Fresh Fish 4/FreshFish_May-June1994.bin/bbs/gnu/gcc-2.3.3-src.lha/src/amiga/gcc-2.3.3/cpp.info-2

GNU Info File | 1994-02-07 | 47.1 KB | 1,209 lines

This is Info file cpp.info, produced by Makeinfo-1.49 from the input
file cpp.texi.

   This file documents the GNU C Preprocessor.

   Copyright (C) 1987, 1989, 1991, 1992 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.


File: cpp.info,  Node: Swallow Semicolon,  Next: Side Effects,  Prev: Macro Parentheses,  Up: Macro Pitfalls

Swallowing the Semicolon
........................

   Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument `p' says where to find it) across whitespace
characters:

     #define SKIP_SPACES (p, limit)  \
     { register char *lim = (limit); \
       while (p != lim) {            \
         if (*p++ != ' ') {          \
           p--; break; }}}

Here Backslash-Newline is used to split the macro definition, which must
be a single line, so that it resembles the way such C code would be
laid out if not part of a macro definition.

   A call to this macro might be `SKIP_SPACES (p, lim)'.  Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it.  But it looks like a
function call.  So it minimizes confusion if you can use it like a
function call, writing a semicolon afterward, as in `SKIP_SPACES (p,
lim);'

   But this can cause trouble before `else' statements, because the
semicolon is actually a null statement.  Suppose you write

     if (*p != 0)
       SKIP_SPACES (p, lim);
     else ...

The presence of two statements--the compound statement and a null
statement--in between the `if' condition and the `else' makes invalid C
code.

   The definition of the macro `SKIP_SPACES' can be altered to solve
this problem, using a `do ... while' statement.  Here is how:

     #define SKIP_SPACES (p, limit)     \
     do { register char *lim = (limit); \
          while (p != lim) {            \
            if (*p++ != ' ') {          \
              p--; break; }}}           \
     while (0)

   Now `SKIP_SPACES (p, lim);' expands into

     do {...} while (0);

which is one statement.


File: cpp.info,  Node: Side Effects,  Next: Self-Reference,  Prev: Swallow Semicolon,  Up: Macro Pitfalls

Duplication of Side Effects
...........................

   Many C programs define a macro `min', for "minimum", like this:

     #define min(X, Y)  ((X) < (Y) ? (X) : (Y))

   When you use this macro with an argument containing a side effect,
as shown here,

     next = min (x + y, foo (z));

it expands as follows:

     next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));

where `x + y' has been substituted for `X' and `foo (z)' for `Y'.

   The function `foo' is used only once in the statement as it appears
in the program, but the expression `foo (z)' has been substituted twice
into the macro expansion.  As a result, `foo' might be called two times
when the statement is executed.  If it has side effects or if it takes
a long time to compute, the results might not be what you intended.  We
say that `min' is an "unsafe" macro.

   The best solution to this problem is to define `min' in a way that
computes the value of `foo (z)' only once.  The C language offers no
standard way to do this, but it can be done with GNU C extensions as
follows:

     #define min(X, Y)                     \
     ({ typeof (X) __x = (X), __y = (Y);   \
        (__x < __y) ? __x : __y; })

   If you do not wish to use GNU C extensions, the only solution is to
be careful when *using* the macro `min'.  For example, you can
calculate the value of `foo (z)', save it in a variable, and use that
variable in `min':

     #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
     ...
     {
       int tem = foo (z);
       next = min (x + y, tem);
     }

(where we assume that `foo' returns type `int').


File: cpp.info,  Node: Self-Reference,  Next: Argument Prescan,  Prev: Side Effects,  Up: Macro Pitfalls

Self-Referential Macros
.......................

   A "self-referential" macro is one whose name appears in its
definition. A special feature of ANSI Standard C is that the
self-reference is not considered a macro call.  It is passed into the
preprocessor output unchanged.

   Let's consider an example:

     #define foo (4 + foo)

where `foo' is also a variable in your program.

   Following the ordinary rules, each reference to `foo' will expand
into `(4 + foo)'; then this will be rescanned and will expand into `(4
+ (4 + foo))'; and so on until it causes a fatal error (memory full) in
the preprocessor.

   However, the special rule about self-reference cuts this process
short after one step, at `(4 + foo)'.  Therefore, this macro definition
has the possibly useful effect of causing the program to add 4 to the
value of `foo' wherever `foo' is referred to.

   In most cases, it is a bad idea to take advantage of this feature.  A
person reading the program who sees that `foo' is a variable will not
expect that it is a macro as well.  The reader will come across the
identifier `foo' in the program and think its value should be that of
the variable `foo', whereas in fact the value is four greater.

   The special rule for self-reference applies also to "indirect"
self-reference.  This is the case where a macro X expands to use a
macro `y', and the expansion of `y' refers to the macro `x'.  The
resulting reference to `x' comes indirectly from the expansion of `x',
so it is a self-reference and is not further expanded.  Thus, after

     #define x (4 + y)
     #define y (2 * x)

`x' would expand into `(4 + (2 * x))'.  Clear?

