C Closures as a Library

A common idiom is C is the callback function pointer, either to deliver information (i.e. a visitor or handler) or to customize the function’s behavior (e.g. a comparator). Examples of the latter in the C standard library are qsort() and bsearch(), each requiring a comparator function in order to operate on arbitrary types.

void qsort(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *));

void *bsearch(const void *key, const void *base,
              size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));

A problem with these functions is that there’s no way to pass context to the callback. The callback may need information beyond the two element pointers when making its decision, or to update a result. For example, suppose I have a structure representing a two-dimensional coordinate, and a coordinate distance function.

struct coord {
    float x;
    float y;
};

static inline float
distance(const struct coord *a, const struct coord *b)
{
    float dx = a->x - b->x;
    float dy = a->y - b->y;
    return sqrtf(dx * dx + dy * dy);
}

If I have an array of coordinates and I want to sort them based on their distance from some target, the comparator needs to know the target. However, the qsort() interface has no way to directly pass this information. Instead it has to be passed by another means, such as a global variable.

struct coord *target;

int
coord_cmp(const void *a, const void *b)
{
    float dist_a = distance(a, target);
    float dist_b = distance(b, target);
    if (dist_a < dist_b)
        return -1;
    else if (dist_a > dist_b)
        return 1;
    else
        return 0;
}

And its usage:

    size_t ncoords = /* ... */;
    struct coords *coords = /* ... */;
    struct current_target = { /* ... */ };
    // ...
    target = &current_target
    qsort(coords, ncoords, sizeof(coords[0]), coord_cmp);

Potential problems are that it’s neither thread-safe nor re-entrant. Two different threads cannot use this comparator at the same time. Also, on some platforms and configurations, repeatedly accessing a global variable in a comparator may have a significant cost. A common workaround for thread safety is to make the global variable thread-local by allocating it in thread-local storage (TLS):

_Thread_local struct coord *target;       // C11
__thread struct coord *target;            // GCC and Clang
__declspec(thread) struct coord *target;  // Visual Studio

This makes the comparator thread-safe. However, it’s still not re-entrant (usually unimportant) and accessing thread-local variables on some platforms is even more expensive — which is the situation for Pthreads TLS, though not a problem for native x86-64 TLS.

Modern libraries usually provide some sort of “user data” pointer — a generic pointer that is passed to the callback function as an additional argument. For example, the GNU C Library has long had qsort_r(): re-entrant qsort.

void qsort_r(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *, void *),
           void *arg);

The new comparator looks like this:

int
coord_cmp_r(const void *a, const void *b, void *target)
{
    float dist_a = distance(a, target);
    float dist_b = distance(b, target);
    if (dist_a < dist_b)
        return -1;
    else if (dist_a > dist_b)
        return 1;
    else
        return 0;
}

And its usage:

    void *arg = &current_target;
    qsort_r(coords, ncoords, sizeof(coords[0]), coord_cmp_r, arg);

User data arguments are thread-safe, re-entrant, performant, and perfectly portable. They completely and cleanly solve the entire problem with virtually no drawbacks. If every library did this, there would be nothing left to discuss and this article would be boring.

The closure solution

In order to make things more interesting, suppose you’re stuck calling a function in some old library that takes a callback but doesn’t support a user data argument. A global variable is insufficient, and the thread-local storage solution isn’t viable for one reason or another. What do you do?

The core problem is that a function pointer is just an address, and it’s the same address no matter the context for any particular callback. On any particular call, the callback has three ways to distinguish this call from other calls. These align with the three solutions above:

  1. Inspect some global state: the global variable solution. The caller will change this state for some other calls.
  2. Query its unique thread ID: the thread-local storage solution. Calls on different threads will have different thread IDs.
  3. Examine a context argument: the user pointer solution.

A wholly different approach is to use a unique function pointer for each callback. The callback could then inspect its own address to differentiate itself from other callbacks. Imagine defining multiple instances of coord_cmp each getting their context from a different global variable. Using a unique copy of coord_cmp on each thread for each usage would be both re-entrant and thread-safe, and wouldn’t require TLS.

Taking this idea further, I’d like to generate these new functions on demand at run time akin to a JIT compiler. This can be done as a library, mostly agnostic to the implementation of the callback. Here’s an example of what its usage will be like:

void *closure_create(void *f, int nargs, void *userdata);
void  closure_destroy(void *);

The callback to be converted into a closure is f and the number of arguments it takes is nargs. A new closure is allocated and returned as a function pointer. This closure takes nargs - 1 arguments, and it will call the original callback with the additional argument userdata.

So, for example, this code uses a closure to convert coord_cmp_r into a function suitable for qsort():

int (*closure)(const void *, const void *);
closure = closure_create(coord_cmp_r, 3, &current_target);

qsort(coords, ncoords, sizeof(coords[0]), closure);

closure_destroy(closure);

Caveat: This API is utterly insufficient for any sort of portability. The number of arguments isn’t nearly enough information for the library to generate a closure. For practically every architecture and ABI, it’s going to depend on the types of each of those arguments. On x86-64 with the System V ABI — where I’ll be implementing this — this argument will only count integer/pointer arguments. To find out what it takes to do this properly, see the libjit documentation.

