sizeof an object needed?This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/90-056
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_017.html
When is sizeof needed?
Refer to subclause 6.5.2.3, page 62, lines 28-29. When is the size of an incomplete structure needed? An interpreter may not need the size until run time, while some strictly typed memory architecture may not even allow pointers to structures of unknown size.
In subclause 6.5.2.3, Footnote 63 starts off as an example. The last sentence contains a “shall.” Does a violation of this “shall” constitute undefined behavior?
Even though an interpreter may not need the size of a structure until run time
its compiler still has to do some checking, i.e. an unexecuted statement may
contain sizeof an incomplete type; even though the statement is unexecuted the
constraint still has to be detected.
Comment from WG14 on 1997-09-23:
Whether the language processor is an interpreter or a true compiler does not affect the language rules about when the size of an object is needed. Both a compiler and an interpreter must act as if the translation phases in subclause 5.1.1.2 were followed. This is a requirement that an implementation act as if the entire program is translated before the program's execution.
The “shall” in Footnote 63 in subclause 6.5.2.3 carries no special meaning: this footnote, like all other footnotes in the standard, is provided to emphasize the consequences of the rules in the standard. The footnote is not part of the standard.
The Committee believes that a careful reading of the standard shows all of the places that the size of an object is needed, and that the translation phases prevent those requirements from being relaxed by an implementation.