   But suppose `y' is used elsewhere, not from the definition of `x'.
Then the use of `x' in the expansion of `y' is not a self-reference
because `x' is not "in progress".  So it does expand.  However, the
expansion of `x' contains a reference to `y', and that is an indirect
self-reference now because `y' is "in progress". The result is that `y'
expands to `(2 * (4 + y))'.

   It is not clear that this behavior would ever be useful, but it is
specified by the ANSI C standard, so you may need to understand it.


File: cpp.info,  Node: Argument Prescan,  Next: Cascaded Macros,  Prev: Self-Reference,  Up: Macro Pitfalls

Separate Expansion of Macro Arguments
.....................................

   We have explained that the expansion of a macro, including the
substituted actual arguments, is scanned over again for macro calls to
be expanded.

   What really happens is more subtle: first each actual argument text
is scanned separately for macro calls.  Then the results of this are
substituted into the macro body to produce the macro expansion, and the
macro expansion is scanned again for macros to expand.

   The result is that the actual arguments are scanned *twice* to expand
macro calls in them.

   Most of the time, this has no effect.  If the actual argument
contained any macro calls, they are expanded during the first scan.
The result therefore contains no macro calls, so the second scan does
not change it. If the actual argument were substituted as given, with
no prescan, the single remaining scan would find the same macro calls
and produce the same results.

   You might expect the double scan to change the results when a
self-referential macro is used in an actual argument of another macro
(*note Self-Reference::.): the self-referential macro would be expanded
once in the first scan, and a second time in the second scan.  But this
is not what happens.  The self-references that do not expand in the
first scan are marked so that they will not expand in the second scan
either.

   The prescan is not done when an argument is stringified or
concatenated. Thus,

     #define str(s) #s
     #define foo 4
     str (foo)

expands to `"foo"'.  Once more, prescan has been prevented from having
any noticeable effect.

   More precisely, stringification and concatenation use the argument as
written, in un-prescanned form.  The same actual argument would be used
in prescanned form if it is substituted elsewhere without
stringification or concatenation.

     #define str(s) #s lose(s)
     #define foo 4
     str (foo)

   expands to `"foo" lose(4)'.

   You might now ask, "Why mention the prescan, if it makes no
difference? And why not skip it and make the preprocessor faster?"  The
answer is that the prescan does make a difference in three special
cases:

   * Nested calls to a macro.

   * Macros that call other macros that stringify or concatenate.

   * Macros whose expansions contain unshielded commas.

   We say that "nested" calls to a macro occur when a macro's actual
argument contains a call to that very macro.  For example, if `f' is a
macro that expects one argument, `f (f (1))' is a nested pair of calls
to `f'.  The desired expansion is made by expanding `f (1)' and
substituting that into the definition of `f'.  The prescan causes the
expected result to happen. Without the prescan, `f (1)' itself would be
substituted as an actual argument, and the inner use of `f' would appear
during the main scan as an indirect self-reference and would not be
expanded.  Here, the prescan cancels an undesirable side effect (in the
medical, not computational, sense of the term) of the special rule for
self-referential macros.

   But prescan causes trouble in certain other cases of nested macro
calls. Here is an example:

     #define foo  a,b
     #define bar(x) lose(x)
     #define lose(x) (1 + (x))

     bar(foo)

We would like `bar(foo)' to turn into `(1 + (foo))', which would then
turn into `(1 + (a,b))'.  But instead, `bar(foo)' expands into
`lose(a,b)', and you get an error because `lose' requires a single
argument.  In this case, the problem is easily solved by the same
parentheses that ought to be used to prevent misnesting of arithmetic
operations:

     #define foo (a,b)
     #define bar(x) lose((x))

   The problem is more serious when the operands of the macro are not
expressions; for example, when they are statements.  Then parentheses
are unacceptable because they would make for invalid C code:

     #define foo { int a, b; ... }

In GNU C you can shield the commas using the `({...})' construct which
turns a compound statement into an expression:

     #define foo ({ int a, b; ... })

   Or you can rewrite the macro definition to avoid such commas:

     #define foo { int a; int b; ... }

   There is also one case where prescan is useful.  It is possible to
use prescan to expand an argument and then stringify it--if you use two
levels of macros.  Let's add a new macro `xstr' to the example shown
above:

     #define xstr(s) str(s)
     #define str(s) #s
     #define foo 4
     xstr (foo)

   This expands into `"4"', not `"foo"'.  The reason for the difference
is that the argument of `xstr' is expanded at prescan (because `xstr'
does not specify stringification or concatenation of the argument).
The result of prescan then forms the actual argument for `str'.  `str'
uses its argument without prescan because it performs stringification;
but it cannot prevent or undo the prescanning already done by `xstr'.


File: cpp.info,  Node: Cascaded Macros,  Next: Newlines in Args,  Prev: Argument Prescan,  Up: Macro Pitfalls

Cascaded Use of Macros
......................

