Versions

[{“Name”:“C11”,“GroupName”:null}]

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)

Source translation

• The manner in which physical source file multibyte characters are mapped to the source character set (5.1.1.2/1).
• Whether non-empty sequences of non-newline whitespace are replaced by single spaces during translation phase 3 (5.1.1.2/1)
• The execution-set character(s) to which character literals and characters in string constants are converted (during translation phase 5) when there is otherwise no corresponding character (5.1.1.2/1).

Operating environment

• The manner in which the diagnostic messages to be emitted are identified (5.1.1.3/1).
• The name and type of the function called at startup in a freestanding implementation (5.1.2.1/1).
• Which library facilities are available in a freestanding implementation, beyond a specified minimal set (5.1.2.1/1).
• The effect of program termination in a freestanding environment (5.1.2.1/2).