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   This file documents the use and the internals of the GNU compiler.

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File: gcc.info,  Node: Options,  Next: Installation,  Prev: Boycott,  Up: Top

GNU CC Command Options
**********************

   The GNU C compiler uses a command syntax much like the Unix C
compiler.  The `gcc' program accepts options and file names as
operands.  Multiple single-letter options may *not* be grouped: `-dr'
is very different from `-d -r'.

   When you invoke GNU CC, it normally does preprocessing,
compilation, assembly and linking.  File names which end in `.c' are
taken as C source to be preprocessed and compiled; file names ending
in `.i' are taken as preprocessor output to be compiled; compiler
output files plus any input files with names ending in `.s' are
assembled; then the resulting object files, plus any other input
files, are linked together to produce an executable.

   Command options allow you to stop this process at an intermediate
stage.  For example, the `-c' option says not to run the linker. 
Then the output consists of object files output by the assembler.

   Other command options are passed on to one stage of processing. 
Some options control the preprocessor and others the compiler itself.
Yet other options control the assembler and linker; these are not
documented here, but you rarely need to use any of them.

   Here are the options to control the overall compilation process,
including those that say whether to link, whether to assemble, and so
on.

`-o FILE'
     Place output in file FILE.  This applies regardless to whatever
     sort of output is being produced, whether it be an executable
     file, an object file, an assembler file or preprocessed C code.

     If `-o' is not specified, the default is to put an executable
     file in `a.out', the object file `SOURCE.c' in `SOURCE.o', an
     assembler file in `SOURCE.s', and preprocessed C on standard
     output.

`-c'
     Compile or assemble the source files, but do not link.  Produce
     object files with names made by replacing `.c' or `.s' with `.o'
     at the end of the input file names.  Do nothing at all for
     object files specified as input.

`-S'
     Compile into assembler code but do not assemble.  The assembler
     output file name is made by replacing `.c' with `.s' at the end
     of the input file name.  Do nothing at all for assembler source
     files or object files specified as input.

`-E'
     Run only the C preprocessor.  Preprocess all the C source files
     specified and output the results to standard output.

`-v'
     Compiler driver program prints the commands it executes as it
     runs the preprocessor, compiler proper, assembler and linker. 
     Some of these are directed to print their own version numbers.

`-pipe'
     Use pipes rather than temporary files for communication between
     the various stages of compilation.  This fails to work on some
     systems where the assembler is unable to read from a pipe; but
     the GNU assembler has no trouble.

`-BPREFIX'
     Compiler driver program tries PREFIX as a prefix for each
     program it tries to run.  These programs are `cpp', `cc1', `as'
     and `ld'.

     For each subprogram to be run, the compiler driver first tries
     the `-B' prefix, if any.  If that name is not found, or if `-B'
     was not specified, the driver tries two standard prefixes, which
     are `/usr/lib/gcc-' and `/usr/local/lib/gcc-'.  If neither of
     those results in a file name that is found, the unmodified
     program name is searched for using the directories specified in
     your `PATH' environment variable.

     The run-time support file `gnulib' is also searched for using
     the `-B' prefix, if needed.  If it is not found there, the two
     standard prefixes above are tried, and that is all.  The file is
     left out of the link if it is not found by those means.  Most of
     the time, on most machines, you can do without it.

     You can get a similar result from the environment variable;
     `GCC_EXEC_PREFIX' if it is defined, its value is used as a
     prefix in the same way.  If both the `-B' option and the
     `GCC_EXEC_PREFIX' variable are present, the `-B' option is used
     first and the environment variable value second.

`-bPREFIX'
     The argument PREFIX is used as a second prefix for the compiler
     executables and libraries.  This prefix is optional: the
     compiler tries each file first with it, then without it.  This
     prefix follows the prefix specified with `-B' or the default
     prefixes.

     Thus, `-bvax- -Bcc/' in the presence of environment variable
     `GCC_EXEC_PREFIX' with definition `/u/foo/' causes GNU CC to try
     the following file names for the preprocessor executable:

          cc/vax-cpp
          cc/cpp
          /u/foo/vax-cpp
          /u/foo/cpp
          /usr/local/lib/gcc-vax-cpp
          /usr/local/lib/gcc-cpp
          /usr/lib/gcc-vax-cpp
          /usr/lib/gcc-cpp

   These options control the details of C compilation itself.