   A "cascade" of macros is when one macro's body contains a reference
to another macro.  This is very common practice.  For example,

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE

   This is not at all the same as defining `TABLESIZE' to be `1020'.
The `#define' for `TABLESIZE' uses exactly the body you specify--in
this case, `BUFSIZE'--and does not check to see whether it too is the
name of a macro.

   It's only when you *use* `TABLESIZE' that the result of its expansion
is checked for more macro names.

   This makes a difference if you change the definition of `BUFSIZE' at
some point in the source file.  `TABLESIZE', defined as shown, will
always expand using the definition of `BUFSIZE' that is currently in
effect:

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE
     #undef BUFSIZE
     #define BUFSIZE 37

Now `TABLESIZE' expands (in two stages) to `37'.


File: cpp.info,  Node: Newlines in Args,  Prev: Cascaded Macros,  Up: Macro Pitfalls

Newlines in Macro Arguments
---------------------------

   Traditional macro processing carries forward all newlines in macro
arguments into the expansion of the macro.  This means that, if some of
the arguments are substituted more than once, or not at all, or out of
order, newlines can be duplicated, lost, or moved around within the
expansion.  If the expansion consists of multiple statements, then the
effect is to distort the line numbers of some of these statements.  The
result can be incorrect line numbers, in error messages or displayed in
a debugger.

   The GNU C preprocessor operating in ANSI C mode adjusts appropriately
for multiple use of an argument--the first use expands all the
newlines, and subsequent uses of the same argument produce no newlines.
But even in this mode, it can produce incorrect line numbering if
arguments are used out of order, or not used at all.

   Here is an example illustrating this problem:

     #define ignore_second_arg(a,b,c) a; c

     ignore_second_arg (foo (),
                        ignored (),
                        syntax error);

The syntax error triggered by the tokens `syntax error' results in an
error message citing line four, even though the statement text comes
from line five.


File: cpp.info,  Node: Conditionals,  Next: Combining Sources,  Prev: Macros,  Up: Top

Conditionals
============

   In a macro processor, a "conditional" is a command that allows a part
of the program to be ignored during compilation, on some conditions. In
the C preprocessor, a conditional can test either an arithmetic
expression or whether a name is defined as a macro.

   A conditional in the C preprocessor resembles in some ways an `if'
statement in C, but it is important to understand the difference between
them.  The condition in an `if' statement is tested during the execution
of your program.  Its purpose is to allow your program to behave
differently from run to run, depending on the data it is operating on.
The condition in a preprocessor conditional command is tested when your
program is compiled. Its purpose is to allow different code to be
included in the program depending on the situation at the time of
compilation.

* Menu:

* Uses: Conditional Uses.       What conditionals are for.
* Syntax: Conditional Syntax.   How conditionals are written.
* Deletion: Deleted Code.       Making code into a comment.
* Macros: Conditionals-Macros.  Why conditionals are used with macros.
* Assertions::                How and why to use assertions.
* Errors: #error Command.       Detecting inconsistent compilation parameters.


File: cpp.info,  Node: Conditional Uses,  Next: Conditional Syntax,  Up: Conditionals

Why Conditionals are Used
-------------------------

   Generally there are three kinds of reason to use a conditional.

   * A program may need to use different code depending on the machine
     or operating system it is to run on.  In some cases the code for
     one operating system may be erroneous on another operating system;
     for example, it might refer to library routines that do not exist
     on the other system.  When this happens, it is not enough to avoid
     executing the invalid code: merely having it in the program makes
     it impossible to link the program and run it.  With a preprocessor
     conditional, the offending code can be effectively excised from
     the program when it is not valid.

   * You may want to be able to compile the same source file into two
     different programs.  Sometimes the difference between the programs
     is that one makes frequent time-consuming consistency checks on its
     intermediate data while the other does not.

   * A conditional whose condition is always false is a good way to
     exclude code from the program but keep it as a sort of comment for
     future reference.

   Most simple programs that are intended to run on only one machine
will not need to use preprocessor conditionals.


File: cpp.info,  Node: Conditional Syntax,  Next: Deleted Code,  Prev: Conditional Uses,  Up: Conditionals

Syntax of Conditionals
----------------------

   A conditional in the C preprocessor begins with a "conditional
command": `#if', `#ifdef' or `#ifndef'. *Note Conditionals-Macros::,
for information on `#ifdef' and `#ifndef'; only `#if' is explained here.

* Menu:

* If: #if Command.     Basic conditionals using `#if' and `#endif'.
* Else: #else Command. Including some text if the condition fails.
* Elif: #elif Command. Testing several alternative possibilities.


File: cpp.info,  Node: #if Command,  Next: #else Command,  Up: Conditional Syntax

The `#if' Command
.................

   The `#if' command in its simplest form consists of

     #if EXPRESSION
     CONTROLLED TEXT
     #endif /* EXPRESSION */

   The comment following the `#endif' is not required, but it is a good
practice because it helps people match the `#endif' to the
corresponding `#if'.  Such comments should always be used, except in
short conditionals that are not nested.  In fact, you can put anything
at all after the `#endif' and it will be ignored by the GNU C
preprocessor, but only comments are acceptable in ANSI Standard C.

