trouble.texi

理解和实践操作系统的一本好书

@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
@c 1999, 2000, 2001, 2003, 2004, 2005, 2006, 2007 Free Software Foundation,
@c Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Trouble
@chapter Known Causes of Trouble with GCC
@cindex bugs, known
@cindex installation trouble
@cindex known causes of trouble

This section describes known problems that affect users of GCC@.  Most
of these are not GCC bugs per se---if they were, we would fix them.
But the result for a user may be like the result of a bug.

Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.

@menu
* Actual Bugs::		      Bugs we will fix later.
* Cross-Compiler Problems::   Common problems of cross compiling with GCC.
* Interoperation::      Problems using GCC with other compilers,
			   and with certain linkers, assemblers and debuggers.
* Incompatibilities::   GCC is incompatible with traditional C.
* Fixed Headers::       GCC uses corrected versions of system header files.
                           This is necessary, but doesn't always work smoothly.
* Standard Libraries::  GCC uses the system C library, which might not be
                           compliant with the ISO C standard.
* Disappointments::     Regrettable things we can't change, but not quite bugs.
* C++ Misunderstandings::     Common misunderstandings with GNU C++.
* Protoize Caveats::    Things to watch out for when using @code{protoize}.
* Non-bugs::		Things we think are right, but some others disagree.
* Warnings and Errors:: Which problems in your code get warnings,
                         and which get errors.
@end menu

@node Actual Bugs
@section Actual Bugs We Haven't Fixed Yet

@itemize @bullet
@item
The @code{fixincludes} script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while @code{fixincludes} is running.  This would seem to be a
bug in the automounter.  We don't know any good way to work around it.

@item
The @code{fixproto} script will sometimes add prototypes for the
@code{sigsetjmp} and @code{siglongjmp} functions that reference the
@code{jmp_buf} type before that type is defined.  To work around this,
edit the offending file and place the typedef in front of the
prototypes.
@end itemize

@node Cross-Compiler Problems
@section Cross-Compiler Problems

You may run into problems with cross compilation on certain machines,
for several reasons.

@itemize @bullet
@item
At present, the program @file{mips-tfile} which adds debug
support to object files on MIPS systems does not work in a cross
compile environment.
@end itemize

@node Interoperation
@section Interoperation

This section lists various difficulties encountered in using GCC
together with other compilers or with the assemblers, linkers,
libraries and debuggers on certain systems.

@itemize @bullet
@item
On many platforms, GCC supports a different ABI for C++ than do other
compilers, so the object files compiled by GCC cannot be used with object
files generated by another C++ compiler.

An area where the difference is most apparent is name mangling.  The use
of different name mangling is intentional, to protect you from more subtle
problems.
Compilers differ as to many internal details of C++ implementation,
including: how class instances are laid out, how multiple inheritance is
implemented, and how virtual function calls are handled.  If the name
encoding were made the same, your programs would link against libraries
provided from other compilers---but the programs would then crash when
run.  Incompatible libraries are then detected at link time, rather than
at run time.

@item
On some BSD systems, including some versions of Ultrix, use of profiling
causes static variable destructors (currently used only in C++) not to
be run.

@item
On some SGI systems, when you use @option{-lgl_s} as an option,
it gets translated magically to @samp{-lgl_s -lX11_s -lc_s}.
Naturally, this does not happen when you use GCC@.
You must specify all three options explicitly.

@item
On a SPARC, GCC aligns all values of type @code{double} on an 8-byte
boundary, and it expects every @code{double} to be so aligned.  The Sun
compiler usually gives @code{double} values 8-byte alignment, with one
exception: function arguments of type @code{double} may not be aligned.

As a result, if a function compiled with Sun CC takes the address of an
argument of type @code{double} and passes this pointer of type
@code{double *} to a function compiled with GCC, dereferencing the
pointer may cause a fatal signal.

One way to solve this problem is to compile your entire program with GCC@.
Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned.  A third solution is to modify the function
that uses the pointer to dereference it via the following function
@code{access_double} instead of directly with @samp{*}:

@smallexample
inline double
access_double (double *unaligned_ptr)
@{
  union d2i @{ double d; int i[2]; @};

  union d2i *p = (union d2i *) unaligned_ptr;
  union d2i u;

  u.i[0] = p->i[0];
  u.i[1] = p->i[1];

  return u.d;
@}
@end smallexample

@noindent
Storing into the pointer can be done likewise with the same union.

@item
On Solaris, the @code{malloc} function in the @file{libmalloc.a} library
may allocate memory that is only 4 byte aligned.  Since GCC on the
SPARC assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
@file{libmalloc.a} library.

The solution is to not use the @file{libmalloc.a} library.  Use instead
@code{malloc} and related functions from @file{libc.a}; they do not have
this problem.

@item
On the HP PA machine, ADB sometimes fails to work on functions compiled
with GCC@.  Specifically, it fails to work on functions that use
@code{alloca} or variable-size arrays.  This is because GCC doesn't
generate HP-UX unwind descriptors for such functions.  It may even be
impossible to generate them.

@item
Debugging (@option{-g}) is not supported on the HP PA machine, unless you use
the preliminary GNU tools.

@item
Taking the address of a label may generate errors from the HP-UX
PA assembler.  GAS for the PA does not have this problem.

