Strange syntax error C2143 in Visual only (missing ';' before 'type')

Question

I'm getting a strange compilation error for a C code in MSVC only. More precisely:

error C2143: syntax error : missing ';' before 'type'

C2143 is a fairly generic error, and there are myriad of questions on SO around it, but none of them seems to apply so far. The closest one can be found here, and stress the importance of declaring variables at the beginning of a block, which seems to have been respected here.

Here is a sample code:

#define       NB_LL 6
typedef struct { long long ll[NB_LL ]; } stateSpace_t;
#define ALLOCATE_ONSTACK(stateName)  stateSpace_t stateName##_s; void* stateName = (void*) &(stateName##_s);

The following code works well:

void f1()
{
    ALLOCATE_ONSTACK(state1);
    /* do something */
}

This one doesn't:

void f2()
{
    ALLOCATE_ONSTACK(state1);
    ALLOCATE_ONSTACK(state2);   // <--- error C2143: syntax error : missing ';' before 'type'
    /* do something */
}

The second code works well with GCC, so the issue seems restricted to MSVC. My understanding is that macro ALLOCATE_ONSTACK() only do variable declaration and initialization, so it seems to respect C syntax.

Is it?

Answer 1

OK, this one is fairly convoluted.

Look at the

#define ALLOCATE_ONSTACK(stateName)

It ends with a ; character.

Now look at your code :

ALLOCATE_ONSTACK(state1);

It also ends with a ';' character. That means that, on this particular line, you have 2 following ';' characters.

Since MSVC is not C99, it requires all declarations to be done at the beginning of the block. Since you have two ';' characters following each other, it acts as if the declaration area was ended. So, when you declare other variables in :

ALLOCATE_ONSTACK(state2);

it then fails, syntax error.

GCC has no such problem since it is C99.

Either remove the ';' character at the end of the macro, or within your source code. Only one is required. Not sure which solution is better...

[Edit] : As suggested in comments and other answer, removing the semicolon from the macro looks the better solution.