gcc.texinfo

这是完整的gcc源代码

@samp{PATH} environment variable.

The run-time support file @file{gnulib} is also searched for using
the @samp{-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;
@code{GCC_EXEC_PREFIX} if it is defined, its value is used as a prefix
in the same way.  If both the @samp{-B} option and the
@code{GCC_EXEC_PREFIX} variable are present, the @samp{-B} option is
used first and the environment variable value second.

@item -b@var{prefix}
The argument @var{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 @samp{-B} or the default prefixes.

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

@example
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
@end example
@end table

These options control the details of C compilation itself.

@table @samp
@item -ansi
Support all ANSI standard C programs.

This turns off certain features of GNU C that are incompatible with
ANSI C, such as the @code{asm}, @code{inline} and @code{typeof}
keywords, and predefined macros such as @code{unix} and @code{vax}
that identify the type of system you are using.  It also enables the
undesirable and rarely used ANSI trigraph feature.

The alternate keywords @code{__asm__}, @code{__inline__} and
@code{__typeof__} continue to work despite @samp{-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
@samp{-ansi}.  Alternate predefined macros such as @code{__unix__} and
@code{__vax__} are also available, with or without @samp{-ansi}.

The @samp{-ansi} option does not cause non-ANSI programs to be
rejected gratuitously.  For that, @samp{-pedantic} is required in
addition to @samp{-ansi}.

The macro @code{__STRICT_ANSI__} is predefined when the @samp{-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.

@item -traditional
Attempt to support some aspects of traditional C compilers.
Specifically:

@itemize @bullet
@item
All @code{extern} declarations take effect globally even if they
are written inside of a function definition.  This includes implicit
declarations of functions.

@item
The keywords @code{typeof}, @code{inline}, @code{signed}, @code{const}
and @code{volatile} are not recognized.  (You can still use the alternative
keywords such as @code{__typeof__}, @code{__inline__}, and so on.)

@item
Comparisons between pointers and integers are always allowed.

@item
Integer types @code{unsigned short} and @code{unsigned char} promote
to @code{unsigned int}.

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

@item
String ``constants'' are not necessarily constant; they are stored in
writable space, and identical looking constants are allocated
separately.

@item
All automatic variables not declared @code{register} are preserved by
@code{longjmp}.  Ordinarily, GNU C follows ANSI C: automatic variables
not declared @code{volatile} may be clobbered.

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

@item
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.

@item
The predefined macro @code{__STDC__} is not defined when you use
@samp{-traditional}, but @code{__GNUC__} is (since the GNU extensions
which @code{__GNUC__} indicates are not affected by
@samp{-traditional}).  If you need to write header files that work
differently depending on whether @samp{-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.
@end itemize

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

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

With @samp{-O}, the compiler tries to reduce code size and execution
time.

Some of the @samp{-f} options described below turn specific kinds of
optimization on or off.

@item -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 @samp{-g} with
@samp{-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.

@item -gg
Produce debugging information in the old GDB format.  This is obsolete.

@item -w
Inhibit all warning messages.

@item -W
Print extra warning messages for these events:

@itemize @bullet
@item
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 @samp{-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 @code{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:

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

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

@example
@{
  int save_y;
  if (change_y) save_y = y, y = new_y;
  @dots{}
  if (change_y) y = save_y;
@}
@end example

@noindent
This has no bug because @code{save_y} is used only if it is set.

Some spurious warnings can be avoided if you declare as
@code{volatile} all the functions you use that never return.
@xref{Function Attributes}.

@item
A nonvolatile automatic variable might be changed by a call to
@code{longjmp}.  These warnings as well are possible only in
optimizing compilation.

The compiler sees only the calls to @code{setjmp}.  It cannot know
where @code{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 @code{longjmp} cannot
in fact be called at the place which would cause a problem.

@item
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:

@example
foo (a)
@{
  if (a > 0)
    return a;
@}
@end example

Spurious warnings can occur because GNU CC does not realize that
certain functions (including @code{abort} and @code{longjmp})
will never return.

@item
An expression-statement contains no side effects.
@end itemize

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

@item -Wimplicit
Warn whenever a function is implicitly declared.

@item -Wreturn-type
Warn whenever a function is defined with a return-type that defaults
to @code{int}.  Also warn about any @code{return} statement with no
return-value in a function whose return-type is not @code{void}.

@item -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.

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

@item -Wcomment
Warn whenever a comment-start sequence @samp{/*} appears in a comment.

@item -Wtrigraphs
Warn if any trigraphs are encountered (assuming they are enabled).

@item -Wall
All of the above @samp{-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 @samp{-W@dots{}} options below are not implied by @samp{-Wall}
because certain kinds of useful macros are almost impossible to write
without causing those warnings.

@item -Wshadow
Warn whenever a local variable shadows another local variable.

@item -Wid-clash-@var{len}
Warn whenever two distinct identifiers match in the first @var{len}
characters.  This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.

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

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

@item -Wwrite-strings
Give string constants the type @code{const char[@var{length}]} so that
copying the address of one into a non-@code{const} @code{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 @code{const} in
declarations and prototypes.  Otherwise, it will just be a nuisance;
this is why we did not make @samp{-Wall} request these warnings.

@item -p
Generate extra code to write profile information suitable for the
analysis program @code{prof}.

@item -pg
Generate extra code to write profile information suitable for the
analysis program @code{gprof}.

@item -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 @code{tcov}.  Note, however, that
the format of the data is not what @code{tcov} expects.  Eventually GNU
@code{gprof} should be extended to process this data.

@item -l@var{library}
Search a standard list of directories for a library named
@var{library}, which is actually a file named
@file{lib@var{library}.a}.  The linker uses this file as if it
had been specified precisely by name.

The directories searched include several standard system directories