@item
Using floating point parameters for indirect calls to static functions
will not work when using the HP assembler.  There simply is no way for GCC
to specify what registers hold arguments for static functions when using
the HP assembler.  GAS for the PA does not have this problem.

@item
In extremely rare cases involving some very large functions you may
receive errors from the HP linker complaining about an out of bounds
unconditional branch offset.  This used to occur more often in previous
versions of GCC, but is now exceptionally rare.  If you should run
into it, you can work around by making your function smaller.

@item
GCC compiled code sometimes emits warnings from the HP-UX assembler of
the form:

@smallexample
(warning) Use of GR3 when
  frame >= 8192 may cause conflict.
@end smallexample

These warnings are harmless and can be safely ignored.

@item
In extremely rare cases involving some very large functions you may
receive errors from the AIX Assembler complaining about a displacement
that is too large.  If you should run into it, you can work around by
making your function smaller.

@item
The @file{libstdc++.a} library in GCC relies on the SVR4 dynamic
linker semantics which merges global symbols between libraries and
applications, especially necessary for C++ streams functionality.
This is not the default behavior of AIX shared libraries and dynamic
linking.  @file{libstdc++.a} is built on AIX with ``runtime-linking''
enabled so that symbol merging can occur.  To utilize this feature,
the application linked with @file{libstdc++.a} must include the
@option{-Wl,-brtl} flag on the link line.  G++ cannot impose this
because this option may interfere with the semantics of the user
program and users may not always use @samp{g++} to link his or her
application.  Applications are not required to use the
@option{-Wl,-brtl} flag on the link line---the rest of the
@file{libstdc++.a} library which is not dependent on the symbol
merging semantics will continue to function correctly.

@item
An application can interpose its own definition of functions for
functions invoked by @file{libstdc++.a} with ``runtime-linking''
enabled on AIX@.  To accomplish this the application must be linked
with ``runtime-linking'' option and the functions explicitly must be
exported by the application (@option{-Wl,-brtl,-bE:exportfile}).

@item
AIX on the RS/6000 provides support (NLS) for environments outside of
the United States.  Compilers and assemblers use NLS to support
locale-specific representations of various objects including
floating-point numbers (@samp{.} vs @samp{,} for separating decimal
fractions).  There have been problems reported where the library linked
with GCC does not produce the same floating-point formats that the
assembler accepts.  If you have this problem, set the @env{LANG}
environment variable to @samp{C} or @samp{En_US}.

@item
@opindex fdollars-in-identifiers
Even if you specify @option{-fdollars-in-identifiers},
you cannot successfully use @samp{$} in identifiers on the RS/6000 due
to a restriction in the IBM assembler.  GAS supports these
identifiers.

@end itemize

@node Incompatibilities
@section Incompatibilities of GCC
@cindex incompatibilities of GCC
@opindex traditional

There are several noteworthy incompatibilities between GNU C and K&R
(non-ISO) versions of C@.

@itemize @bullet
@cindex string constants
@cindex read-only strings
@cindex shared strings
@item
GCC normally makes string constants read-only.  If several
identical-looking string constants are used, GCC stores only one
copy of the string.

@cindex @code{mktemp}, and constant strings
One consequence is that you cannot call @code{mktemp} with a string
constant argument.  The function @code{mktemp} always alters the
string its argument points to.

@cindex @code{sscanf}, and constant strings
@cindex @code{fscanf}, and constant strings
@cindex @code{scanf}, and constant strings
Another consequence is that @code{sscanf} does not work on some very
old systems when passed a string constant as its format control string
or input.  This is because @code{sscanf} incorrectly tries to write
into the string constant.  Likewise @code{fscanf} and @code{scanf}.

The solution to these problems is to change the program to use
@code{char}-array variables with initialization strings for these
purposes instead of string constants.

@item
@code{-2147483648} is positive.

This is because 2147483648 cannot fit in the type @code{int}, so
(following the ISO C rules) its data type is @code{unsigned long int}.
Negating this value yields 2147483648 again.

@item
GCC does not substitute macro arguments when they appear inside of
string constants.  For example, the following macro in GCC

@smallexample
#define foo(a) "a"
@end smallexample

@noindent
will produce output @code{"a"} regardless of what the argument @var{a} is.

@cindex @code{setjmp} incompatibilities
@cindex @code{longjmp} incompatibilities
@item
When you use @code{setjmp} and @code{longjmp}, the only automatic
variables guaranteed to remain valid are those declared
@code{volatile}.  This is a consequence of automatic register
allocation.  Consider this function:

@smallexample
jmp_buf j;

foo ()
@{
  int a, b;

  a = fun1 ();
  if (setjmp (j))
    return a;

  a = fun2 ();
  /* @r{@code{longjmp (j)} may occur in @code{fun3}.} */
  return a + fun3 ();
@}
@end smallexample

Here @code{a} may or may not be restored to its first value when the
@code{longjmp} occurs.  If @code{a} is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.

@opindex W
If you use the @option{-W} option with the @option{-O} option, you will
get a warning when GCC thinks such a problem might be possible.

@item
Programs that use preprocessing directives in the middle of macro
arguments do not work with GCC@.  For example, a program like this
will not work: