11. Implementation Details

Here we document details of how the preprocessor's implementation affects its user-visible behavior. You should try to avoid undue reliance on behaviour described here, as it is possible that it will change subtly in future implementations.

Also documented here are obsolete features and changes from previous versions of GNU CPP.

11.1 Implementation-defined behavior
11.2 Implementation limits
11.3 Obsolete Features
11.4 Differences from previous versions

11.1 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, 2.2 Include Operation.

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

  See section 2.5 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.

11.2 Implementation limits

GNU CPP has a small number of internal limits. This section lists the limits which the C standard requires to be no lower than some minimum, and all the others we are aware of. We intend there to be as few limits as possible. If you encounter an undocumented or inconvenient limit, please report that to us as a bug. (See the section on reporting bugs in the GCC manual.)

Where we say something is limited only by available memory, that means that internal data structures impose no intrinsic limit, and space is allocated with malloc or equivalent. The actual limit will therefore depend on many things, such as the size of other things allocated by the compiler at the same time, the amount of memory consumed by other processes on the same computer, etc.

• Nesting levels of `#include' files.

  We impose an arbitrary limit of 200 levels, to avoid runaway recursion. The standard requires at least 15 levels.

• Nesting levels of conditional inclusion.

  The C standard mandates this be at least 63. GNU CPP is limited only by available memory.

• Levels of parenthesised expressions within a full expression.

  The C standard requires this to be at least 63. In preprocessor conditional expressions, it is limited only by available memory.

• Significant initial characters in an identifier or macro name.

  The preprocessor treats all characters as significant. The C standard requires only that the first 63 be significant.

• Number of macros simultaneously defined in a single translation unit.

  The standard requires at least 4095 be possible. GNU CPP is limited only by available memory.

• Number of parameters in a macro definition and arguments in a macro call.

  We allow USHRT_MAX, which is no smaller than 65,535. The minimum required by the standard is 127.

• Number of characters on a logical source line.

  The C standard requires a minimum of 4096 be permitted. GNU CPP places no limits on this, but you may get incorrect column numbers reported in diagnostics for lines longer than 65,535 characters.

• Maximum size of a source file.

  The standard does not specify any lower limit on the maximum size of a source file. GNU cpp maps files into memory, so it is limited by the available address space. This is generally at least two gigabytes. Depending on the operating system, the size of physical memory may or may not be a limitation.

11.3 Obsolete Features

GNU CPP has a number of features which are present mainly for compatibility with older programs. We discourage their use in new code. In some cases, we plan to remove the feature in a future version of GCC.

11.3.1 Assertions
11.3.2 Obsolete once-only headers
11.3.3 Miscellaneous obsolete features

11.3.1 Assertions

Assertions are a deprecated alternative to macros in writing conditionals to test what sort of computer or system the compiled program will run on. Assertions are usually predefined, but you can define them with preprocessing directives or command-line options.

Assertions were intended to provide a more systematic way to describe the compiler's target system. However, in practice they are just as unpredictable as the system-specific predefined macros. In addition, they are not part of any standard, and only a few compilers support them. Therefore, the use of assertions is less portable than the use of system-specific predefined macros. We recommend you do not use them at all.

An assertion looks like this:

#predicate (answer)

predicate must be a single identifier. answer can be any sequence of tokens; all characters are significant except for leading and trailing whitespace, and differences in internal whitespace sequences are ignored. (This is similar to the rules governing macro redefinition.) Thus, (x + y) is different from (x+y) but equivalent to ( x + y ). Parentheses do not nest inside an answer.

To test an assertion, you write it in an `#if'. For example, this conditional succeeds if either vax or ns16000 has been asserted as an answer for machine.

#if #machine (vax) || #machine (ns16000)

You can test whether any answer is asserted for a predicate by omitting the answer in the conditional:

#if #machine

Assertions are made with the `#assert' directive. Its sole argument is the assertion to make, without the leading `#' that identifies assertions in conditionals.

#assert predicate (answer)

You may make several assertions with the same predicate and different answers. Subsequent assertions do not override previous ones for the same predicate. All the answers for any given predicate are simultaneously true.

Assertions can be cancelled with the the `#unassert' directive. It has the same syntax as `#assert'. In that form it cancels only the answer which was specified on the `#unassert' line; other answers for that predicate remain true. You can cancel an entire predicate by leaving out the answer:

#unassert predicate

In either form, if no such assertion has been made, `#unassert' has no effect.

You can also make or cancel assertions using command line options. See section 12. Invocation.

11.3.2 Obsolete once-only headers

GNU CPP supports two more ways of indicating that a header file should be read only once. Neither one is as portable as a wrapper `#ifndef', and we recommend you do not use them in new programs.

