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* gcc: (gcc).                  The GNU Compiler Collection.
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   This file documents the use of the GNU compilers.

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File: gcc.info,  Node: C Dialect Options,  Next: C++ Dialect Options,  Prev: Invoking G++,  Up: Invoking GCC

Options Controlling C Dialect
=============================

   The following options control the dialect of C (or languages derived
from C, such as C++ and Objective-C) that the compiler accepts:

`-ansi'
     In C mode, support all ISO C89 programs.  In C++ mode, remove GNU
     extensions that conflict with ISO C++.

     This turns off certain features of GCC that are incompatible with
     ISO C89 (when compiling C code), or of standard C++ (when
     compiling C++ code), such as the `asm' and `typeof' keywords, and
     predefined macros such as `unix' and `vax' that identify the type
     of system you are using.  It also enables the undesirable and
     rarely used ISO trigraph feature.  For the C compiler, it disables
     recognition of C++ style `//' comments as well as the `inline'
     keyword.

     The alternate keywords `__asm__', `__extension__', `__inline__'
     and `__typeof__' continue to work despite `-ansi'.  You would not
     want to use them in an ISO C program, of course, but it is useful
     to put them in header files that might be included in compilations
     done with `-ansi'.  Alternate predefined macros such as `__unix__'
     and `__vax__' are also available, with or without `-ansi'.

     The `-ansi' option does not cause non-ISO programs to be rejected
     gratuitously.  For that, `-pedantic' is required in addition to
     `-ansi'.  *Note Warning Options::.

     The macro `__STRICT_ANSI__' is predefined when the `-ansi' option
     is used.  Some header files may notice this macro and refrain from
     declaring certain functions or defining certain macros that the
     ISO standard doesn't call for; this is to avoid interfering with
     any programs that might use these names for other things.

     Functions which would normally be built in but do not have
     semantics defined by ISO C (such as `alloca' and `ffs') are not
     built-in functions with `-ansi' is used.  *Note Other built-in
     functions provided by GCC: Other Builtins, for details of the
     functions affected.

`-std='
     Determine the language standard.  This option is currently only
     supported when compiling C.  A value for this option must be
     provided; possible values are

    `c89'
    `iso9899:1990'
          ISO C89 (same as `-ansi').

    `iso9899:199409'
          ISO C89 as modified in amendment 1.

    `c99'
    `c9x'
    `iso9899:1999'
    `iso9899:199x'
          ISO C99.  Note that this standard is not yet fully supported;
          see `http://gcc.gnu.org/gcc-3.1/c99status.html' for more
          information.  The names `c9x' and `iso9899:199x' are
          deprecated.

    `gnu89'
          Default, ISO C89 plus GNU extensions (including some C99
          features).

    `gnu99'

    `gnu9x'
          ISO C99 plus GNU extensions.  When ISO C99 is fully
          implemented in GCC, this will become the default.  The name
          `gnu9x' is deprecated.

     Even when this option is not specified, you can still use some of
     the features of newer standards in so far as they do not conflict
     with previous C standards.  For example, you may use
     `__restrict__' even when `-std=c99' is not specified.

     The `-std' options specifying some version of ISO C have the same
     effects as `-ansi', except that features that were not in ISO C89
     but are in the specified version (for example, `//' comments and
     the `inline' keyword in ISO C99) are not disabled.

     *Note Language Standards Supported by GCC: Standards, for details
     of these standard versions.

`-aux-info FILENAME'
     Output to the given filename prototyped declarations for all
     functions declared and/or defined in a translation unit, including
     those in header files.  This option is silently ignored in any
     language other than C.

     Besides declarations, the file indicates, in comments, the origin
     of each declaration (source file and line), whether the
     declaration was implicit, prototyped or unprototyped (`I', `N' for
     new or `O' for old, respectively, in the first character after the
     line number and the colon), and whether it came from a declaration
     or a definition (`C' or `F', respectively, in the following
     character).  In the case of function definitions, a K&R-style list
     of arguments followed by their declarations is also provided,
     inside comments, after the declaration.

`-fno-asm'
     Do not recognize `asm', `inline' or `typeof' as a keyword, so that
     code can use these words as identifiers.  You can use the keywords
     `__asm__', `__inline__' and `__typeof__' instead.  `-ansi' implies
     `-fno-asm'.

