This section describes known problems that affect users of GNU CC. Most of these are not GNU CC 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.
protoize.
fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while fixincludes is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
fixproto script will sometimes add prototypes for the
sigsetjmp and siglongjmp functions that reference the
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.
This is a list of problems (and some apparent problems which don't really mean anything is wrong) that show up during installation of GNU CC.
CC can interfere with the functioning of make.
fixincludes if the
System V file system doesn't support symbolic links. These problems
result in a failure to fix the declaration of size_t in
`sys/types.h'. If you find that size_t is a signed type and
that type mismatches occur, this could be the cause.
The solution is not to use such a directory for building GNU CC.
gcc driver program looked for
as and ld in various places; for example, in files
beginning with `/usr/local/lib/gcc-'. GNU CC version 2 looks for
them in the directory
`/usr/local/lib/gcc-lib/target/version'.
Thus, to use a version of as or ld that is not the system
default, for example gas or GNU ld, you must put them in
that directory (or make links to them from that directory).
make. These failures, which
are often due to files that were not found, are expected, and can safely
be ignored.
make recompiles parts of the compiler when installing
the compiler. In one case, this was traced down to a bug in
make. Either ignore the problem or switch to GNU Make.
enquire, which is part of building
GNU CC. The fix is to get rid of the file real-ld which purify
installs--so that GNU CC won't try to use it.
__GNU_LIBRARY__ conditional
around line 31 to `#if 1'.
enquire hangs due to a hardware problem in the motherboard--it
reports floating point exceptions to the kernel incorrectly. You can
install GNU CC except for `float.h' by patching out the command to
run enquire. You may also be able to fix the problem for real by
getting a replacement motherboard. This problem was observed in
Revision E of the Micronics motherboard, and is fixed in Revision F.
It has also been observed in the MYLEX MXA-33 motherboard.
If you encounter this problem, you may also want to consider removing
the FPU from the socket during the compilation. Alternatively, if you
are running SCO Unix, you can reboot and force the FPU to be ignored.
To do this, type `hd(40)unix auto ignorefpu'.
ln /etc/emulator.rel1 /etc/emulatorand then reboot the system. (The default emulator file remains present under the name `emulator.dflt'.) Try using `/etc/emulator.att', if you have such a problem on the SCO system. Another system which has this problem is Esix. We don't know whether it has an alternate emulator that works. On NetBSD 0.8, a similar problem manifests itself as these error messages:
enquire.c: In function `fprop': enquire.c:2328: floating overflow
genflags or genoutput while building GNU CC. This is said to
be due to a bug in sh. You can probably get around it by running
genflags or genoutput manually and then retrying the
make.
pkginfo command. To add an optional package, use the
pkgadd command. For further details, see the Solaris
documentation.
For Solaris 2.0 and 2.1, GNU CC needs six packages: `SUNWarc',
`SUNWbtool', `SUNWesu', `SUNWhea', `SUNWlibm', and
`SUNWtoo'.
For Solaris 2.2, GNU CC needs an additional seventh package: `SUNWsprot'.
PATH.
add.d.
fixincludes. This causes
problems in building GNU CC. Once GNU CC is installed, the problems go
away.
To work around this problem, when making the stage 1 compiler, specify
this option to Make:
GCC_FOR_TARGET="./xgcc -B./ -I./include"When making stage 2 and stage 3, specify this option:
CFLAGS="-g -I./include"
alloca against shared
libraries on RISC-OS 5.0, and DEC's OSF/1 systems. This is a bug
in the linker, that is supposed to be fixed in future revisions.
To protect against this, GNU CC passes `-non_shared' to the
linker unless you pass an explicit `-shared' or
`-call_shared' switch.
ld fatal: failed to write symbol name something in strings table for file whateverThis probably indicates that the disk is full or your ULIMIT won't allow the file to be as large as it needs to be. This problem can also result because the kernel parameter
MAXUMEM
is too small. If so, you must regenerate the kernel and make the value
much larger. The default value is reported to be 1024; a value of 32768
is said to work. Smaller values may also work.
