#4138 Old-style (K&R) function definitions silently accept void parameters
1: Sample code that reproduces the problem.
void f(value)
void value;
{
(void)sizeof &value;
}
2: Exact command used to run SDCC on this sample code.
sdcc -mz80 --std=c89 -S repro.c
This is an old-style (K&R) function definition. --std=c89 selects a
language mode that permits that syntax; the same behaviour also reproduces
with --std=c11.
3: SDCC version tested.
SDCC : mcs51/z80/z180/r2k/r2ka/r3ka/r4k/r5k/r6k/sm83/tlcs90/ez80/z80n/r800/ds390/pic16/pic14/TININative/ds400/hc08/s08/stm8/pdk13/pdk14/pdk15/mos6502/mos65c02/huc6280/f8/f8l TD- 4.6.4 #16997 (Linux)
4: Observed and expected behaviour.
SDCC exits successfully and emits no diagnostic on Z80, STM8 and MCS51 with --stack-auto. The parameter declaration gives value type void; it does not denote a function with no parameters, as f(void) would. The body asks only for the size of a pointer to the parameter, avoiding an unreferenced-parameter warning and SDCC's separate diagnostic when casting a void expression to void.
GCC 16.2.0 rejects the same source with:
error: parameter 'value' declared with void type
It would be useful for SDCC to diagnose the invalid parameter consistently, before target-dependent allocation. MCS51 default/large, PDK14 and S08 already reject the sample with error 66:
attempt to allocate variable of unknown size 'value'
C11 draft N1570,
6.9.1 paragraph 7, requires each adjusted parameter type in a function definition, including an identifier-list definition, to be a complete object type. void does not satisfy this requirement.
If the declarator includes an identifier list, the types of the parameters shall be declared in a following declaration list. In either case, the type of each parameter is adjusted as described in 6.7.6.3 for a parameter type list; the resulting type shall be a complete object type.
5: Additional checks and Compiler Explorer comparison.
Compiler Explorer: SDCC r16997 beside GCC 16.2.
Both panes use the sample above. SDCC uses -mz80 --std=c89; GCC uses -std=c89 -pedantic-errors -fsyntax-only. Compiler versions, compilation results, shared source and saved compiler options were checked on 12 October 2026. SDCC is silent; GCC diagnoses the void parameter.
A typedef alias for void behaves the same way. Old-style definitions with an int parameter or a void * parameter, and a definition using f(void), are valid cases: all three compile without diagnostics on all seven local configurations and are accepted by GCC 16.2 on Compiler Explorer.
| Configuration | Direct void parameter, C89 and C11 modes | Typedef for void, C89 mode |
|---|---|---|
| Z80 | Accepted silently | Accepted silently |
| STM8 | Accepted silently | Accepted silently |
| MCS51 stack-auto | Accepted silently | Accepted silently |
| MCS51 default | Error 66 | Error 66 |
| MCS51 large | Error 66 | Error 66 |
| PDK14 | Error 66 | Error 66 |
| S08 | Error 66 | Error 66 |
The current candidate for bugs:#4135 does not change these results. That candidate checks completeness in checkFunction, called by createFunctionDecl; old-style parameter declarations are merged afterwards by mergeKRDeclListIntoFuncDecl. The parser's existing void-parameter checks handle parameter-type lists, but do not catch this declaration-list form.
