called. If it isn't called, we do not "expect" anything from a function. Thus, its pure existence does not raise any conflict: both cases (bypassing or non-bypassing) have a well defined semantic.
Problem
The C standard prohibits a goto into a function scope where a VLA exists. A VLA and the call to alloca function should have the same result on low level. (I could be wrong, as I'm just a C, not a low level programmer, but in my imagin that appears to be witty) So will the following snippet be also undefined behaivng? ``` int main() { char *p; goto label1; { p = _alloca(1); label1: p = NULL; } } ``` Ofcourse I cant refference `p`, but whats about the behaviour?