Question

I want to initialize a linked list with pointer arguments like so:

/* 
 * Initialize a linked list using variadic arguments
 * Returns the number of structures initialized
 */
int init_structures(struct structure *first, ...) 
{
    struct structure *s;
    unsigned int count = 0;
    va_list va;
    va_start(va, first);

    for (s = first; s != NULL; s = va_arg(va, (struct structure *))) {
        if ((s = malloc(sizeof(struct structure))) == NULL) {
            perror("malloc");
            exit(EXIT_FAILURE);
        }
        count++;
    }

    va_end(va);

    return count;
}

The problem is that clang errors type name requires a specifier or qualifier at va_arg(va, (struct structure *)), and says that the type specifier defaults to int. It also notes instantiated form at (struct structure *) and struct structure *. This, what seems to be getting assigned to s is int (struct structure *).

It compiles fine when parentheses are removed from (struct structure *), but the structures that are supposed to be initialized are inaccessible.

Why is int assumed when parentheses are around the type argument passed to va_arg? How can I fix this?

Answer 1

va_arg is a macro on many systems, and evidently the parentheses around struct structure * causes the macro to expand so something unparseable. So don’t do that.

This has nothing to do with the reason that your initialized structures are “inaccessible”. You are allocating structures and assigning them to s, but s is a local variable. You can’t affect a value in the caller by assigning to a local variable. To accomplish what you want to do, the caller needs to pass a pointer-to-a-pointer, which you can then initialize

int init_structures(struct structure **first, ...) 
{
    struct structure **s;
    unsigned int count = 0;
    va_list va;
    va_start(va, first);

    for (s = first; s != NULL; s = va_arg(va, struct structure **)) {
        if ((*s = malloc(sizeof(struct structure))) == NULL) {
            perror("malloc");
            exit(EXIT_FAILURE);
        }
        count++;
    }

    va_end(va);

    return count;
}

And the caller should do:

struct structure *a, *b;
init_structures(&a, &b, NULL);