Memory design

This implementation will be for x86-64 Linux, though the high level details will be the same for any program running in virtual memory. My closures will span exactly two consecutive pages (typically 8kB), though it’s possible to use exactly one page depending on the desired trade-offs. The reason I need two pages are because each page will have different protections.

Native code — the thunk — lives in the upper page. The user data pointer and callback function pointer lives at the high end of the lower page. The two pointers could really be anywhere in the lower page, and they’re only at the end for aesthetic reasons. The thunk code will be identical for all closures of the same number of arguments.

The upper page will be executable and the lower page will be writable. This allows new pointers to be set without writing to executable thunk memory. In the future I expect operating systems to enforce W^X (“write xor execute”), and this code will already be compliant. Alternatively, the pointers could be “baked in” with the thunk page and immutable, but since creating closure requires two system calls, I figure it’s better that the pointers be mutable and the closure object reusable.

The address for the closure itself will be the upper page, being what other functions will call. The thunk will load the user data pointer from the lower page as an additional argument, then jump to the actual callback function also given by the lower page.

Thunk assembly

The x86-64 thunk assembly for a 2-argument closure calling a 3-argument callback looks like this:

user:  dq 0
func:  dq 0
;; --- page boundary here ---
thunk2:
        mov  rdx, [rel user]
        jmp  [rel func]

As a reminder, the integer/pointer argument register order for the System V ABI calling convention is: rdi, rsi, rdx, rcx, r8, r9. The third argument is passed through rdx, so the user pointer is loaded into this register. Then it jumps to the callback address with the original arguments still in place, plus the new argument. The user and func values are loaded RIP-relative (rel) to the address of the code. The thunk is using the callback address (its own address) to determine the context.

The assembled machine code for the thunk is just 13 bytes:

unsigned char thunk2[16] = {
    // mov  rdx, [rel user]
    0x48, 0x8b, 0x15, 0xe9, 0xff, 0xff, 0xff,
    // jmp  [rel func]
    0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
}

All closure_create() has to do is allocate two pages, copy this buffer into the upper page, adjust the protections, and return the address of the thunk. Since closure_create() will work for nargs number of arguments, there will actually be 6 slightly different thunks, one for each of the possible register arguments (rdi through r9).

static unsigned char thunk[6][13] = {
    {
        0x48, 0x8b, 0x3d, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    }, {
        0x48, 0x8b, 0x35, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    }, {
        0x48, 0x8b, 0x15, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    }, {
        0x48, 0x8b, 0x0d, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    }, {
        0x4C, 0x8b, 0x05, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    }, {
        0x4C, 0x8b, 0x0d, 0xe9, 0xff, 0xff, 0xff,
        0xff, 0x25, 0xeb, 0xff, 0xff, 0xff
    },
};

Given a closure pointer returned from closure_create(), here are the setter functions for setting the closure’s two pointers.

void
closure_set_data(void *closure, void *data)
{
    void **p = closure;
    p[-2] = data;
}

void
closure_set_function(void *closure, void *f)
{
    void **p = closure;
    p[-1] = f;
}

In closure_create(), allocation is done with an anonymous mmap(), just like in my JIT compiler. It’s initially mapped writable in order to copy the thunk, then the thunk page is set to executable.

void *
closure_create(void *f, int nargs, void *userdata)
{
    long page_size = sysconf(_SC_PAGESIZE);
    int prot = PROT_READ | PROT_WRITE;
    int flags = MAP_ANONYMOUS | MAP_PRIVATE;
    char *p = mmap(0, page_size * 2, prot, flags, -1, 0);
    if (p == MAP_FAILED)
        return 0;

    void *closure = p + page_size;
    memcpy(closure, thunk[nargs - 1], sizeof(thunk[0]));
    mprotect(closure, page_size, PROT_READ | PROT_EXEC);

    closure_set_function(closure, f);
    closure_set_data(closure, userdata);
    return closure;
}

Destroying a closure is done by computing the lower page address and calling munmap() on it:

void
closure_destroy(void *closure)
{
    long page_size = sysconf(_SC_PAGESIZE);
    munmap((char *)closure - page_size, page_size * 2);
}

And that’s it! You can see the entire demo here:

It’s a lot simpler for x86-64 than it is for x86, where there’s no RIP-relative addressing and arguments are passed on the stack. The arguments must all be copied back onto the stack, above the new argument, and it cannot be a tail call since the stack has to be fixed before returning. Here’s what the thunk looks like for a 2-argument closure:

data:	dd 0
func:	dd 0
;; --- page boundary here ---
thunk2:
        call .rip2eax
.rip2eax:
        pop eax
        push dword [eax - 13]
        push dword [esp + 12]
        push dword [esp + 12]
        call [eax - 9]
        add esp, 12
        ret

Exercise for the reader: Port the closure demo to a different architecture or to the the Windows x64 ABI.

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Chris Wellons

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