Implementation-defined behavior

This is how GNU CPP behaves in all the cases which the C standard describes as implementation-defined. This term means that the implementation is free to do what it likes, but must document its choice and stick to it.

• The mapping of physical source file multi-byte characters to the execution character set.

  Currently, GNU cpp only supports character sets that are strict supersets of ASCII, and performs no translation of characters.

• Non-empty sequences of whitespace characters.

  In textual output, each whitespace sequence is collapsed to a single space. For aesthetic reasons, the first token on each non-directive line of output is preceded with sufficient spaces that it appears in the same column as it did in the original source file.

• The numeric value of character constants in preprocessor expressions.

  The preprocessor and compiler interpret character constants in the same way; escape sequences such as \a are given the values they would have on the target machine.

  Multi-character character constants are interpreted a character at a time, shifting the previous result left by the number of bits per character on the host, and adding the new character. For example, 'ab' on an 8-bit host would be interpreted as 'a' * 256 + 'b'. If there are more characters in the constant than can fit in the widest native integer type on the host, usually a long, the excess characters are ignored and a diagnostic is given.

• Source file inclusion.

  For a discussion on how the preprocessor locates header files, Include Operation.

• Interpretation of the filename resulting from a macro-expanded #include directive.

  See Computed Includes.

• Treatment of a #pragma directive that after macro-expansion results in a standard pragma.

  No macro expansion occurs on any #pragma directive line, so the question does not arise.

  Note that GCC does not yet implement any of the standard pragmas.