   EXPRESSION is a C expression of integer type, subject to stringent
restrictions.  It may contain

   * Integer constants, which are all regarded as `long' or `unsigned
     long'.

   * Character constants, which are interpreted according to the
     character set and conventions of the machine and operating system
     on which the preprocessor is running.  The GNU C preprocessor uses
     the C data type `char' for these character constants; therefore,
     whether some character codes are negative is determined by the C
     compiler used to compile the preprocessor.  If it treats `char' as
     signed, then character codes large enough to set the sign bit will
     be considered negative; otherwise, no character code is considered
     negative.

   * Arithmetic operators for addition, subtraction, multiplication,
     division, bitwise operations, shifts, comparisons, and `&&' and
     `||'.

   * Identifiers that are not macros, which are all treated as zero(!).

   * Macro calls.  All macro calls in the expression are expanded before
     actual computation of the expression's value begins.

   Note that `sizeof' operators and `enum'-type values are not allowed.
`enum'-type values, like all other identifiers that are not taken as
macro calls and expanded, are treated as zero.

   The CONTROLLED TEXT inside of a conditional can include preprocessor
commands.  Then the commands inside the conditional are obeyed only if
that branch of the conditional succeeds.  The text can also contain
other conditional groups.  However, the `#if' and `#endif' commands
must balance.


File: cpp.info,  Node: #else Command,  Next: #elif Command,  Prev: #if Command,  Up: Conditional Syntax

The `#else' Command
...................

   The `#else' command can be added to a conditional to provide
alternative text to be used if the condition is false.  This is what it
looks like:

     #if EXPRESSION
     TEXT-IF-TRUE
     #else /* Not EXPRESSION */
     TEXT-IF-FALSE
     #endif /* Not EXPRESSION */

   If EXPRESSION is nonzero, and thus the TEXT-IF-TRUE is active, then
`#else' acts like a failing conditional and the TEXT-IF-FALSE is
ignored.  Contrariwise, if the `#if' conditional fails, the
TEXT-IF-FALSE is considered included.


File: cpp.info,  Node: #elif Command,  Prev: #else Command,  Up: Conditional Syntax

The `#elif' Command
...................

   One common case of nested conditionals is used to check for more
than two possible alternatives.  For example, you might have

     #if X == 1
     ...
     #else /* X != 1 */
     #if X == 2
     ...
     #else /* X != 2 */
     ...
     #endif /* X != 2 */
     #endif /* X != 1 */

   Another conditional command, `#elif', allows this to be abbreviated
as follows:

     #if X == 1
     ...
     #elif X == 2
     ...
     #else /* X != 2 and X != 1*/
     ...
     #endif /* X != 2 and X != 1*/

   `#elif' stands for "else if".  Like `#else', it goes in the middle
of a `#if'-`#endif' pair and subdivides it; it does not require a
matching `#endif' of its own.  Like `#if', the `#elif' command includes
an expression to be tested.

   The text following the `#elif' is processed only if the original
`#if'-condition failed and the `#elif' condition succeeds. More than
one `#elif' can go in the same `#if'-`#endif' group.  Then the text
after each `#elif' is processed only if the `#elif' condition succeeds
after the original `#if' and any previous `#elif' commands within it
have failed.  `#else' is equivalent to `#elif 1', and `#else' is
allowed after any number of `#elif' commands, but `#elif' may not follow
`#else'.


File: cpp.info,  Node: Deleted Code,  Next: Conditionals-Macros,  Prev: Conditional Syntax,  Up: Conditionals

Keeping Deleted Code for Future Reference
-----------------------------------------

   If you replace or delete a part of the program but want to keep the
old code around as a comment for future reference, the easy way to do
this is to put `#if 0' before it and `#endif' after it.

   This works even if the code being turned off contains conditionals,
but they must be entire conditionals (balanced `#if' and `#endif').


File: cpp.info,  Node: Conditionals-Macros,  Next: Assertions,  Prev: Deleted Code,  Up: Conditionals

Conditionals and Macros
-----------------------

   Conditionals are useful in connection with macros or assertions,
because those are the only ways that an expression's value can vary
from one compilaton to another.  A `#if' command whose expression uses
no macros or assertions is equivalent to `#if 1' or `#if 0'; you might
as well determine which one, by computing the value of the expression
yourself, and then simplify the program.

   For example, here is a conditional that tests the expression
`BUFSIZE == 1020', where `BUFSIZE' must be a macro.

     #if BUFSIZE == 1020
       printf ("Large buffers!\n");
     #endif /* BUFSIZE is large */

   (Programmers often wish they could test the size of a variable or
data type in `#if', but this does not work.  The preprocessor does not
understand `sizeof', or typedef names, or even the type keywords such
as `int'.)

