ISO/IEC JTC 1/SC22/WG14 N1254

1 September 2007

Thomas Plum,

Saturation in C1x

WG14 N1254: Saturation in C1x


Within the safety and security communities, there is an important need to determine that certain computed results have not been contaminated by integer overflow. Languages that compete with C have traditionally met that need using methods that are awkward for C; this category would include the throwing of exceptions (or signals), encoding ranges into the integer types, and the use of arbitrary-precision (“BigInt”) libraries. (Call this category the “awkward methods”.) However, in C1x we could meet this need by providing saturation semantics for overflow-handling.


There has been discussion within both WG14 and the SC22 Other Working Group on Vulnerabilities [OWGV] of the CERT Secure Coding Standards (see [CERT-SCS], [N1209], and [N1255] ). Rule INT08-A is easy to state, and challenging to automate: “Integer operations must result in an integer value within the range of the integer type (that is, the resulting value is the same as the result produced by unlimited-range integers).” (We need a good name for this property of an integer value. Unfortunately, “in-range” carries the connotation of arbitrary range-qualification, as in Ada or Pascal. For now, I'll call it the “correctly-represented” property.)

In the C standard, integer overflow produces undefined behavior; i.e., any behavior is permitted, and in particular, producing a saturated “MAX” result is permitted. There is currently no way of requiring a saturated result. If in C1x we defined a new standard pragma such as _Pragma(STDC SAT), then without impacting existing code we could provide the necessary semantics.

The basic idea is to permit static analyzers to determine that specific calculated values have the “correctly-represented” property, without burdening the programmer with unnecessary runtime overhead or a one-macro-per-arithmetic-operator ugliness. The languages that provide the “awkward methods” don't automatically provide fully-automated proofs of safety or security properties, and a 21st-century C language could compete quite well, if it is adequately static-analyzable.

Clearly, the compiler could deliver saturation semantics at a runtime cost that beats any user-programmed solution. For some result types, the hardware may already provide saturating operators, for minimal overhead.

For completeness, we probably would want a pragma for modwrap and for “default” as well; I leave the details for future deliberations. The important point is to provide some portable guaranteed method to obtain saturation semantics.


[OWGV] ISO/IEC JTC 1/SC 22/OWG:Vulnerabilities

[CERT-SCS] CERT Secure Coding Standards