`-ansi'
     Support all ANSI standard C programs.

     This turns off certain features of GNU C that are incompatible
     with ANSI C, such as the `asm', `inline' 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 ANSI trigraph feature.

     The alternate keywords `__asm__', `__inline__' and `__typeof__'
     continue to work despite `-ansi'.  You would not want to use
     them in an ANSI C program, of course, but it 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-ANSI programs to be
     rejected gratuitously.  For that, `-pedantic' is required in
     addition to `-ansi'.

     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 ANSI standard doesn't call for; this is to avoid
     interfering with any programs that might use these names for
     other things.

`-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 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.

        * String "constants" are not necessarily constant; they are
          stored in writable space, and identical looking constants
          are allocated separately.

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

        * In the preprocessor, comments convert to nothing at all,
          rather than to a space.  This allows traditional token
          concatenation.

        * In the preprocessor, macro arguments are recognized within
          string constants in a macro definition (and their values
          are stringified, though without additional quote marks,
          when they appear in such a context).  The preprocessor
          always considers a string constant to end at a newline.

        * The predefined macro `__STDC__' is not defined when you use
          `-traditional', but `__GNUC__' is (since the GNU extensions
          which `__GNUC__' indicates are not affected by
          `-traditional').  If you need to write header files that
          work differently depending on whether `-traditional' is in
          use, by testing both of these predefined macros you can
          distinguish four situations: GNU C, traditional GNU C,
          other ANSI C compilers, and other old C compilers.

`-O'
     Optimize.  Optimizing compilation takes somewhat more time, and
     a lot more memory for a large function.

     Without `-O', the compiler's goal is to reduce the cost of
     compilation and to make debugging produce the expected results. 
     Statements are independent: if you stop the program with a
     breakpoint between statements, you can then assign a new value
     to any variable or change the program counter to any other
     statement in the function and get exactly the results you would
     expect from the source code.

     Without `-O', only variables declared `register' are allocated
     in registers.  The resulting compiled code is a little worse
     than produced by PCC without `-O'.

     With `-O', the compiler tries to reduce code size and execution
     time.

     Some of the `-f' options described below turn specific kinds of
     optimization on or off.

`-g'
     Produce debugging information in the operating system's native
     format (for DBX or SDB).  GDB also can work with this debugging
     information.

     Unlike most other C compilers, GNU CC allows you to use `-g'
     with `-O'.  The shortcuts taken by optimized code may
     occasionally produce surprising results: some variables you
     declared may not exist at all; flow of control may briefly move
     where you did not expect it; some statements may not be executed
     because they compute constant results or their values were
     already at hand; some statements may execute in different places
     because they were moved out of loops.  Nevertheless it proves
     possible to debug optimized output.  This makes it reasonable to
     use the optimizer for programs that might have bugs.

`-gg'
     Produce debugging information in the old GDB format.  This is
     obsolete.

`-w'
     Inhibit all warning messages.

`-W'
     Print extra warning messages for these events:

        * An automatic variable is used without first being
          initialized.

          These warnings are possible only in optimizing compilation,
          because they require data flow information that is computed
          only when optimizing.  If you don't specify `-O', you
          simply won't get these warnings.

          These warnings occur only for variables that are candidates
          for register allocation.  Therefore, they do not occur for
          a variable that is declared `volatile', or whose address is
          taken, or whose size is other than 1, 2, 4 or 8 bytes. 
          Also, they do not occur for structures, unions or arrays,
          even when they are in registers.

          Note that there may be no warning about a variable that is
          used only to compute a value that itself is never used,
          because such computations may be deleted by data flow
          analysis before the warnings are printed.

          These warnings are made optional because GNU CC is not
          smart enough to see all the reasons why the code might be
          correct despite appearing to have an error.  Here is one
          example of how this can happen:

               {
                 int x;
                 switch (y)
                   {
                   case 1: x = 1;
                     break;
                   case 2: x = 4;
                     break;
                   case 3: x = 5;
                   }
                 foo (x);
               }

          If the value of `y' is always 1, 2 or 3, then `x' is always
          initialized, but GNU CC doesn't know this.  Here is another
          common case:

               {
                 int save_y;
                 if (change_y) save_y = y, y = new_y;
                 ...
                 if (change_y) y = save_y;
               }

          This has no bug because `save_y' is used only if it is set.