     In C++, this switch only affects the `typeof' keyword, since `asm'
     and `inline' are standard keywords.  You may want to use the
     `-fno-gnu-keywords' flag instead, which has the same effect.  In
     C99 mode (`-std=c99' or `-std=gnu99'), this switch only affects
     the `asm' and `typeof' keywords, since `inline' is a standard
     keyword in ISO C99.

`-fno-builtin'
`-fno-builtin-FUNCTION (C and Objective-C only)'
     Don't recognize built-in functions that do not begin with
     `__builtin_' as prefix.  *Note Other built-in functions provided
     by GCC: Other Builtins, for details of the functions affected,
     including those which are not built-in functions when `-ansi' or
     `-std' options for strict ISO C conformance are used because they
     do not have an ISO standard meaning.

     GCC normally generates special code to handle certain built-in
     functions more efficiently; for instance, calls to `alloca' may
     become single instructions that adjust the stack directly, and
     calls to `memcpy' may become inline copy loops.  The resulting
     code is often both smaller and faster, but since the function
     calls no longer appear as such, you cannot set a breakpoint on
     those calls, nor can you change the behavior of the functions by
     linking with a different library.

     In C++, `-fno-builtin' is always in effect.  The `-fbuiltin'
     option has no effect.  Therefore, in C++, the only way to get the
     optimization benefits of built-in functions is to call the function
     using the `__builtin_' prefix.  The GNU C++ Standard Library uses
     built-in functions to implement many functions (like
     `std::strchr'), so that you automatically get efficient code.

     With the `-fno-builtin-FUNCTION' option, not available when
     compiling C++, only the built-in function FUNCTION is disabled.
     FUNCTION must not begin with `__builtin_'.  If a function is named
     this is not built-in in this version of GCC, this option is
     ignored.  There is no corresponding `-fbuiltin-FUNCTION' option;
     if you wish to enable built-in functions selectively when using
     `-fno-builtin' or `-ffreestanding', you may define macros such as:

          #define abs(n)          __builtin_abs ((n))
          #define strcpy(d, s)    __builtin_strcpy ((d), (s))

`-fhosted'
     Assert that compilation takes place in a hosted environment.  This
     implies `-fbuiltin'.  A hosted environment is one in which the
     entire standard library is available, and in which `main' has a
     return type of `int'.  Examples are nearly everything except a
     kernel.  This is equivalent to `-fno-freestanding'.

`-ffreestanding'
     Assert that compilation takes place in a freestanding environment.
     This implies `-fno-builtin'.  A freestanding environment is one
     in which the standard library may not exist, and program startup
     may not necessarily be at `main'.  The most obvious example is an
     OS kernel.  This is equivalent to `-fno-hosted'.

     *Note Language Standards Supported by GCC: Standards, for details
     of freestanding and hosted environments.

`-trigraphs'
     Support ISO C trigraphs.  The `-ansi' option (and `-std' options
     for strict ISO C conformance) implies `-trigraphs'.

`-no-integrated-cpp'
     Invoke the external cpp during compilation.  The default is to use
     the integrated cpp (internal cpp).  This option also allows a
     user-supplied cpp via the `-B' option.  This flag is applicable in
     both C and C++ modes.

     We do not guarantee to retain this option in future, and we may
     change its semantics.

`-traditional'
     Attempt to support some aspects of traditional C compilers.
     Specifically:

        * All `extern' declarations take effect globally even if they
          are written inside of a function definition.  This includes
          implicit declarations of functions.

        * The newer keywords `typeof', `inline', `signed', `const' and
          `volatile' are not recognized.  (You can still use the
          alternative keywords such as `__typeof__', `__inline__', and
          so on.)

        * Comparisons between pointers and integers are always allowed.

        * Integer types `unsigned short' and `unsigned char' promote to
          `unsigned int'.

        * Out-of-range floating point literals are not an error.

        * Certain constructs which ISO regards as a single invalid
          preprocessing number, such as `0xe-0xd', are treated as
          expressions instead.

        * String "constants" are not necessarily constant; they are
          stored in writable space, and identical looking constants are
          allocated separately.  (This is the same as the effect of
          `-fwritable-strings'.)

        * All automatic variables not declared `register' are preserved
          by `longjmp'.  Ordinarily, GNU C follows ISO C: automatic
          variables not declared `volatile' may be clobbered.

