Page 195 - C-Language

Chapter 28: Implementation-defined

behaviour

Remarks

Overview

The C standard describes the language syntax, the functions provided by the standard library, and
the behavior of conforming C processors (roughly speaking, compilers) and conforming C
programs. With respect to behavior, the standard for the most part specifies particular behaviors
for programs and processors. On the other hand, some operations have explicit or implicit
undefined behavior -- such operations are always to be avoided, as you cannot rely on anything
about them. In between, there are a variety of implementation defined behaviors. These behaviors
may vary between C processors, runtimes, and standard libraries (collectively, implementations),
but they are consistent and reliable for any given implementation, and conforming implementations
document their behavior in each of these areas.

It is sometimes reasonable for a program to rely on implementation-defined behavior. For
example, if the program is anyway specific to a particular operating environment then relying on
implementation-defined behaviors general to the common processors for that environment is
unlikely to be a problem. Alternatively, one can use conditional compilation directives to select
implementation-defined behaviors appropriate for the implementation in use. In any case, it is
essential to know which operations have implementation defined behavior, so as to either avoid
them or to make an informed decision about whether and how to use them.

The balance of these remarks constitute a list of all the implementation-defined behaviors and
characteristics specified in the C2011 standard, with references to the standard. Many of them use
the terminology of the standard. Some others rely more generally on the context of the standard,
such as the eight stages of translating source code into a program, or the difference between
hosted and freestanding implementations. Some that may be particularly surprising or notable are
presented in bold typeface. Not all the behaviors described are supported by earlier C standards,
but generally speaking, they have implementation-defined behavior in all versions of the standard
that support them.

Programs and Processors

General

• The number of bits in one byte (3.6/3). At least 8, the actual value can be queried with the
macro CHAR_BIT.

• Which output messages are considered "diagnostic messages" (3.10/1)

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