   The special operator `defined' is used in `#if' expressions to test
whether a certain name is defined as a macro.  Either `defined NAME' or
`defined (NAME)' is an expression whose value is 1 if NAME is defined
as macro at the current point in the program, and 0 otherwise.  For the
`defined' operator it makes no difference what the definition of the
macro is; all that matters is whether there is a definition.  Thus, for
example,

     #if defined (vax) || defined (ns16000)

would include the following code if either of the names `vax' and
`ns16000' is defined as a macro.  You can test the same condition using
assertions (*note Assertions::.), like this:

     #if #cpu (vax) || #cpu (ns16000)

   If a macro is defined and later undefined with `#undef', subsequent
use of the `defined' operator returns 0, because the name is no longer
defined.  If the macro is defined again with another `#define',
`defined' will recommence returning 1.

   Conditionals that test just the definedness of one name are very
common, so there are two special short conditional commands for this
case.

`#ifdef NAME'
     is equivalent to `#if defined (NAME)'.

`#ifndef NAME'
     is equivalent to `#if ! defined (NAME)'.

   Macro definitions can vary between compilations for several reasons.

   * Some macros are predefined on each kind of machine.  For example,
     on a Vax, the name `vax' is a predefined macro.  On other
     machines, it would not be defined.

   * Many more macros are defined by system header files.  Different
     systems and machines define different macros, or give them
     different values.  It is useful to test these macros with
     conditionals to avoid using a system feature on a machine where it
     is not implemented.

   * Macros are a common way of allowing users to customize a program
     for different machines or applications.  For example, the macro
     `BUFSIZE' might be defined in a configuration file for your
     program that is included as a header file in each source file.  You
     would use `BUFSIZE' in a preprocessor conditional in order to
     generate different code depending on the chosen configuration.

   * Macros can be defined or undefined with `-D' and `-U' command
     options when you compile the program.  You can arrange to compile
     the same source file into two different programs by choosing a
     macro name to specify which program you want, writing conditionals
     to test whether or how this macro is defined, and then controlling
     the state of the macro with compiler command options. *Note
     Invocation::.

   Assertions are usually predefined, but can be defined with
preprocessor commands or command-line options.


File: cpp.info,  Node: Assertions,  Next: #error Command,  Prev: Conditionals-Macros,  Up: Conditionals

Assertions
----------

   "Assertions" are a more systematic 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 preprocessor commands or command-line options.

   The macros traditionally used to describe the type of target are not
classified in any way according to which question they answer; they may
indicate a hardware architecture, a particular hardware model, an
operating system, a particular version of an operating system, or
specific configuration options.  These are jumbled together in a single
namespace.  In contrast, each assertion consists of a named question and
an answer.  The question is usually called the "predicate". An
assertion looks like this:

     #PREDICATE (ANSWER)

You must use a properly formed identifier for PREDICATE.  The value of
ANSWER can be any sequence of words; all characters are significant
except for leading and trailing whitespace, and differences in internal
whitespace sequences are ignored.  Thus, `x + y' is different from
`x+y' but equivalent to `x + y'.  `)' is not allowed in an answer.

   Here is a conditional to test whether the answer ANSWER is asserted
for the predicate PREDICATE:

     #if #PREDICATE (ANSWER)

There may be more than one answer asserted for a given predicate.  If
you omit the answer, you can test whether *any* answer is asserted for
PREDICATE:

     #if #PREDICATE

   Most of the time, the assertions you test will be predefined
assertions. GNU C provides three predefined predicates: `system', `cpu',
and `machine'.  `system' is for assertions about the type of software,
`cpu' describes the type of computer architecture, and `machine' gives
more information about the computer.  For example, on a GNU system, the
following assertions would be true:

     #system (gnu)
     #system (mach)
     #system (mach 3)
     #system (mach 3.SUBVERSION)
     #system (hurd)
     #system (hurd VERSION)

and perhaps others.  On a Unix system, you would find `#system (unix)'
and either `#system (unix bsd)' or `#system (unix sysv)', with possible
version numbers following.  The alternatives with more or less version
information let you ask more or less detailed questions about the type
of system software.

   *Portability note:* Many Unix C compilers provide only one answer
for the `system' assertion: `#system (unix)', if they support
assertions at all.  This is less than useful.

   An assertion with a multi-word answer is completely different from
several assertions with individual single-word answers.  For example,
the presence of `system (mach 3.0)' does not mean that `system (3.0)'
is true. It also does not directly imply `system (mach)', but in GNU C,
that last will normally be asserted as well.

   You can create assertions within a C program using `#assert', like
this:

     #assert PREDICATE (ANSWER)

(Note the absence of a `#' before PREDICATE.)

   Each time you do this, you assert a new true answer for PREDICATE.
Asserting one answer does not invalidate previously asserted answers;
they all remain true.  The only way to remove an assertion is with
`#unassert'.  `#unassert' has the same syntax as `#assert'.  You can
also remove all assertions about PREDICATE like this:

     #unassert PREDICATE

   You can also add or cancel assertions using command options when you
run `gcc' or `cpp'.  *Note Invocation::.