          Some spurious warnings can be avoided if you declare as
          `volatile' all the functions you use that never return. 
          *Note Function Attributes::.

        * A nonvolatile automatic variable might be changed by a call
          to `longjmp'.  These warnings as well are possible only in
          optimizing compilation.

          The compiler sees only the calls to `setjmp'.  It cannot
          know where `longjmp' will be called; in fact, a signal
          handler could call it at any point in the code.  As a
          result, you may get a warning even when there is in fact no
          problem because `longjmp' cannot in fact be called at the
          place which would cause a problem.

        * A function can return either with or without a value. 
          (Falling off the end of the function body is considered
          returning without a value.)  For example, this function
          would evoke such a warning:

               foo (a)
               {
                 if (a > 0)
                   return a;
               }

          Spurious warnings can occur because GNU CC does not realize
          that certain functions (including `abort' and `longjmp')
          will never return.

        * An expression-statement contains no side effects.

     In the future, other useful warnings may also be enabled by this
     option.

`-Wimplicit'
     Warn whenever a function is implicitly declared.

`-Wreturn-type'
     Warn whenever a function is defined with a return-type that
     defaults to `int'.  Also warn about any `return' statement with
     no return-value in a function whose return-type is not `void'.

`-Wunused'
     Warn whenever a local variable is unused aside from its
     declaration, whenever a function is declared static but never
     defined, and whenever a statement computes a result that is
     explicitly not used.

`-Wswitch'
     Warn whenever a `switch' statement has an index of enumeral type
     and lacks a `case' for one or more of the named codes of that
     enumeration.  (The presence of a `default' label prevents this
     warning.)  `case' labels outside the enumeration range also
     provoke warnings when this option is used.

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

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

`-Wall'
     All of the above `-W' options combined.  These are all the
     options which pertain to usage that we recommend avoiding and
     that we believe is easy to avoid, even in conjunction with macros.

     The other `-W...' options below are not implied by `-Wall'
     because certain kinds of useful macros are almost impossible to
     write without causing those warnings.

`-Wshadow'
     Warn whenever a local variable shadows another local variable.

`-Wid-clash-LEN'
     Warn whenever two distinct identifiers match in the first LEN
     characters.  This may help you prepare a program that will
     compile with certain obsolete, brain-damaged compilers.

`-Wpointer-arith'
     Warn about anything that depends on the "size of" a function
     type or of `void'.  GNU C assigns these types a size of 1, for
     convenience in calculations with `void *' pointers and pointers
     to functions.

`-Wcast-qual'
     Warn whenever a pointer is cast so as to remove a type qualifier
     from the target type.  For example, warn if a `const char *' is
     cast to an ordinary `char *'.

`-Wwrite-strings'
     Give string constants the type `const char[LENGTH]' so that
     copying the address of one into a non-`const' `char *' pointer
     will get a warning.  These warnings will help you find at
     compile time code that can try to write into a string constant,
     but only if you have been very careful about using `const' in
     declarations and prototypes.  Otherwise, it will just be a
     nuisance; this is why we did not make `-Wall' request these
     warnings.

`-p'
     Generate extra code to write profile information suitable for
     the analysis program `prof'.

`-pg'
     Generate extra code to write profile information suitable for
     the analysis program `gprof'.

`-a'
     Generate extra code to write profile information for basic
     blocks, which will record the number of times each basic block
     is executed.  This data could be analyzed by a program like
     `tcov'.  Note, however, that the format of the data is not what
     `tcov' expects.  Eventually GNU `gprof' should be extended to
     process this data.

`-lLIBRARY'
     Search a standard list of directories for a library named
     LIBRARY, which is actually a file named `libLIBRARY.a'.  The
     linker uses this file as if it had been specified precisely by
     name.

     The directories searched include several standard system
     directories plus any that you specify with `-L'.

     Normally the files found this way are library files--archive
     files whose members are object files.  The linker handles an
     archive file by scanning through it for members which define
     symbols that have so far been referenced but not defined.  But
     if the file that is found is an ordinary object file, it is
     linked in the usual fashion.  The only difference between using
     an `-l' option and specifying a file name is that `-l' searches
     several directories.