/usr/local/lib/bison.simple: In function `yyparse': /usr/local/lib/bison.simple:625: virtual memory exhaustedthat too indicates a problem with disk space, ULIMIT, or
MAXUMEM.
MAXUMEM = 4096
_floatdisf cc1: warning: `-g' option not supported on this version of GCC cc1: warning: `-g1' option not supported on this version of GCC ./xgcc: Internal compiler error: program as got fatal signal 11A patched version of the assembler is available by anonymous ftp from
altdorf.ai.mit.edu as the file
`archive/cph/hpux-8.0-assembler'. If you have HP software support,
the patch can also be obtained directly from HP, as described in the
following note:
This patch is also known as PHCO_4484.This is the patched assembler, to patch SR#1653-010439, where the assembler aborts on floating point constants.
The bug is not really in the assembler, but in the shared library version of the function "cvtnum(3c)". The bug on "cvtnum(3c)" is SR#4701-078451. Anyway, the attached assembler uses the archive library version of "cvtnum(3c)" and thus does not exhibit the bug.
fixproto shell script triggers a bug in the system shell.
If you encounter this problem, upgrade your operating system or
use BASH (the GNU shell) to run fixproto.
muldi3 in file `libgcc2.c'.
You may be able to succeed by getting GNU CC version 1, installing it,
and using it to compile GNU CC version 2. The bug in the Pyramid C
compiler does not seem to affect GNU CC version 1.
va_arg when you build GNU CC.
If this happens, then you need to link most programs with the library
`iclib.a'. You must also modify `stdio.h' as follows: before
the lines
#if defined(__i860__) && !defined(_VA_LIST) #include <va_list.h>insert the line
#if __PGC__and after the lines
extern int vprintf(const char *, va_list ); extern int vsprintf(char *, const char *, va_list ); #endifinsert the line
#endif /* __PGC__ */These problems don't exist in operating system version 1.1.
./fixproto: sh internal 1K buffer overflowTo fix this, change the first line of the fixproto script to look like:
#!/bin/ksh
You may run into problems with cross compilation on certain machines, for several reasons.
REAL_VALUE_TYPE. But doing so is a substantial amount of
work for each target machine.
See section Cross Compilation and Floating Point.
This section lists various difficulties encountered in using GNU C or GNU C++ together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.
fixincludes. The corrected header files go in a new
directory; GNU CC searches this directory before `/usr/include'.
If you use `-I/usr/include', this tells GNU CC to search
`/usr/include' earlier on, before the corrected headers. The
result is that you get the uncorrected header files.
Instead, you should use these options (when compiling C programs):
-I/usr/local/lib/gcc-lib/target/version/include -I/usr/includeFor C++ programs, GNU CC also uses a special directory that defines C++ interfaces to standard C subroutines. This directory is meant to be searched before other standard include directories, so that it takes precedence. If you are compiling C++ programs and specifying include directories explicitly, use this option first, then the two options above:
-I/usr/local/lib/g++-include
double on an 8-byte
boundary, and it expects every double to be so aligned. The Sun
compiler usually gives double values 8-byte alignment, with one
exception: function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type double and passes this pointer of type
double * to a function compiled with GNU CC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GNU
CC. 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
access_double instead of directly with `*':
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;
}
Storing into the pointer can be done likewise with the same union.
malloc function in the `libmalloc.a' library
may allocate memory that is only 4 byte aligned. Since GNU CC 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
`libmalloc.a' library.
The solution is to not use the `libmalloc.a' library. Use instead
malloc and related functions from `libc.a'; they do not have
this problem.