        * The character escape sequences `\x' and `\a' evaluate as the
          literal characters `x' and `a' respectively.  Without
          `-traditional', `\x' is a prefix for the hexadecimal
          representation of a character, and `\a' produces a bell.

     This option is deprecated and may be removed.

     You may wish to use `-fno-builtin' as well as `-traditional' if
     your program uses names that are normally GNU C built-in functions
     for other purposes of its own.

     You cannot use `-traditional' if you include any header files that
     rely on ISO C features.  Some vendors are starting to ship systems
     with ISO C header files and you cannot use `-traditional' on such
     systems to compile files that include any system headers.

     The `-traditional' option also enables `-traditional-cpp'.

`-traditional-cpp'
     Attempt to support some aspects of traditional C preprocessors.
     See the GNU CPP manual for details.

`-fcond-mismatch'
     Allow conditional expressions with mismatched types in the second
     and third arguments.  The value of such an expression is void.
     This option is not supported for C++.

`-funsigned-char'
     Let the type `char' be unsigned, like `unsigned char'.

     Each kind of machine has a default for what `char' should be.  It
     is either like `unsigned char' by default or like `signed char' by
     default.

     Ideally, a portable program should always use `signed char' or
     `unsigned char' when it depends on the signedness of an object.
     But many programs have been written to use plain `char' and expect
     it to be signed, or expect it to be unsigned, depending on the
     machines they were written for.  This option, and its inverse, let
     you make such a program work with the opposite default.

     The type `char' is always a distinct type from each of `signed
     char' or `unsigned char', even though its behavior is always just
     like one of those two.

`-fsigned-char'
     Let the type `char' be signed, like `signed char'.

     Note that this is equivalent to `-fno-unsigned-char', which is the
     negative form of `-funsigned-char'.  Likewise, the option
     `-fno-signed-char' is equivalent to `-funsigned-char'.

`-fsigned-bitfields'
`-funsigned-bitfields'
`-fno-signed-bitfields'
`-fno-unsigned-bitfields'
     These options control whether a bit-field is signed or unsigned,
     when the declaration does not use either `signed' or `unsigned'.
     By default, such a bit-field is signed, because this is
     consistent: the basic integer types such as `int' are signed types.

     However, when `-traditional' is used, bit-fields are all unsigned
     no matter what.

`-fwritable-strings'
     Store string constants in the writable data segment and don't
     uniquize them.  This is for compatibility with old programs which
     assume they can write into string constants.  The option
     `-traditional' also has this effect.

     Writing into string constants is a very bad idea; "constants"
     should be constant.

`-fallow-single-precision'
     Do not promote single precision math operations to double
     precision, even when compiling with `-traditional'.

     Traditional K&R C promotes all floating point operations to double
     precision, regardless of the sizes of the operands.   On the
     architecture for which you are compiling, single precision may be
     faster than double precision.   If you must use `-traditional',
     but want to use single precision operations when the operands are
     single precision, use this option.   This option has no effect
     when compiling with ISO or GNU C conventions (the default).

`-fshort-wchar'
     Override the underlying type for `wchar_t' to be `short unsigned
     int' instead of the default for the target.  This option is useful
     for building programs to run under WINE.


File: gcc.info,  Node: C++ Dialect Options,  Next: Objective-C Dialect Options,  Prev: C Dialect Options,  Up: Invoking GCC

Options Controlling C++ Dialect
===============================

   This section describes the command-line options that are only
meaningful for C++ programs; but you can also use most of the GNU
compiler options regardless of what language your program is in.  For
example, you might compile a file `firstClass.C' like this:

     g++ -g -frepo -O -c firstClass.C

In this example, only `-frepo' is an option meant only for C++
programs; you can use the other options with any language supported by
GCC.

   Here is a list of options that are _only_ for compiling C++ programs:

`-fno-access-control'
     Turn off all access checking.  This switch is mainly useful for
     working around bugs in the access control code.

`-fcheck-new'
     Check that the pointer returned by `operator new' is non-null
     before attempting to modify the storage allocated.  The current
     Working Paper requires that `operator new' never return a null
     pointer, so this check is normally unnecessary.

     An alternative to using this option is to specify that your
     `operator new' does not throw any exceptions; if you declare it
     `throw()', G++ will check the return value.  See also `new
     (nothrow)'.

`-fconserve-space'
     Put uninitialized or runtime-initialized global variables into the
     common segment, as C does.  This saves space in the executable at
     the cost of not diagnosing duplicate definitions.  If you compile
     with this flag and your program mysteriously crashes after
     `main()' has completed, you may have an object that is being
     destroyed twice because two definitions were merged.