`-LDIR'
     Add directory DIR to the list of directories to be searched for
     `-l'.

`-nostdlib'
     Don't use the standard system libraries and startup files when
     linking.  Only the files you specify will be passed to the linker.

`-mMACHINESPEC'
     Machine-dependent option specifying something about the type of
     target machine.  These options are defined by the macro
     `TARGET_SWITCHES' in the machine description.  The default for
     the options is also defined by that macro, which enables you to
     change the defaults.

     These are the `-m' options defined in the 68000 machine
     description:

    `-m68020'
    `-mc68020'
          Generate output for a 68020 (rather than a 68000).  This is
          the default if you use the unmodified sources.

    `-m68000'
    `-mc68000'
          Generate output for a 68000 (rather than a 68020).

    `-m68881'
          Generate output containing 68881 instructions for floating
          point.  This is the default if you use the unmodified
          sources.

    `-mfpa'
          Generate output containing Sun FPA instructions for
          floating point.

    `-msoft-float'
          Generate output containing library calls for floating point.

    `-mshort'
          Consider type `int' to be 16 bits wide, like `short int'.

    `-mnobitfield'
          Do not use the bit-field instructions.  `-m68000' implies
          `-mnobitfield'.

    `-mbitfield'
          Do use the bit-field instructions.  `-m68020' implies
          `-mbitfield'.  This is the default if you use the
          unmodified sources.

    `-mrtd'
          Use a different function-calling convention, in which
          functions that take a fixed number of arguments return with
          the `rtd' instruction, which pops their arguments while
          returning.  This saves one instruction in the caller since
          there is no need to pop the arguments there.

          This calling convention is incompatible with the one
          normally used on Unix, so you cannot use it if you need to
          call libraries compiled with the Unix compiler.

          Also, you must provide function prototypes for all
          functions that take variable numbers of arguments
          (including `printf'); otherwise incorrect code will be
          generated for calls to those functions.

          In addition, seriously incorrect code will result if you
          call a function with too many arguments.  (Normally, extra
          arguments are harmlessly ignored.)

          The `rtd' instruction is supported by the 68010 and 68020
          processors, but not by the 68000.

     These `-m' options are defined in the Vax machine description:

    `-munix'
          Do not output certain jump instructions (`aobleq' and so
          on) that the Unix assembler for the Vax cannot handle
          across long ranges.

    `-mgnu'
          Do output those jump instructions, on the assumption that
          you will assemble with the GNU assembler.

    `-mg'
          Output code for g-format floating point numbers instead of
          d-format.

     These `-m' switches are supported on the Sparc:

    `-mfpu'
          Generate output containing floating point instructions. 
          This is the default if you use the unmodified sources.

    `-mno-epilogue'
          Generate separate return instructions for `return'
          statements.  This has both advantages and disadvantages; I
          don't recall what they are.

     These `-m' options are defined in the Convex machine description:

    `-mc1'
          Generate output for a C1.  This is the default when the
          compiler is configured for a C1.

    `-mc2'
          Generate output for a C2.  This is the default when the
          compiler is configured for a C2.

    `-margcount'
          Generate code which puts an argument count in the word
          preceding each argument list.  Some nonportable Convex and
          Vax programs need this word.  (Debuggers don't; this info
          is in the symbol table.)

    `-mnoargcount'
          Omit the argument count word.  This is the default if you
          use the unmodified sources.

`-fFLAG'
     Specify machine-independent flags.  Most flags have both
     positive and negative forms; the negative form of `-ffoo' would
     be `-fno-foo'.  In the table below, only one of the forms is
     listed--the one which is not the default.  You can figure out
     the other form by either removing `no-' or adding it.

    `-fpcc-struct-return'
          Use the same convention for returning `struct' and `union'
          values that is used by the usual C compiler on your system.
          This convention is less efficient for small structures, and
          on many machines it fails to be reentrant; but it has the
          advantage of allowing intercallability between GCC-compiled
          code and PCC-compiled code.

    `-ffloat-store'
          Do not store floating-point variables in registers.  This
          prevents undesirable excess precision on machines such as
          the 68000 where the floating registers (of the 68881) keep
          more precision than a `double' is supposed to have.

