|The C Preprocessor: Implementation-defined behavior|
This is how 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 input character set can be specified using the -finput-charset option, while the execution character set may be controlled using the -fexec-charset and -fwide-exec-charset options.
The C and C++ standards allow identifiers to be composed of ‘_’ and the alphanumeric characters. C++ also allows universal character names. C99 and later C standards permit both universal character names and implementation-defined characters.
GCC allows the ‘$’ character in identifiers as an extension for most targets. This is true regardless of the std= switch, since this extension cannot conflict with standards-conforming programs. When preprocessing assembler, however, dollars are not identifier characters by default.
Currently the targets that by default do not permit ‘$’ are AVR, IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX operating system.
You can override the default with -fdollars-in-identifiers or fno-dollars-in-identifiers. See fdollars-in-identifiers.
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 preprocessor and compiler interpret character constants in the same way; i.e. escape sequences such as ‘\a’ are given the values they would have on the target machine.
The compiler evaluates a multi-character character constant a character
at a time, shifting the previous value left by the number of bits per
target character, and then or-ing in the bit-pattern of the new
character truncated to the width of a target character. The final
bit-pattern is given type
int, and is therefore signed,
regardless of whether single characters are signed or not.
If there are more
characters in the constant than would fit in the target
compiler issues a warning, and the excess leading characters are
'ab' for a target with an 8-bit
char would be
interpreted as ‘(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')’, and
'\234a' as ‘(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')’.
For a discussion on how the preprocessor locates header files, Include Operation.
See Computed Includes.
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.