_dlclose, _dlsym or _dlopen
when linking, compile and link against the file
`mit/util/misc/dlsym.c' from the MIT version of X windows.
cc does not
compile GNU CC correctly. We do not yet know why. However, GNU CC
compiled on earlier HP-UX versions works properly on HP-UX 9.01 and can
compile itself properly on 9.01.
alloca or variable-size arrays. This is because GNU CC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
(warning) Use of GR3 when frame >= 8192 may cause conflict.These warnings are harmless and can be safely ignored.
as -u < /dev/nullIf the command exits normally, the assembler fix already is installed. If the assembler complains that "-u" is an unknown flag, you need to order the fix.
extern int foo; ... foo ... static int foo;will cause the linker to report an undefined symbol
foo.
Although this behavior differs from most other systems, it is not a
bug because redefining an extern variable as static
is undefined in ANSI C.
size_t. You should change `sys/types.h' by adding these
lines around the definition of size_t:
#ifndef _SIZE_T #define _SIZE_T actual typedef here #endif
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
-L/usr/local/lib/gcc-lib/we32k-att-sysv/2.7.1 -lgcc -lc_sThe first specifies where to find the library `libgcc.a' specified with the `-lgcc' option. GNU CC does linking by invoking
ld, just as cc does, and
there is no reason why it should matter which compilation program
you use to invoke ld. If someone tracks this problem down,
it can probably be fixed easily.
ecvt, fcvt and gcvt. Given valid
floating point numbers, they sometimes print `NaN'.
Certain programs have problems compiling.
#ifdef __STDC__ #define NeedFunctionPrototypes 0 #endif
-traditional -Dvolatile=__volatile__ -I/usr/include/sun -I/usr/ucbinclude -fpcc-struct-returnmost of which are unnecessary with GCC 2.4.5 and newer versions. You can make a properly working Perl by setting
ccflags to
`-fwritable-strings' (implied by the `-traditional' in the
original options) and cppflags to empty in `config.sh', then
typing `./doSH; make depend; make'.
MALLOC=/usr/local/lib/libgmalloc.aAlternatively, if you have compiled `gmalloc.c' from Emacs 19, copy the object file to `gmalloc.o' and use this option when you relink GNU CC:
MALLOC=gmalloc.o
There are several noteworthy incompatibilities between GNU C and most existing (non-ANSI) versions of C. The `-traditional' option eliminates many of these incompatibilities, but not all, by telling GNU C to behave like the other C compilers.
mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.
Another consequence is that sscanf does not work on some systems
when passed a string constant as its format control string or input.
This is because sscanf incorrectly tries to write into the string
constant. Likewise fscanf and scanf.
The best solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the `-fwritable-strings' flag, which directs GNU CC
to handle string constants the same way most C compilers do.
`-traditional' also has this effect, among others.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so
(following the ANSI C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.
#define foo(a) "a"will produce output
"a" regardless of what the argument a is.
The `-traditional' option directs GNU CC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.
setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* longjmp (j) may occur in fun3. */
return a + fun3 ();
}
Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the `-W' option with the `-O' option, you will
get a warning when GNU CC thinks such a problem might be possible.
The `-traditional' option directs GNU C to put variables in
the stack by default, rather than in registers, in functions that
call setjmp. This results in the behavior found in
traditional C compilers.
foobar (
#define luser
hack)
ANSI C does not permit such a construct. It would make sense to support
it when `-traditional' is used, but it is too much work to
implement.
extern declaration affects all the
rest of the file even if it happens within a block.
The `-traditional' option directs GNU C to treat all extern
declarations as global, like traditional compilers.
long, etc., with a typedef name,
as shown here:
typedef int foo; typedef long foo bar;In ANSI C, this is not allowed:
long and other type modifiers
require an explicit int. Because this criterion is expressed
by Bison grammar rules rather than C code, the `-traditional'
flag cannot alter it.
#if 0 You can't expect this to work. #endifThe best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'. However, `-traditional' suppresses these error messages.
time, so it did not matter what type your program declared it to
return. But in systems with ANSI C headers, time is declared to
return time_t, and if that is not the same as long, then
`long time ();' is erroneous.