File: cpp.info,  Node: #error Command,  Prev: Assertions,  Up: Conditionals

The `#error' and `#warning' Commands
------------------------------------

   The command `#error' causes the preprocessor to report a fatal
error.  The rest of the line that follows `#error' is used as the error
message.

   You would use `#error' inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a Vax, you might write

     #ifdef vax
     #error Won't work on Vaxen.  See comments at get_last_object.
     #endif

*Note Nonstandard Predefined::, for why this works.

   If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with `#error'.  For example,

     #if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
         || HASH_TABLE_SIZE % 5 == 0
     #error HASH_TABLE_SIZE should not be divisible by a small prime
     #endif

   The command `#warning' is like the command `#error', but causes the
preprocessor to issue a warning and continue preprocessing.  The rest of
the line that follows `#warning' is used as the warning message.

   You might use `#warning' in obsolete header files, with a message
directing the user to the header file which should be used instead.


File: cpp.info,  Node: Combining Sources,  Next: Other Commands,  Prev: Conditionals,  Up: Top

Combining Source Files
======================

   One of the jobs of the C preprocessor is to inform the C compiler of
where each line of C code came from: which source file and which line
number.

   C code can come from multiple source files if you use `#include';
both `#include' and the use of conditionals and macros can cause the
line number of a line in the preprocessor output to be different from
the line's number in the original source file.  You will appreciate the
value of making both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source file.

   The C preprocessor builds on this feature by offering a command by
which you can control the feature explicitly.  This is useful when a
file for input to the C preprocessor is the output from another program
such as the `bison' parser generator, which operates on another file
that is the true source file.  Parts of the output from `bison' are
generated from scratch, other parts come from a standard parser file.
The rest are copied nearly verbatim from the source file, but their
line numbers in the `bison' output are not the same as their original
line numbers. Naturally you would like compiler error messages and
symbolic debuggers to know the original source file and line number of
each line in the `bison' input.

   `bison' arranges this by writing `#line' commands into the output
file.  `#line' is a command that specifies the original line number and
source file name for subsequent input in the current preprocessor input
file.  `#line' has three variants:

`#line LINENUM'
     Here LINENUM is a decimal integer constant.  This specifies that
     the line number of the following line of input, in its original
     source file, was LINENUM.

`#line LINENUM FILENAME'
     Here LINENUM is a decimal integer constant and FILENAME is a
     string constant.  This specifies that the following line of input
     came originally from source file FILENAME and its line number there
     was LINENUM.  Keep in mind that FILENAME is not just a file name;
     it is surrounded by doublequote characters so that it looks like a
     string constant.

`#line ANYTHING ELSE'
     ANYTHING ELSE is checked for macro calls, which are expanded. The
     result should be a decimal integer constant followed optionally by
     a string constant, as described above.

   `#line' commands alter the results of the `__FILE__' and `__LINE__'
predefined macros from that point on.  *Note Standard Predefined::.

   The output of the preprocessor (which is the input for the rest of
the compiler) contains commands that look much like `#line' commands.
They start with just `#' instead of `#line', but this is followed by a
line number and file name as in `#line'.  *Note Output::.


File: cpp.info,  Node: Other Commands,  Next: Output,  Prev: Combining Sources,  Up: Top

Miscellaneous Preprocessor Commands
===================================

   This section describes three additional preprocessor commands.  They
are not very useful, but are mentioned for completeness.

   The "null command" consists of a `#' followed by a Newline, with
only whitespace (including comments) in between.  A null command is
understood as a preprocessor command but has no effect on the
preprocessor output.  The primary significance of the existence of the
null command is that an input line consisting of just a `#' will
produce no output, rather than a line of output containing just a `#'.
Supposedly some old C programs contain such lines.

   The ANSI standard specifies that the `#pragma' command has an
arbitrary, implementation-defined effect.  In the GNU C preprocessor,
`#pragma' commands are ignored, except for `#pragma once' (*note
Once-Only::.).

   The `#ident' command is supported for compatibility with certain
other systems.  It is followed by a line of text.  On some systems, the
text is copied into a special place in the object file; on most systems,
the text is ignored and this command has no effect.  Typically `#ident'
is only used in header files supplied with those systems where it is
meaningful.


File: cpp.info,  Node: Output,  Next: Invocation,  Prev: Other Commands,  Up: Top

C Preprocessor Output
=====================

   The output from the C preprocessor looks much like the input, except
that all preprocessor command lines have been replaced with blank lines
and all comments with spaces.  Whitespace within a line is not altered;
however, a space is inserted after the expansions of most macro calls.

   Source file name and line number information is conveyed by lines of
the form

     # LINENUM FILENAME FLAGS

which are inserted as needed into the middle of the input (but never
within a string or character constant).  Such a line means that the
following line originated in file FILENAME at line LINENUM.

   After the file name comes zero or more flags, which are `1', `2' or
`3'.  If there are multiple flags, spaces separate them.  Here is what
the flags mean:

`1'
     This indicates the start of a new file.

`2'
     This indicates returning to a file (after having included another
     file).