     This option is no longer useful on most targets, now that support
     has been added for putting variables into BSS without making them
     common.

`-fno-const-strings'
     Give string constants type `char *' instead of type `const char
     *'.  By default, G++ uses type `const char *' as required by the
     standard.  Even if you use `-fno-const-strings', you cannot
     actually modify the value of a string constant, unless you also use
     `-fwritable-strings'.

     This option might be removed in a future release of G++.  For
     maximum portability, you should structure your code so that it
     works with string constants that have type `const char *'.

`-fdollars-in-identifiers'
     Accept `$' in identifiers.  You can also explicitly prohibit use of
     `$' with the option `-fno-dollars-in-identifiers'.  (GNU C allows
     `$' by default on most target systems, but there are a few
     exceptions.)  Traditional C allowed the character `$' to form part
     of identifiers.  However, ISO C and C++ forbid `$' in identifiers.

`-fno-elide-constructors'
     The C++ standard allows an implementation to omit creating a
     temporary which is only used to initialize another object of the
     same type.  Specifying this option disables that optimization, and
     forces G++ to call the copy constructor in all cases.

`-fno-enforce-eh-specs'
     Don't check for violation of exception specifications at runtime.
     This option violates the C++ standard, but may be useful for
     reducing code size in production builds, much like defining
     `NDEBUG'.  The compiler will still optimize based on the exception
     specifications.

`-fexternal-templates'
     Cause `#pragma interface' and `implementation' to apply to
     template instantiation; template instances are emitted or not
     according to the location of the template definition.  *Note
     Template Instantiation::, for more information.

     This option is deprecated.

`-falt-external-templates'
     Similar to `-fexternal-templates', but template instances are
     emitted or not according to the place where they are first
     instantiated.  *Note Template Instantiation::, for more
     information.

     This option is deprecated.

`-ffor-scope'
`-fno-for-scope'
     If `-ffor-scope' is specified, the scope of variables declared in
     a for-init-statement is limited to the `for' loop itself, as
     specified by the C++ standard.  If `-fno-for-scope' is specified,
     the scope of variables declared in a for-init-statement extends to
     the end of the enclosing scope, as was the case in old versions of
     G++, and other (traditional) implementations of C++.

     The default if neither flag is given to follow the standard, but
     to allow and give a warning for old-style code that would
     otherwise be invalid, or have different behavior.

`-fno-gnu-keywords'
     Do not recognize `typeof' as a keyword, so that code can use this
     word as an identifier.  You can use the keyword `__typeof__'
     instead.  `-ansi' implies `-fno-gnu-keywords'.

`-fno-implicit-templates'
     Never emit code for non-inline templates which are instantiated
     implicitly (i.e. by use); only emit code for explicit
     instantiations.  *Note Template Instantiation::, for more
     information.

`-fno-implicit-inline-templates'
     Don't emit code for implicit instantiations of inline templates,
     either.  The default is to handle inlines differently so that
     compiles with and without optimization will need the same set of
     explicit instantiations.

`-fno-implement-inlines'
     To save space, do not emit out-of-line copies of inline functions
     controlled by `#pragma implementation'.  This will cause linker
     errors if these functions are not inlined everywhere they are
     called.

`-fms-extensions'
     Disable pedantic warnings about constructs used in MFC, such as
     implicit int and getting a pointer to member function via
     non-standard syntax.

`-fno-nonansi-builtins'
     Disable built-in declarations of functions that are not mandated by
     ANSI/ISO C.  These include `ffs', `alloca', `_exit', `index',
     `bzero', `conjf', and other related functions.

`-fno-operator-names'
     Do not treat the operator name keywords `and', `bitand', `bitor',
     `compl', `not', `or' and `xor' as synonyms as keywords.

`-fno-optional-diags'
     Disable diagnostics that the standard says a compiler does not
     need to issue.  Currently, the only such diagnostic issued by G++
     is the one for a name having multiple meanings within a class.

`-fpermissive'
     Downgrade messages about nonconformant code from errors to
     warnings.  By default, G++ effectively sets `-pedantic-errors'
     without `-pedantic'; this option reverses that.  This behavior and
     this option are superseded by `-pedantic', which works as it does
     for GNU C.

