GNU Info
(gcc-295.info)Incompatibilities
Incompatibilities of GCC
========================
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.
* GCC normally makes string constants read-only. If several
identical-looking string constants are used, GCC stores only one
copy of the string.
One consequence is that you cannot call `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 GCC 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.
* GCC does not substitute macro arguments when they appear inside of
string constants. For example, the following macro in GCC
#define foo(a) "a"
will produce output `"a"' regardless of what the argument A is.
The `-traditional' option directs GCC to handle such cases (among
others) in the old-fashioned (non-ANSI) fashion.
* When you use `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 GCC 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.
* 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:
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.
* Declarations of external variables and functions within a block
apply only to the block containing the declaration. In other
words, they have the same scope as any other declaration in the
same place.
In some other C compilers, a `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.
* In traditional C, you can combine `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.
* PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.
* PCC allows whitespace in the middle of compound assignment
operators such as `+='. GCC, following the ANSI standard, does not
allow this. The difficulty described immediately above applies
here too.
* GCC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if
these comments contain apostrophes, GCC will probably report an
error. For example, this code would produce an error:
#if 0
You can't expect this to work.
#endif
The best solution to such a problem is to put the text into an
actual C comment delimited by `/*...*/'. However, `-traditional'
suppresses these error messages.
* Many user programs contain the declaration `long time ();'. In the
past, the system header files on many systems did not actually
declare `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'.
* When compiling functions that return `float', PCC converts it to a
double. GCC 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.
* When compiling functions that return structures or unions, GCC
output code normally uses a method different from that used on most
versions of Unix. As a result, code compiled with GCC cannot call
a structure-returning function compiled with PCC, and vice versa.
The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or
union with any other size is stored into an address supplied by
the caller (usually in a special, fixed register, but on some
machines it is passed on the stack). The machine-description
macros `STRUCT_VALUE' and `STRUCT_INCOMING_VALUE' tell GCC 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. GCC 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. GCC 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 GCC to use a compatible convention for all structure
and union returning with the option `-fpcc-struct-return'.
* GNU C complains about program fragments such as `0x74ae-0x4000'
which appear to be two hexadecimal constants separated by the minus
operator. Actually, this string is a single "preprocessing token".
Each such token must correspond to one token in C. Since this
does not, GNU C prints an error message. Although it may appear
obvious that what is meant is an operator and two values, the ANSI
C standard specifically requires that this be treated as erroneous.
A "preprocessing token" is a "preprocessing number" if it begins
with a digit and is followed by letters, underscores, digits,
periods and `e+', `e-', `E+', or `E-' character sequences.
To make the above program fragment valid, place whitespace in
front of the minus sign. This whitespace will end the
preprocessing number.
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