In the Objective-C language, there is a variant of `#include' called `#import' which includes a file, but does so at most once. If you use `#import' instead of `#include', then you don't need the conditionals inside the header file to prevent multiple inclusion of the contents. GCC permits the use of `#import' in C and C++ as well as Objective-C. However, it is not in standard C or C++ and should therefore not be used by portable programs.

`#import' is not a well designed feature. It requires the users of a header file to know that it should only be included once. It is much better for the header file's implementor to write the file so that users don't need to know this. Using a wrapper `#ifndef' accomplishes this goal.

In the present implementation, a single use of `#import' will prevent the file from ever being read again, by either `#import' or `#include'. You should not rely on this; do not use both `#import' and `#include' to refer to the same header file.

Another way to prevent a header file from being included more than once is with the `#pragma once' directive. If `#pragma once' is seen when scanning a header file, that file will never be read again, no matter what.

`#pragma once' does not have the problems that `#import' does, but it is not recognized by all preprocessors, so you cannot rely on it in a portable program.

11.3.3 Miscellaneous obsolete features

Here are a few more obsolete features.

• Attempting to paste two tokens which together do not form a valid preprocessing token.

  The preprocessor currently warns about this and outputs the two tokens adjacently, which is probably the behavior the programmer intends. It may not work in future, though.

  Most of the time, when you get this warning, you will find that `##' is being used superstitiously, to guard against whitespace appearing between two tokens. It is almost always safe to delete the `##'.

• #pragma poison

  This is the same as #pragma GCC poison. The version without the GCC prefix is deprecated. See section 7. Pragmas.

• Multi-line string constants

  GCC currently allows a string constant to extend across multiple logical lines of the source file. This extension is deprecated and will be removed in a future version of GCC. Such string constants are already rejected in all directives apart from `#define'.

  Instead, make use of ISO C concatenation of adjacent string literals, or use `\n' followed by a backslash-newline.

11.4 Differences from previous versions

This section details behavior which has changed from previous versions of GNU CPP. We do not plan to change it again in the near future, but we do not promise not to, either.

The "previous versions" discussed here are 2.95 and before. The behavior of GCC 3.0 is mostly the same as the behavior of the widely used 2.96 and 2.97 development snapshots. Where there are differences, they generally represent bugs in the snapshots.

• Order of evaluation of `#' and `##' operators

  The standard does not specify the order of evaluation of a chain of `##' operators, nor whether `#' is evaluated before, after, or at the same time as `##'. You should therefore not write any code which depends on any specific ordering. It is possible to guarantee an ordering, if you need one, by suitable use of nested macros.

  An example of where this might matter is pasting the arguments `1', `e' and `-2'. This would be fine for left-to-right pasting, but right-to-left pasting would produce an invalid token `e-2'.

  GCC 3.0 evaluates `#' and `##' at the same time and strictly left to right. Older versions evaluated all `#' operators first, then all `##' operators, in an unreliable order.

• The form of whitespace betwen tokens in preprocessor output

  See section 9. Preprocessor Output, for the current textual format. This is also the format used by stringification. Normally, the preprocessor communicates tokens directly to the compiler's parser, and whitespace does not come up at all.

  Older versions of GCC preserved all whitespace provided by the user and inserted lots more whitespace of their own, because they could not accurately predict when extra spaces were needed to prevent accidental token pasting.

• Optional argument when invoking rest argument macros

  As an extension, GCC permits you to omit the variable arguments entirely when you use a variable argument macro. This is forbidden by the 1999 C standard, and will provoke a pedantic warning with GCC 3.0. Previous versions accepted it silently.

• `##' swallowing preceding text in rest argument macros

  Formerly, in a macro expansion, if `##' appeared before a variable arguments parameter, and the set of tokens specified for that argument in the macro invocation was empty, previous versions of GNU CPP would back up and remove the preceding sequence of non-whitespace characters (not the preceding token). This extension is in direct conflict with the 1999 C standard and has been drastically pared back.

  In the current version of the preprocessor, if `##' appears between a comma and a variable arguments parameter, and the variable argument is omitted entirely, the comma will be removed from the expansion. If the variable argument is empty, or the token before `##' is not a comma, then `##' behaves as a normal token paste.

• Traditional mode and GNU extensions

  Traditional mode used to be implemented in the same program as normal preprocessing. Therefore, all the GNU extensions to the preprocessor were still available in traditional mode. It is now a separate program and does not implement any of the GNU extensions, except for a partial implementation of assertions. Even those may be removed in a future release.