`-frepo'
     Enable automatic template instantiation at link time.  This option
     also implies `-fno-implicit-templates'.  *Note Template
     Instantiation::, for more information.

`-fno-rtti'
     Disable generation of information about every class with virtual
     functions for use by the C++ runtime type identification features
     (`dynamic_cast' and `typeid').  If you don't use those parts of
     the language, you can save some space by using this flag.  Note
     that exception handling uses the same information, but it will
     generate it as needed.

`-fstats'
     Emit statistics about front-end processing at the end of the
     compilation.  This information is generally only useful to the G++
     development team.

`-ftemplate-depth-N'
     Set the maximum instantiation depth for template classes to N.  A
     limit on the template instantiation depth is needed to detect
     endless recursions during template class instantiation.  ANSI/ISO
     C++ conforming programs must not rely on a maximum depth greater
     than 17.

`-fuse-cxa-atexit'
     Register destructors for objects with static storage duration with
     the `__cxa_atexit' function rather than the `atexit' function.
     This option is required for fully standards-compliant handling of
     static destructors, but will only work if your C library supports
     `__cxa_atexit'.

`-fvtable-gc'
     Emit special relocations for vtables and virtual function
     references so that the linker can identify unused virtual
     functions and zero out vtable slots that refer to them.  This is
     most useful with `-ffunction-sections' and `-Wl,--gc-sections', in
     order to also discard the functions themselves.

     This optimization requires GNU as and GNU ld.  Not all systems
     support this option.  `-Wl,--gc-sections' is ignored without
     `-static'.

`-fno-weak'
     Do not use weak symbol support, even if it is provided by the
     linker.  By default, G++ will use weak symbols if they are
     available.  This option exists only for testing, and should not be
     used by end-users; it will result in inferior code and has no
     benefits.  This option may be removed in a future release of G++.

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

   In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:

`-fno-default-inline'
     Do not assume `inline' for functions defined inside a class scope.
     *Note Options That Control Optimization: Optimize Options.  Note
     that these functions will have linkage like inline functions; they
     just won't be inlined by default.

`-Wctor-dtor-privacy (C++ only)'
     Warn when a class seems unusable, because all the constructors or
     destructors in a class are private and the class has no friends or
     public static member functions.

`-Wnon-virtual-dtor (C++ only)'
     Warn when a class declares a non-virtual destructor that should
     probably be virtual, because it looks like the class will be used
     polymorphically.

`-Wreorder (C++ only)'
     Warn when the order of member initializers given in the code does
     not match the order in which they must be executed.  For instance:

          struct A {
            int i;
            int j;
            A(): j (0), i (1) { }
          };

     Here the compiler will warn that the member initializers for `i'
     and `j' will be rearranged to match the declaration order of the
     members.

   The following `-W...' options are not affected by `-Wall'.

`-Weffc++ (C++ only)'
     Warn about violations of the following style guidelines from Scott
     Meyers' `Effective C++' book:

        * Item 11:  Define a copy constructor and an assignment
          operator for classes with dynamically allocated memory.

        * Item 12:  Prefer initialization to assignment in constructors.

        * Item 14:  Make destructors virtual in base classes.

        * Item 15:  Have `operator=' return a reference to `*this'.

        * Item 23:  Don't try to return a reference when you must
          return an object.


     and about violations of the following style guidelines from Scott
     Meyers' `More Effective C++' book:

        * Item 6:  Distinguish between prefix and postfix forms of
          increment and decrement operators.

        * Item 7:  Never overload `&&', `||', or `,'.


     If you use this option, you should be aware that the standard
     library headers do not obey all of these guidelines; you can use
     `grep -v' to filter out those warnings.

`-Wno-deprecated (C++ only)'
     Do not warn about usage of deprecated features.  *Note Deprecated
     Features::.

`-Wno-non-template-friend (C++ only)'
     Disable warnings when non-templatized friend functions are declared
     within a template.  With the advent of explicit template
     specification support in G++, if the name of the friend is an
     unqualified-id (i.e., `friend foo(int)'), the C++ language
     specification demands that the friend declare or define an
     ordinary, nontemplate function.  (Section 14.5.3).  Before G++
     implemented explicit specification, unqualified-ids could be
     interpreted as a particular specialization of a templatized
     function.  Because this non-conforming behavior is no longer the
     default behavior for G++, `-Wnon-template-friend' allows the
     compiler to check existing code for potential trouble spots, and
     is on by default.  This new compiler behavior can be turned off
     with `-Wno-non-template-friend' which keeps the conformant
     compiler code but disables the helpful warning.

