Submitter: Joseph Myers (UK)
Submission Date: 2007-03-24
Source: Joseph Myers <joseph@codesourcery.com>
Reference Document: ISO/IEC WG14
N1120
Version: 1.0
Date: 2007-03-24
Subject: Variably modified compound literals
Summary
Consider the code:
extern int a;
void *p = &(int (*)[a]){ 0 };
Does such a variably modified compound literal at file scope violate any constraint? 6.7.5.2#2 says:
[#2] Only an ordinary identifier (as defined in 6.2.3) with both block scope or function prototype scope and no linkage shall have a variably modified type. If an identifier is declared to be an object with static storage duration, it shall not have a variable length array type.
However, this only seems to constrain declarations of identifiers,
not any other expression with variably modified type (such as a
compound literal, inside or outside a function). If however the above
code is valid, when is a evaluated for the purposes of
the requirement in 6.7.5.2#5 that "each time it is evaluated it shall
have a value greater than zero"? Must a have positive
value throughout execution of the program, or is it only the initial
value of a which must be positive? (I think the
initializer is a constant expression, being the
address of an object of static storage duration.)
The variably modified compound literal is an object, and I think it
should be treated like other objects outside functions and required
not to have variably modified type (even if inside sizeof, just like
the initializers of compound literals outside functions must be
constant even if inside sizeof).
Suggested Technical Corrigendum
6.5.2.5 paragraph 3, after "shall consist of constant expressions" add
"and the type name shall not specify a variably modified type".