https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86418
Bug ID: 86418
Summary: warn about mismatch in type between argument and
parameter type for declaration without prototype
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vries at gcc dot gnu.org
Target Milestone: ---
Keywords: diagnostic
Consider this test-case, based on pr48063.c:
...
extern void abort (void);
static void bar ();
int
main (void)
{
bar (1);
return 0;
}
static void
bar (double i)
{
if (i)
abort ();
}
...
The test-case contains undefined behaviour:
- bar is declared without prototype
- the call to bar in main is before the definition of bar, so the call is
considered calling a function that does not include a prototype (as opposed
to the situation where we move the definition of bar to before main, and the
call uses the prototype provided by the definition of bar)
- the int argument in the call is incompatible with the double parameter of the
definition of bar
In words of the c standard:
...
[ 6.9.1 Function definitions ]
If the declarator includes a parameter type list, the list also specifies the
types of all the parameters; such a declarator also serves as a function
prototype for later calls to the same function in the same translation unit.
...
and:
...
[ 6.5.2.2 Function calls ]
If the expression that denotes the called function has a type that does not
include a prototype, the integer promotions are performed on each argument, and
arguments that have type float are promoted to double. These are called the
default argument promotions. If the number of arguments does not equal the
number of parameters, the behavior is undefined. If the function is defined
with a type that includes a prototype, and either the prototype ends with an
ellipsis (, ...) or the types of the arguments after promotion are not
compatible with the types of the parameters, the behavior is undefined.
...
and:
...
[ J.2 Undefined behavior ]
For call to a function without a function prototype in scope where the function
is defined with a function prototype, either the prototype ends with an
ellipsis or the types of the arguments after promotion are not compatible with
the types of the parameters (6.5.2.2).
...
Atm, the only warning we get is when cloning:
...
$ gcc -O2 pr48063.c -Wall
pr48063.c: In function ‘bar.constprop’:
pr48063.c:18:6: warning: ‘i’ is used uninitialized in this function
[-Wuninitialized]
if (i)
^
...
The cloned version has no parameters, and replaces parameter i with local
variable i, which is uninitialized:
...
bar.constprop ()
{
double i;
<bb 2>
if (i_1(D) != 0.0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>
abort ();
<bb 4>
return;
}
...
