Page 122 - C-Language

Chapter 15: Constraints

Remarks

Constraints are a term used in all of the existing C specifications (recently ISO-IEC 9899-2011).
They are one of the three parts of the language described in clause 6 of the standard (along side
syntax and semantics).

ISO-IEC 9899-2011 defines a constraint as a:

restriction, either syntactic or semantic, by which the exposition of language elements
is to be interpreted

(Please also note, in terms of the C standard, a "runtime-constraint" is not a kind of constraint and
has extensively different rules.)

In other words a constraint describes a rule of the language which would make an otherwise
syntactically valid program illegal. In this respect constraints are somewhat like undefined
behavior, any program which does not follow them is not defined in terms of the C language.

Constraints on the other hand have a very significant difference from Undefined Behaviors.
Namely an implementation is required to provide a diagnostic message during the translation
phase (part of compilation) if a constraint is breached, this message may be a warning or may halt
the compilation.

Examples

Duplicate variable names in the same scope

An example of a constraint as expressed in the C standard is having two variables of the same
1)
name declared in a scope , for example:

void foo(int bar)
{
int var;
double var;
}

This code breaches the constraint and must produce a diagnostic message at compile time. This
is very useful as compared to undefined behavior as the developer will be informed of the issue
before the program is run, potentially doing anything.

Constraints thus tend to be errors which are easily detectable at compile time such as this, issues
which result in undefined behavior but would be difficult or impossible to detect at compile time are
thus not constraints.

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