`-Wold-style-cast (C++ only)'
     Warn if an old-style (C-style) cast to a non-void type is used
     within a C++ program.  The new-style casts (`static_cast',
     `reinterpret_cast', and `const_cast') are less vulnerable to
     unintended effects, and much easier to grep for.

`-Woverloaded-virtual (C++ only)'
     Warn when a function declaration hides virtual functions from a
     base class.  For example, in:

          struct A {
            virtual void f();
          };
          
          struct B: public A {
            void f(int);
          };

     the `A' class version of `f' is hidden in `B', and code like this:

          B* b;
          b->f();

     will fail to compile.

`-Wno-pmf-conversions (C++ only)'
     Disable the diagnostic for converting a bound pointer to member
     function to a plain pointer.

`-Wsign-promo (C++ only)'
     Warn when overload resolution chooses a promotion from unsigned or
     enumeral type to a signed type over a conversion to an unsigned
     type of the same size.  Previous versions of G++ would try to
     preserve unsignedness, but the standard mandates the current
     behavior.

`-Wsynth (C++ only)'
     Warn when G++'s synthesis behavior does not match that of cfront.
     For instance:

          struct A {
            operator int ();
            A& operator = (int);
          };
          
          main ()
          {
            A a,b;
            a = b;
          }

     In this example, G++ will synthesize a default `A& operator =
     (const A&);', while cfront will use the user-defined `operator ='.


File: gcc.info,  Node: Objective-C Dialect Options,  Next: Language Independent Options,  Prev: C++ Dialect Options,  Up: Invoking GCC

Options Controlling Objective-C Dialect
=======================================

   This section describes the command-line options that are only
meaningful for Objective-C programs; but you can also use most of the
GNU compiler options regardless of what language your program is in.
For example, you might compile a file `some_class.m' like this:

     gcc -g -fgnu-runtime -O -c some_class.m

In this example, only `-fgnu-runtime' is an option meant only for
Objective-C programs; you can use the other options with any language
supported by GCC.

   Here is a list of options that are _only_ for compiling Objective-C
programs:

`-fconstant-string-class=CLASS-NAME'
     Use CLASS-NAME as the name of the class to instantiate for each
     literal string specified with the syntax `@"..."'.  The default
     class name is `NXConstantString'.

`-fgnu-runtime'
     Generate object code compatible with the standard GNU Objective-C
     runtime.  This is the default for most types of systems.

`-fnext-runtime'
     Generate output compatible with the NeXT runtime.  This is the
     default for NeXT-based systems, including Darwin and Mac OS X.

`-gen-decls'
     Dump interface declarations for all classes seen in the source
     file to a file named `SOURCENAME.decl'.

`-Wno-protocol'
     Do not warn if methods required by a protocol are not implemented
     in the class adopting it.

`-Wselector'
     Warn if a selector has multiple methods of different types defined.


File: gcc.info,  Node: Language Independent Options,  Next: Warning Options,  Prev: Objective-C Dialect Options,  Up: Invoking GCC

Options to Control Diagnostic Messages Formatting
=================================================

   Traditionally, diagnostic messages have been formatted irrespective
of the output device's aspect (e.g. its width, ...).  The options
described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often
source location information should be reported.  Right now, only the
C++ front end can honor these options.  However it is expected, in the
near future, that the remaining front ends would be able to digest them
correctly.

`-fmessage-length=N'
     Try to format error messages so that they fit on lines of about N
     characters.  The default is 72 characters for `g++' and 0 for the
     rest of the front ends supported by GCC.  If N is zero, then no
     line-wrapping will be done; each error message will appear on a
     single line.

`-fdiagnostics-show-location=once'
     Only meaningful in line-wrapping mode.  Instructs the diagnostic
     messages reporter to emit _once_ source location information; that
     is, in case the message is too long to fit on a single physical
     line and has to be wrapped, the source location won't be emitted
     (as prefix) again, over and over, in subsequent continuation
     lines.  This is the default behavior.

`-fdiagnostics-show-location=every-line'
     Only meaningful in line-wrapping mode.  Instructs the diagnostic
     messages reporter to emit the same source location information (as
     prefix) for physical lines that result from the process of breaking
     a message which is too long to fit on a single line.