          For most programs, the excess precision does only good, but
          a few programs rely on the precise definition of IEEE
          floating point.  Use `-ffloat-store' for such programs.

    `-fno-asm'
          Do not recognize `asm', `inline' or `typeof' as a keyword. 
          These words may then be used as identifiers.  You can use
          `__asm__', `__inline__' and `__typeof__' instead.

    `-fno-defer-pop'
          Always pop the arguments to each function call as soon as
          that function returns.  Normally the compiler (when
          optimizing) lets arguments accumulate on the stack for
          several function calls and pops them all at once.

    `-fstrength-reduce'
          Perform the optimizations of loop strength reduction and
          elimination of iteration variables.

    `-fcombine-regs'
          Allow the combine pass to combine an instruction that
          copies one register into another.  This might or might not
          produce better code when used in addition to `-O'.  I am
          interested in hearing about the difference this makes.

    `-fforce-mem'
          Force memory operands to be copied into registers before
          doing arithmetic on them.  This may produce better code by
          making all memory references potential common
          subexpressions.  When they are not common subexpressions,
          instruction combination should eliminate the separate
          register-load.  I am interested in hearing about the
          difference this makes.

    `-fforce-addr'
          Force memory address constants to be copied into registers
          before doing arithmetic on them.  This may produce better
          code just as `-fforce-mem' may.  I am interested in hearing
          about the difference this makes.

    `-fomit-frame-pointer'
          Don't keep the frame pointer in a register for functions
          that don't need one.  This avoids the instructions to save,
          set up and restore frame pointers; it also makes an extra
          register available in many functions.  *It also makes
          debugging impossible.*

          On some machines, such as the Vax, this flag has no effect,
          because the standard calling sequence automatically handles
          the frame pointer and nothing is saved by pretending it
          doesn't exist.  The machine-description macro
          `FRAME_POINTER_REQUIRED' controls whether a target machine
          supports this flag.  *Note Registers::.

    `-finline-functions'
          Integrate all simple functions into their callers.  The
          compiler heuristically decides which functions are simple
          enough to be worth integrating in this way.

          If all calls to a given function are integrated, and the
          function is declared `static', then the function is
          normally not output as assembler code in its own right.

    `-fcaller-saves'
          Enable values to be allocated in registers that will be
          clobbered by function calls, by emitting extra instructions
          to save and restore the registers around such calls.  Such
          allocation is done only when it seems to result in better
          code than would otherwise be produced.

          This option is enabled by default on certain machines,
          usually those which have no call-preserved registers to use
          instead.

    `-fkeep-inline-functions'
          Even if all calls to a given function are integrated, and
          the function is declared `static', nevertheless output a
          separate run-time callable version of the function.

    `-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.
          `-traditional' also has this effect.

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

    `-fcond-mismatch'
          Allow conditional expressions with mismatched types in the
          second and third arguments.  The value of such an
          expression is void.

    `-fno-function-cse'
          Do not put function addresses in registers; make each
          instruction that calls a constant function contain the
          function's address explicitly.

          This option results in less efficient code, but some
          strange hacks that alter the assembler output may be
          confused by the optimizations performed when this option is
          not used.

    `-fvolatile'
          Consider all memory references through pointers to be
          volatile.

    `-fshared-data'
          Requests that the data and non-`const' variables of this
          compilation be shared data rather than private data.  The
          distinction makes sense only on certain operating systems,
          where shared data is shared between processes running the
          same program, while private data exists in one copy per
          process.

    `-funsigned-char'
          Let the type `char' be the 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.  (Actually, at present, the
          default is always signed.)

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

          Note that this is equivalent to `-fno-signed-char', which
          is the negative form of `-fsigned-char'.

    `-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'.

    `-fdelayed-branch'
          If supported for the target machine, attempt to reorder
          instructions to exploit instruction slots available after
          delayed branch instructions.

    `-ffixed-REG'
          Treat the register named REG as a fixed register; generated
          code should never refer to it (except perhaps as a stack
          pointer, frame pointer or in some other fixed role).

          REG must be the name of a register.  The register names
          accepted are machine-specific and are defined in the
          `REGISTER_NAMES' macro in the machine description macro file.