`3'
     This indicates that the following text comes from a system header
     file, so certain warnings should be suppressed.


File: cpp.info,  Node: Invocation,  Next: Concept Index,  Prev: Output,  Up: Top

Invoking the C Preprocessor
===========================

   Most often when you use the C preprocessor you will not have to
invoke it explicitly: the C compiler will do so automatically.
However, the preprocessor is sometimes useful individually.

   The C preprocessor expects two file names as arguments, INFILE and
OUTFILE.  The preprocessor reads INFILE together with any other files
it specifies with `#include'.  All the output generated by the combined
input files is written in OUTFILE.

   Either INFILE or OUTFILE may be `-', which as INFILE means to read
from standard input and as OUTFILE means to write to standard output.
Also, if OUTFILE or both file names are omitted, the standard output
and standard input are used for the omitted file names.

   Here is a table of command options accepted by the C preprocessor.
These options can also be given when compiling a C program; they are
passed along automatically to the preprocessor when it is invoked by the
compiler.

`-P'
     Inhibit generation of `#'-lines with line-number information in
     the output from the preprocessor (*note Output::.).  This might be
     useful when running the preprocessor on something that is not C
     code and will be sent to a program which might be confused by the
     `#'-lines.

`-C'
     Do not discard comments: pass them through to the output file.
     Comments appearing in arguments of a macro call will be copied to
     the output before the expansion of the macro call.

`-traditional'
     Try to imitate the behavior of old-fashioned C, as opposed to ANSI
     C.

        * Traditional macro expansion pays no attention to singlequote
          or doublequote characters; macro argument symbols are
          replaced by the argument values even when they appear within
          apparent string or character constants.

        * Traditionally, it is permissable for a macro expansion to end
          in the middle of a string or character constant.  The
          constant continues into the text surrounding the macro call.

        * However, traditionally the end of the line terminates a
          string or character constant, with no error.

        * In traditional C, a comment is equivalent to no text at all.
          (In ANSI C, a comment counts as whitespace.)

        * Traditional C does not have the concept of a "preprocessing
          number". It considers `1.0e+4' to be three tokens: `1.0e',
          `+', and `4'.

        * A macro is not suppressed within its own definition, in
          traditional C. Thus, any macro that is used recursively
          inevitably causes an error.

        * The character `#' has no special meaning within a macro
          definition in traditional C.

        * In traditional C, the text at the end of a macro expansion
          can run together with the text after the macro call, to
          produce a single token. (This is impossible in ANSI C.)

        * Traditionally, `\' inside a macro argument suppresses the
          syntactic significance of the following character.

`-trigraphs'
     Process ANSI standard trigraph sequences.  These are
     three-character sequences, all starting with `??', that are
     defined by ANSI C to stand for single characters.  For example,
     `??/' stands for `\', so `'??/n'' is a character constant for a
     newline. Strictly speaking, the GNU C preprocessor does not
     support all programs in ANSI Standard C unless `-trigraphs' is
     used, but if you ever notice the difference it will be with relief.

     You don't want to know any more about trigraphs.

`-pedantic'
     Issue warnings required by the ANSI C standard in certain cases
     such as when text other than a comment follows `#else' or `#endif'.

`-pedantic-errors'
     Like `-pedantic', except that errors are produced rather than
     warnings.

`-Wtrigraphs'
     Warn if any trigraphs are encountered (assuming they are enabled).

`-Wcomment'
     Warn whenever a comment-start sequence `/*' appears in a comment.

`-Wall'
     Requests both `-Wtrigraphs' and `-Wcomment' (but not
     `-Wtraditional').

`-Wtraditional'
     Warn about certain constructs that behave differently in
     traditional and ANSI C.

`-I DIRECTORY'
     Add the directory DIRECTORY to the end of the list of directories
     to be searched for header files (*note Include Syntax::.). This
     can be used to override a system header file, substituting your
     own version, since these directories are searched before the system
     header file directories.  If you use more than one `-I' option,
     the directories are scanned in left-to-right order; the standard
     system directories come after.

`-I-'
     Any directories specified with `-I' options before the `-I-'
     option are searched only for the case of `#include "FILE"'; they
     are not searched for `#include <FILE>'.

     If additional directories are specified with `-I' options after
     the `-I-', these directories are searched for all `#include'
     commands.

     In addition, the `-I-' option inhibits the use of the current
     directory as the first search directory for `#include "FILE"'.
     Therefore, the current directory is searched only if it is
     requested explicitly with `-I.'.  Specifying both `-I-' and `-I.'
     allows you to control precisely which directories are searched
     before the current one and which are searched after.

`-nostdinc'
     Do not search the standard system directories for header files.
     Only the directories you have specified with `-I' options (and the
     current directory, if appropriate) are searched.

`-nostdinc++'
     Do not search for header files in the C++-specific standard
     directories, but do still search the other standard directories.
     (This option is used when building `libg++'.)

`-D NAME'
     Predefine NAME as a macro, with definition `1'.

