syntaxError for __VA_ARGS__

[Zufu Liu]

extern int printf(const char *fmt, ...);
#if defined(__GNUC__) || defined(__clang__)
#define p1(fmt, ...)    printf((fmt), ##__VA_ARGS__)
#else
#define p1(fmt, ...)    printf((fmt), __VA_ARGS__)
#endif

void test() {
    p1("hello");
}

cppcheck.exe test.c
Checking test.c ...
test.c:9:5: error: syntax error [syntaxError]
    p1("hello");
    ^
Checking test.c: __GNUC__;__clang__...

[Daniel Marjamäki]

ok thanks.. is there some compiler that allows the code printf("hello",); ?

[Zufu Liu]

I think it's not supported by any compiler, but the macro expansion for p1("hello") is different than directly call printf("hello",), it's expanded into printf("hello") without trialling comma, though different compiler accepts different syntax on different platform.

https://godbolt.org/z/x4X_BA

[Daniel Marjamäki]

I am talking about the second macro because Cppcheck writes a syntax error when that is used:

#define p1(fmt, ...)    printf((fmt), __VA_ARGS__)

For that macro, the preprocessor should add the trailing comma as far as I know. For instance "gcc -E" outputs a comma when that macro is used.

If it's not supported by any compiler, I am happy about the Cppcheck warning. If you only care about gcc/clang I suggest -f -D__GNUC__.

[Zufu Liu]

The second one is supported by MSVC and clang-cl, possible Intel C++ for Visual Studio.
first one is supported by GCC, clang, Intel, and MSVC 2017/2019:

https://godbolt.org/z/x4X_BA

https://devblogs.microsoft.com/cppblog/msvc-preprocessor-progress-towards-conformance/

Behavior 4 [comma elision in variadic macros]

[Zufu Liu]

Anothor issue is C++20's __VA_OPT__:

#include <cstdio>
#define p1(fmt, ...)        printf(fmt __VA_OPT__(,) __VA_ARGS__)

void test() {
    p1("hello");
    p1("%s", "hello");
}

Cppcheck 2.0 (not tested master code):

cppcheck.exe test.cpp
Checking test.cpp ...
test.cpp:6:2: error: There is an unknown macro here somewhere. Configuration is required. If __VA_OPT__ is a macro then please configure it. [unknownMacro]
 p1("%s", "hello");
 ^

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0306r4.html
https://devblogs.microsoft.com/cppblog/msvc-preprocessor-progress-towards-conformance/

[Zufu Liu]

for following code:

extern int printf(const char *fmt, ...);
#define p1(fmt, ...)    printf((fmt), ##__VA_ARGS__)
#define p2(fmt, ...)    printf((fmt), __VA_ARGS__)
#define p3(fmt, ...)    printf((fmt) __VA_OPT__(,) __VA_ARGS__)

void test() {
    p1("hello");
    p2("hello");
    p3("hello");
}

g++ -E test.cpp or gcc -x c -E test.cpp (with g++ 10.1 from msys2):

# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.cpp"
extern int printf(const char *fmt, ...);

void test() {
    printf(("hello"));
    printf(("hello"), );
    printf(("hello") );
}

clang++ -E test.cpp (Clang 11 Windows default target msvc):

# 1 "test.cpp"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 367 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "test.cpp" 2
extern int printf(const char *fmt, ...);

void test() {
    printf(("hello"));
    printf(("hello"));
    printf(("hello") __VA_OPT__(,) );
}

clang++ -E -std=c++20 test.cpp

# 1 "test.cpp"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 394 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "test.cpp" 2
extern int printf(const char *fmt, ...);

void test() {
    printf(("hello"));
    printf(("hello"));
    printf(("hello") );
}

cppcheck (master) shows syntax error for p2 and p3.

[Daniel Marjamäki]

It seems to me that __VA_OPT__ should be handled in the preprocessor directly so I created https://github.com/danmar/simplecpp/issues/191 for that.

Well for the other inconsistency it seems to me the standard compliant way is to keep the comma so maybe we should keep simplecpp as it is. maybe it's best that cppcheck removes the comma. I created ticket https://trac.cppcheck.net/ticket/9754 .

[Daniel Marjamäki]

About removing the comma.. hmm that is a problem that we have not bumped into before and it's a deprecated feature. I spontaneosly feels that the payoff would not be that high for it.

[Zufu Liu]

latest doc for gcc and msvc:
https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview#comma-elision-in-variadic-macros

__VA_ARGS__ is the traditional msvc way, ##__VA_ARGS__ (msvc 2017 and later is supported in most compiler), __VA_OPT__ is the standard way (c++20, c2x) .
I think __VA_ARGS__ can be reported as non-portable code (not necessary syntax error at least for msvc project), so cppcheck continue checking remaining file after the trailing comma, currently cppechek stops once a syntax error is met.