The solution is to change your program to use time_t as the return
type of time.
float, PCC converts it to
a double. GNU CC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.
STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GNU CC where to pass this address.
By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. GNU CC does not use this method because it is
slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GNU CC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.
You can tell GNU CC to use a compatible convention for all structure and
union returning with the option `-fpcc-struct-return'.
GNU CC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GNU CC unless they are changed. Some have bugs, some are incompatible with ANSI C, and some depend on special features of other compilers.
Installing GNU CC automatically creates and installs the fixed header
files, by running a program called fixincludes (or for certain
targets an alternative such as fixinc.svr4). Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
fixincludes script to fail.
This means you will encounter problems due to bugs in the system header
files. It may be no comfort that they aren't GNU CC's fault, but it
does mean that there's nothing for us to do about them.
GNU CC by itself attempts to be what the ISO/ANSI C standard calls a conforming freestanding implementation. This means all ANSI C language features are available, as well as the contents of `float.h', `limits.h', `stdarg.h', and `stddef.h'. The rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using `-Wall') that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns
char * while the C standard says that sprintf returns an
int. The fixincludes program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return char *.
If you need a Standard compliant library, then you need to find one, as
GNU CC does not provide one. The GNU C library (called glibc)
has been ported to a number of operating systems, and provides ANSI/ISO,
POSIX, BSD and SystemV compatibility. You could also ask your operating
system vendor if newer libraries are available.
These problems are perhaps regrettable, but we don't know any practical way around them.
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
This code really is erroneous, because the scope of struct
mumble in the prototype is limited to the argument list containing it.
It does not refer to the struct mumble defined with file scope
immediately below--they are two unrelated types with similar names in
different scopes.
But in the definition of foo, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ANSI standard specifies.
It is easy enough for you to make your code work by moving the
definition of struct mumble above the prototype. It's not worth
being incompatible with ANSI C just to avoid an error for the example
shown above.
include; then do
`make install' again.
double in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the `-ffloat-store'
option (see section Options That Control Optimization).
C++ is a complex language and an evolving one, and its standard definition (the ANSI C++ draft standard) is also evolving. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.
When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo
{
...
void method();
static int bar;
};
This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define both
method and bar elsewhere. According to the draft ANSI
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++ from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++ reports as undefined
symbols any static data members that lack definitions.
It is dangerous to use pointers or references to portions of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like the libg++
String class, that define a conversion function to type
char * or const char *. However, any class that returns
a pointer to some internal structure is potentially subject to this
problem.
For example, a program may use a function strfunc that returns
String objects, and another function charfunc that
operates on pointers to char:
String strfunc (); void charfunc (const char *);
In this situation, it may seem natural to write `charfunc
(strfunc ());' based on the knowledge that class String has an
explicit conversion to char pointers. However, what really
happens is akin to `charfunc (strfunc ().convert ());',
where the convert method is a function to do the same data
conversion normally performed by a cast. Since the last use of the
temporary String object is the call to the conversion function,
the compiler may delete that object before actually calling
charfunc. The compiler has no way of knowing that deleting the
String object will invalidate the pointer. The pointer then
points to garbage, so that by the time charfunc is called, it
gets an invalid argument.
Code like this may run successfully under some other compilers, especially those that delete temporaries relatively late. However, the GNU C++ behavior is also standard-conforming, so if your program depends on late destruction of temporaries it is not portable.
If you think this is surprising, you should be aware that the ANSI C++ committee continues to debate the lifetime-of-temporaries problem.
For now, at least, the safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:
String& tmp = strfunc (); charfunc (tmp);
protoize
The conversion programs protoize and unprotoize can
sometimes change a source file in a way that won't work unless you
rearrange it.
protoize can insert references to a type name or type tag before
the definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the new
references are, so fixing the file by hand is straightforward.
protoize cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. protoize
inserts a comment containing `???' each time it finds such a
variable; so you can find all such variables by searching for this
string. ANSI C does not require declaring the argument types of
pointer-to-function types.
unprotoize can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure unprotoize is safe is when
you are removing prototypes that were made with protoize; if
the program worked before without any prototypes, it will work again
without them.
