Question

While I am reading the FreeBSD source code about the hlist in the file list.h, I am confused about this macro:

#define hlist_for_each_entry_safe(tp, p, n, head, field)        \
for (p = (head)->first; p ?                 \
    (n = p->next) | (tp = hlist_entry(p, typeof(*tp), field)) : \
    NULL; p = n)

and I am trying use the similar style in my function as below:

int *a;
int *b;
int *c;

if(a ? (b = (int *)0x0c) | (c = (int *)malloc(sizeof(int)) : NULL){

    printf("test\n");
}

when I use GCC compile it, the compiler give me the error, says:

invalid operands to binary "|"

I write my function in that way, because I think the condition of the “for” circulation returns “true” or “false”, but in fact the compiler seems to tell me they are not the same.And then I do not understand the condition between “for” and “if”.

PS:My mother tongue is not English, maybe you could not understand it well, I am sorry about that.

Answer 1

You can’t | with pointers. You have to use an integral type. I don’t know about the FreeBSD code you cite, but they may use uintptr_t.

Regardless, littleadv is right. Don’t ever do this. It is unreadable and unmaintainable gobbledygook.