Plain "C" static initializer macro with variable length tail.

Question

I have a struct defined as:

typedef struct coro_context {
    int id;
    jmp_buf env;
    list_head list;
    jmp_buf waiter;
    long timeout;
    void *private;
    char stack[0];
} coro_context;

which I need to initialize with three values

• id = 0
• private = function pointer
• append to end of struct a suitable char array of length size

My current attempt looks like (the "unsigned long" bit is to get the right alignment):

#define CORO_CONTEXT(name,size) \
        unsigned long __##name##_ctx[(sizeof(coro_context)+size)/sizeof(unsigned long)]={0};\
        coro_context *name##_ctx = (coro_context *)__##name##_ctx

This works, but has two problems (ok, problems are one and a half ;) ):

• it is UGLY (half problem).
• I see no way to statically initialize private = name.

Note: I insist on "static initialization" because I want to use this in plain "C" (c11 is ok, if required) because I need to use these initialization outside function context.

Answer 1

I'd use a union overlaying the struct coro_context instance with a char buffer:

#include <setjmp.h>
typedef struct coro_context {
    int id;
    jmp_buf env;
    list_head list;
    jmp_buf waiter;
    long timeout;
    void *private;
    char stack[]; /*should be [] in C11, [0] is not valid C */
} coro_context;

/*shouldn't start globals with underscore*/
/*shouldn't violate aliasing rules*/
/*C11 compound literals obviate the need for an extra global identifier*/
#define CORO_CONTEXT(name,size) \
    coro_context *name##_ctx = &(union { coro_context ctx; char buf[sizeof(coro_context)+size]; }){ .ctx={ .private=&name } }.ctx;

int my_name;

CORO_CONTEXT(my_name,32)

and then take the address of .ctx. That should also get rid off the aliasing issues your solution has.

You'll need to initialize with an address of a global, because the value of a global is not usable in static initializations.

If you want something C99 compatible, you will need the extra identifier, but it shouldn't start with two underscores:

#include <setjmp.h>
typedef struct coro_context {
    int id;
    jmp_buf env;
    list_head list;
    jmp_buf waiter;
    long timeout;
    void *private;
    char stack[1]; /*c99 doesn't have flexible array members*/
} coro_context;

#define CORO_CONTEXT(name,size) \
    union { coro_context ctx; char buf[sizeof(coro_context)+size]; } name##_ctx__ = { { 0, {0},{0},{0}, 0, &name } }; \
    coro_context *name##_ctx = &name##_ctx__.ctx;

int my_name;

CORO_CONTEXT(my_name,32)