This is doc/gcc.info, produced by makeinfo version 4.5 from doc/gcc.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gcc: (gcc). The GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the use of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  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).  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. `-Wabi (C++ only)' Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible. You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers. The known incompatibilites at this point include: * Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example: struct A { virtual void f(); int f1 : 1; }; struct B : public A { int f2 : 1; }; In this case, G++ will place `B::f2' into the same byte as`A::f1'; other compilers will not. You can avoid this problem by explicitly padding `A' so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout `B' identically. * Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example: struct A { virtual void f(); char c1; }; struct B { B(); char c2; }; struct C : public A, public virtual B {}; In this case, G++ will not place `B' into the tail-padding for `A'; other compilers will. You can avoid this problem by explicitly padding `A' so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout `C' identically. `-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.