`-D NAME=DEFINITION'
     Predefine NAME as a macro, with definition DEFINITION. There are
     no restrictions on the contents of DEFINITION, but if you are
     invoking the preprocessor from a shell or shell-like program you
     may need to use the shell's quoting syntax to protect characters
     such as spaces that have a meaning in the shell syntax.  If you
     use more than one `-D' for the same NAME, the rightmost definition
     takes effect.

`-U NAME'
     Do not predefine NAME.  If both `-U' and `-D' are specified for
     one name, the `-U' beats the `-D' and the name is not predefined.

`-A PREDICATE(ANSWER)'
     Make an assertion with the predicate PREDICATE and answer ANSWER.
     *Note Assertions::.

     You can use `-A-' to disable all predefined assertions; it also
     undefines all predefined macros that identify the type of target
     system.

`-dM'
     Instead of outputting the result of preprocessing, output a list of
     `#define' commands for all the macros defined during the execution
     of the preprocessor, including predefined macros.  This gives you
     a way of finding out what is predefined in your version of the
     preprocessor; assuming you have no file `foo.h', the command

          touch foo.h; cpp -dM foo.h

     will show the values of any predefined macros.

`-dD'
     Like `-dM' except in two respects: it does *not* include the
     predefined macros, and it outputs *both* the `#define' commands
     and the result of preprocessing.  Both kinds of output go to the
     standard output file.

`-M'
     Instead of outputting the result of preprocessing, output a rule
     suitable for `make' describing the dependencies of the main source
     file.  The preprocessor outputs one `make' rule containing the
     object file name for that source file, a colon, and the names of
     all the included files.  If there are many included files then the
     rule is split into several lines using `\'-newline.

     This feature is used in automatic updating of makefiles.

`-MM'
     Like `-M' but mention only the files included with `#include
     "FILE"'.  System header files included with `#include <FILE>' are
     omitted.

`-MD'
     Like `-M' but the dependency information is written to files with
     names made by replacing `.c' with `.d' at the end of the input
     file names.  This is in addition to compiling the file as
     specified--`-MD' does not inhibit ordinary compilation the way
     `-M' does.

     In Mach, you can use the utility `md' to merge the `.d' files into
     a single dependency file suitable for using with the `make'
     command.

`-MMD'
     Like `-MD' except mention only user header files, not system
     header files.

`-H'
     Print the name of each header file used, in addition to other
     normal activities.

`-imacros FILE'
     Process FILE as input, discarding the resulting output, before
     processing the regular input file.  Because the output generated
     from FILE is discarded, the only effect of `-imacros FILE' is to
     make the macros defined in FILE available for use in the main
     input.

`-include FILE'
     Process FILE as input, and include all the resulting output,
     before processing the regular input file.

`-lang-c'
`-lang-c++'
`-lang-objc'
`-lang-objc++'
     Specify the source language.  `-lang-c++' makes the preprocessor
     handle C++ comment syntax, and includes extra default include
     directories for C++, and `-lang-objc' enables the Objective C
     `#import' command.  `-lang-c' explicitly turns off both of these
     extensions, and `-lang-objc++' enables both.

     These options are generated by the compiler driver `gcc', but not
     passed from the `gcc' command line.

`-lint'
     Look for commands to the program checker `lint' embedded in
     comments, and emit them preceded by `#pragma lint'.  For example,
     the comment `/* NOTREACHED */' becomes `#pragma lint NOTREACHED'.

     This option is available only when you call `cpp' directly; `gcc'
     will not pass it from its command line.

`-$'
     Forbid the use of `$' in identifiers.  This is required for ANSI
     conformance.  `gcc' automatically supplies this option to the
     preprocessor if you specify `-ansi', but `gcc' doesn't recognize
     the `-$' option itself--to use it without the other effects of
     `-ansi', you must call the preprocessor directly.


File: cpp.info,  Node: Concept Index,  Next: Index,  Prev: Invocation,  Up: Top

Concept Index
*************

* Menu:

* assertions:                           Assertions.
* assertions, undoing:                  Assertions.
* cascaded macros:                      Cascaded Macros.
* commands:                             Commands.
* concatenation:                        Concatenation.
* conditionals:                         Conditionals.
* header file:                          Header Files.
* inheritance:                          Inheritance.
* line control:                         Combining Sources.
* macro body uses macro:                Cascaded Macros.
* null command:                         Other Commands.
* options:                              Invocation.
* output format:                        Output.
* overriding a header file:             Inheritance.
* predefined macros:                    Predefined.
* predicates:                           Assertions.
* preprocessor commands:                Commands.
* redefining macros:                    Redefining.
* repeated inclusion:                   Once-Only.
* retracting assertions:                Assertions.
* self-reference:                       Self-Reference.
* semicolons (after macro calls):       Swallow Semicolon.
* side effects (in macro arguments):    Side Effects.
* stringification:                      Stringification.
* testing predicates:                   Assertions.
* unassert:                             Assertions.
* undefining macros:                    Undefining.
* unsafe macros:                        Side Effects.