          This flag does not have a negative form, because it
          specifies a three-way choice.

    `-fcall-used-REG'
          Treat the register named REG as an allocatable register
          that is clobbered by function calls.  It may be allocated
          for temporaries or variables that do not live across a call.
          Functions compiled this way will not save and restore the
          register REG.

          Use of this flag for a register that has a fixed pervasive
          role in the machine's execution model, such as the stack
          pointer or frame pointer, will produce disastrous results.

          This flag does not have a negative form, because it
          specifies a three-way choice.

    `-fcall-saved-REG'
          Treat the register named REG as an allocatable register
          saved by functions.  It may be allocated even for
          temporaries or variables that live across a call. 
          Functions compiled this way will save and restore the
          register REG if they use it.

          Use of this flag for a register that has a fixed pervasive
          role in the machine's execution model, such as the stack
          pointer or frame pointer, will produce disastrous results.

          A different sort of disaster will result from the use of
          this flag for a register in which function values may be
          returned.

          This flag does not have a negative form, because it
          specifies a three-way choice.

`-dLETTERS'
     Says to make debugging dumps at times specified by LETTERS. 
     Here are the possible letters:

    `r'
          Dump after RTL generation.

    `j'
          Dump after first jump optimization.

    `s'
          Dump after CSE (including the jump optimization that
          sometimes follows CSE).

    `L'
          Dump after loop optimization.

    `f'
          Dump after flow analysis.

    `c'
          Dump after instruction combination.

    `l'
          Dump after local register allocation.

    `g'
          Dump after global register allocation.

    `d'
          Dump after delayed branch scheduling.

    `J'
          Dump after last jump optimization.

    `m'
          Print statistics on memory usage, at the end of the run.

`-pedantic'
     Issue all the warnings demanded by strict ANSI standard C;
     reject all programs that use forbidden extensions.

     Valid ANSI standard C programs should compile properly with or
     without this option (though a rare few will require `-ansi'). 
     However, without this option, certain GNU extensions and
     traditional C features are supported as well.  With this option,
     they are rejected.  There is no reason to use this option; it
     exists only to satisfy pedants.

     `-pedantic' does not cause warning messages for use of the
     alternate keywords whose names begin and end with `__'.  *Note
     Alternate Keywords::.

`-static'
     On Suns running version 4, this prevents linking with the shared
     libraries.  (`-g' has the same effect.)

   These options control the C preprocessor, which is run on each C
source file before actual compilation.  If you use the `-E' option,
nothing is done except C preprocessing.  Some of these options make
sense only together with `-E' because they request preprocessor
output that is not suitable for actual compilation.

`-C'
     Tell the preprocessor not to discard comments.  Used with the
     `-E' option.

`-IDIR'
     Search directory DIR for include files.

`-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'
     directives.  (Ordinarily *all* `-I' directories are used this
     way.)

     In addition, the `-I-' option inhibits the use of the current
     directory (where the current input file came from) as the first
     search directory for `#include "FILE"'.  There is no way to
     override this effect of `-I-'.  With `-I.' you can specify
     searching the directory which was current when the compiler was
     invoked.  That is not exactly the same as what the preprocessor
     does by default, but it is often satisfactory.

     `-I-' does not inhibit the use of the standard system
     directories for header files.  Thus, `-I-' and `-nostdinc' are
     independent.

`-i 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 `-i FILE' is to make
     the macros defined in FILE available for use in the main input.

`-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.

     Between `-nostdinc' and `-I-', you can eliminate all directories
     from the search path except those you specify.

`-M'
     Tell the preprocessor to output a rule suitable for `make'
     describing the dependencies of each object file.  For each
     source file, the preprocessor outputs one `make'-rule whose
     target is the object file name for that source file and whose
     dependencies are all the files `#include'd in it.  This rule may
     be a single line or may be continued with `\'-newline if it is
     long.

     `-M' implies `-E'.

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

     `-MM' implies `-E'.

`-DMACRO'
     Define macro MACRO with the  string `1' as its definition.

`-DMACRO=DEFN'
     Define macro MACRO as DEFN.

`-UMACRO'
     Undefine macro MACRO.

`-trigraphs'
     Support ANSI C trigraphs.  You don't want to know about this
     brain-damage.  The `-ansi' option also has this effect.

