Partial functions in C This program illustrates a hack to create partial functions in C. The way it works is that it generates a template function (partial_template_function) with known pointers, that is later copied to a region of memory obtained with mmap(), patched up with the address and data to be passed to the real function, and then made …

partial.c

/*
 * Partial applied functions in C
 * Leandro Pereira <leandro@tia.mat.br>
 */
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <stdbool.h>
#include <sys/mman.h>

struct Partial {
  void *caller;
  size_t caller_len;
};

#define PARAMETER_CONSTANT 0xFEEDBEEF
#define FUNCTION_CONSTANT 0xABAD1DEA

static bool
patch_pointer(void *code_addr, size_t code_len, void *look_for, void *patch_with)
{
    unsigned char *code = code_addr;
    intptr_t look = (intptr_t)look_for;

    do {
        if (*((intptr_t *)code) == look) {
            union {
              unsigned char octet[sizeof(void *)];
              void *ptr;
            } patch;

            patch.ptr = patch_with;
            code[0] = patch.octet[0];
            code[1] = patch.octet[1];
            code[2] = patch.octet[2];
            code[3] = patch.octet[3];

            return true;
        }

        code++;
    } while (code_len--);

    return false;
}

/*
 * Make sure this function is always declared before partial_new().
 */
static void
partial_template_function(void)
{
    ((void (*)(void *))FUNCTION_CONSTANT)((void *)PARAMETER_CONSTANT);
}

/*
 * Make sure no functions get declared here.
 */

struct Partial *
partial_new(void (*func)(void *data), void *data)
{
    struct Partial *t;

    if (!func) return NULL;

    t = calloc(1, sizeof(*t));
    if (!t) return NULL;

    t->caller_len = (size_t)((intptr_t)partial_new - (intptr_t)partial_template_function);
    assert(t->caller_len > 0);

    /*
     * Map it as RW, without EXEC bits, and only turn it on after pointers
     * have been patched up
     */ 
    t->caller = mmap(0, t->caller_len, PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (t->caller == MAP_FAILED) goto no_mmap;

    /*
     * Copy the code the compiler generated to call an arbitrary function with
     * an arbitrary parameter
     */
    memcpy(t->caller, partial_template_function, t->caller_len);

    /*
     * Change the arbitrary constants to the ones passed as arguments to this
     * function. This thing will only work on 32-bit, little endian
     */
    if (!patch_pointer(t->caller, t->caller_len, (void *)FUNCTION_CONSTANT, func)) goto no_patch;
    if (!patch_pointer(t->caller, t->caller_len, (void *)PARAMETER_CONSTANT, data)) goto no_patch;

    /* Make sure no one can change this later, and that we can execute it */
    mprotect(t->caller, t->caller_len, PROT_EXEC | PROT_READ);

    return t;

no_patch:
    munmap(t->caller, t->caller_len);

no_mmap:
    free(t);

    return NULL;
}

void
partial_del(struct Partial *t)
{
    if (!t) return;

    munmap(t->caller, t->caller_len);
    free(t);
}

void
(*partial_to_function(struct Partial *t))()
{
    if (!t) return NULL;

    return (void (*)(void *))(t)->caller;
}

static void
test(void *data)
{
    printf("it worked! received data: %p\n", data);
}

static void
another_test(void *data)
{
    printf("another test with different data: %p\n", data);
}

int main(void)
{
    struct Partial *t, *t2;
    void (*func)();
    void (*func2)();

    t = partial_new(test, (void *)0x12341337);
    if (!t) goto end;
    t2 = partial_new(another_test, (void *)0xbebac0ca);
    if (!t2) goto end;

    func = (void *)partial_to_function(t);
    if (!func) goto end;
    func2 = partial_to_function(t2);
    if (!func2) goto end;

    /*
     * Now pass `func` around as a callback. When it is called, the
     * parameter passed to partial_new() will be passed to the original
     * function.  Magic.
     */
    atexit(func);

    /* Or call it directly. */
    func2();

    /*
     * atexit() callbacks are called in the dtor, so make sure partial_del(t)
     * does not run.
     */
    partial_del(t2);

end:
    return 0;
}

What witchcraft is this? You are playing with dark magic :P

Nicely done, sir!

how can this possibly be useful?

#define FUNCTION_CONSTANT 0xABAD1DEA

You have good taste and you should feel good.

Hahaha :) Reminds me of the good ol' IA32 asm code, this kind of tricks was just used over and over - because of, no choice.
Well nice piece !
Now you say you wouldn't recommend it (I wouldn't either) but with some more work, I think it's even packageable in a (dangerous) library. Just have to make freaking sure you get the good length for partial_template_functions (why not use labels in the code for that ?) and also be a lil' more generic for IA32/64, big or little endian, and you're good to go !
BUT you could also use a global.

👍

Nice trick.
I don't see the point/need of this cast inside partial_to_function:

return (void (*)(void *))(t)->caller;

if you keep the cast shouldn't it be:

return (void (*)())(t)->caller;