You can find all the places where this problem might occur by compiling
the program with the `-Wconversion' option. It prints a warning
whenever an argument is converted.
protoize cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing conditionals.
When this happens, protoize changes nothing in regard to such
a function. protoize tries to detect such instances and warn
about them.
You can generally work around this problem by using protoize step
by step, each time specifying a different set of `-D' options for
compilation, until all of the functions have been converted. There is
no automatic way to verify that you have got them all, however.
unprotoize can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such choices
of formal parameter names.
This section lists changes that people frequently request, but which we do not make because we think GNU CC is better without them.
void whenever the value isn't
useful.
int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed
dialect with `-fsigned-bitfields' and the unsigned dialect with
`-funsigned-bitfields'. However, this leaves open the question of
which dialect to use by default.
Currently, the preferred dialect makes plain bitfields signed, because
this is simplest. Since int is the same as signed int in
every other context, it is cleanest for them to be the same in bitfields
as well.
Some computer manufacturers have published Application Binary Interface
standards which specify that plain bitfields should be unsigned. It is
a mistake, however, to say anything about this issue in an ABI. This is
because the handling of plain bitfields distinguishes two dialects of C.
Both dialects are meaningful on every type of machine. Whether a
particular object file was compiled using signed bitfields or unsigned
is of no concern to other object files, even if they access the same
bitfields in the same data structures.
A given program is written in one or the other of these two dialects.
The program stands a chance to work on most any machine if it is
compiled with the proper dialect. It is unlikely to work at all if
compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently on
certain machines.
Occasionally users write programs intended only for a particular machine
type. On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine. But such applications are rare. And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.
This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).
There are some arguments for making bitfields unsigned by default on all
machines. If, for example, this becomes a universal de facto standard,
it would make sense for GNU CC to go along with it. This is something
to be considered in the future.
(Of course, users strongly concerned about portability should indicate
explicitly in each bitfield whether it is signed or not. In this way,
they write programs which have the same meaning in both C dialects.)
__STDC__ when `-ansi' is not used.
Currently, GNU CC defines __STDC__ as long as you don't use
`-traditional'. This provides good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain `gcc' supports
all the features of ANSI C, the correct answer to these questions is
"yes".
Some users try to use __STDC__ to check for the availability of
certain library facilities. This is actually incorrect usage in an ANSI
C program, because the ANSI C standard says that a conforming
freestanding implementation should define __STDC__ even though it
does not have the library facilities. `gcc -ansi -pedantic' is a
conforming freestanding implementation, and it is therefore required to
define __STDC__, even though it does not come with an ANSI C
library.
Sometimes people say that defining __STDC__ in a compiler that
does not completely conform to the ANSI C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ANSI C, such as `gcc -ansi'---not for other
compilers such as plain `gcc'. Whatever the ANSI C standard says
is relevant to the design of plain `gcc' without `-ansi' only
for pragmatic reasons, not as a requirement.
__STDC__ in C++.
Programs written to compile with C++-to-C translators get the
value of __STDC__ that goes with the C compiler that is
subsequently used. These programs must test __STDC__
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ANSI C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ANSI
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
void func (int, int); int i = 2; func (i++, i++);There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first.
func might get the
arguments `2, 3', or it might get `3, 2', or even `2, 2'.
The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the `-W' options (for instance, `-Wall' requests a variety of useful warnings).
GNU CC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The `-pedantic' option tells GNU CC to issue warnings in such cases; `-pedantic-errors' says to make them errors instead. This does not mean that all non-ANSI constructs get warnings or errors.
See section Options to Request or Suppress Warnings, for more detail on these and related command-line options.