In the description of Lua' Pluto library, it says it lib persist functions and threads.
Can persist any Lua function
Can persist threads
Works with any Lua chunkreader/chunkwriter
Support for "invariant" permanent objects, of all datatypes
Hmm, I can't imagine how the functions and threads to be persisted. Can I have some explanation about this feature?
The source code is relatively easy to follow and very commented.
What the lib does is determine what parts compose the functions and/or threads, and then store every part separately.
If you skip the code and just read the comments, here's how the two relevant functions look:
static void persistfunction(PersistInfo *pi)
{
...
if(cl->c.isC) {
/* It's a C function. For now, we aren't going to allow
* persistence of C closures, even if the "C proto" is
* already in the permanents table. */
lua_pushstring(pi->L, "Attempt to persist a C function");
lua_error(pi->L);
} else { /* It's a Lua closure. */
/* Persist prototype */
...
/* Persist upvalue values (not the upvalue objects themselves) */
...
/* Persist function environment */
...
}
}
static void persistthread(PersistInfo *pi)
{
...
/* Persist the stack */
...
/* Now, persist the CallInfo stack. */
...
/* Serialize the state's other parameters, with the exception of upval stuff */
...
/* Finally, record upvalues which need to be reopened */
...
}
So, as you can see, a function can be considered as a composition of a prototype, a group of upvalues and an environment (a table). A thread is two "stacks" (the call stack and the memory stack, I think), the state information (excluding upvalues), which is basically what variables had which values when the thread was defined, and the upvalues.
You may read more about upvalues in PiL 27.3.3
Can persist any Lua function
This means that Pluto can persist any Lua function by persisting it's bytecode and all required upvalues. See here and here for source. When you unpersist it, you can call the function as usual. Note that it cannot persist C functions registered in Lua.
Can persist threads
It persists the thread's stack and activation records, so when you unpersist it, you can resume where the stack was executing. Code is here.
Related
I am writing some functions for a C extension module for python and need to import a module I wrote directly in python for access to a custom python type. I use PyImport_ImportModule() in the body of my C function, then PyObject_GetAttrString() on the module to get the custom python type. This executes every time the C function is called and seems like it's not very efficient and may not be best practice. I'm looking for a way to have access to the python custom type as a PyObject* or PyTypeObject* in my source code for efficiency and I may need the type in more than one C function also.
Right now the function looks something like
static PyObject* foo(PyObject* self, PyObject* args)
{
PyObject* myPythonModule = PyImport_ImportModule("my.python.module");
if (!myPythonModule)
return NULL;
PyObject* myPythonType = PyObject_GetAttrString(myPythonModule, "MyPythonType");
if (!myPythonType) {
Py_DECREF(myPythonModule);
return NULL;
}
/* more code to create and return a MyPythonType instance */
}
To avoid retrieving myPythonType every function call I tried adding a global variable to hold the object at the top of my C file
static PyObject* myPythonType;
and initialized it in the module init function similar to the old function body
PyMODINIT_FUNC
PyInit_mymodule(void)
{
/* more initializing here */
PyObject* myPythonModule = PyImport_ImportModule("my.python.module");
if (!myPythonModule) {
/* clean-up code here */
return NULL;
}
// set the static global variable here
myPythonType = PyObject_GetAttrString(myPythonModule, "MyPythonType");
Py_DECREF(myPythonModule);
if (!myPythonType) {
/* clean-up code here */
return NULL;
/* finish initializing module */
}
which worked, however I am unsure how to Py_DECREF the global variable whenever the module is finished being used. Is there a way to do that or even a better way to solve this whole problem I am overlooking?
First, just calling import each time probably isn't as bad as you think - Python does internally keep a list of imported modules, so the second time you call it on the same module the cost is much lower. So this might be an acceptable solution.
Second, the global variable approach should work, but you're right that it doesn't get cleaned up. This is rarely a problem because modules are rarely unloaded (and most extension modules don't really support it), but it isn't great. It also won't work with isolated sub-interpreters (which isn't much of a concern now, but may become more more popular in future).
The most robust way to do it needs multi-phase initialization of your module. To quickly summarise what you should do:
You should define a module state struct containing this type of information,
Your module spec should contain the size of the module state struct,
You need to initialize this struct within the Py_mod_exec slot.
You need to create an m_free function (and ideally the other GC functions) to correctly decref your state during de-initialization.
Within a global module function, self will be your module object, and so you can get the state with PyModule_GetState(self)
Here is the function that is defined within the C code:
typedef void (*spAnimationStateListener)(spAnimationState *state, spEventType type, spTrackEntry *entry, spEvent *event);
struct spTrackEntry {
...
spAnimationStateListener listener;
...
};
And, I am trying to use it in Swift like this:
class TrackEntry {
private let ptr: UnsafeMutablePointer<spTrackEntry>!
public func addEventListener(_ eventListener: #escaping () -> ()) {
let listener: spAnimationStateListener = { (state, type, entry, event) in
eventListener() //cannot use this
}
ptr.pointee.listener = listener
}
}
I saw some suggestion like passing "self" as a parameter but I could not figure out how to do that. I cannot change the C code and it is accepting four parameters. I also tried unsafeBitCast by using convention(block) and it compiled but data were broken.
I cannot use it as a static variable because listeners are separate between different TrackEntry instances.
As mentioned, C function pointers can only be called from static or global context.
There is a good reason : in C, functions are only declared in global context - ie. in the memory dedicated to instructions storage, not in the memory dedicated to data storage. You cannot have instances of a function. A function pointer can just point to one function or another. You cannot create code on-the-fly, neither pass it by value. You can only point to one part of the instructions or another.
So, if only one listener can be used at a time, you could declare a global variable for the closure, and change it before the listener is created, pointing to this listener ; and when another listener is created replace it.
But if you have several listeners at once, well you will have to globally create a closure for each of them (if you do not have too much, and not too generic).
I wonder if there's a language sugar/SDK utility function in Dart that allows to protect a certain code from running more than once?
E.g.
void onUserLogin() {
...
runOnce(() {
handleInitialMessage();
});
...
}
I know I can add a global or class static boolean flag to check but it would be accessible in other functions of the same scope with a risk of accidental mixup in the future.
In C++ I could e.g. use a local static bool for this.
There is no built-in functionality to prevent code from running more than once. You need some kind of external state to know whether it actually did run.
You can't just remember whether the function itself has been seen before, because you use a function expression ("lambda") here, and every evaluation of that creates a new function object which is not even equal to other function objects created by the same expression.
So, you need something to represent the location of the call.
I guess you could hack up something using stack traces. I will not recommend that (very expensive for very little advantage).
So, I'd recommend something like:
class RunOnce {
bool _hasRun = false;
void call(void Function() function) {
if (_hasRun) return;
// Set after calling if you don't want a throw to count as a run.
_hasRun = true;
function();
}
}
...
static final _runOnce = RunOnce();
void onUserLogin() {
_runOnce(handleInitialMessage);
}
It's still just a static global that can be accidentally reused.
I'd like to create a library, written in Java, callable from C, with simple method signatures:
int addThree(int in) {
return in + 3;
}
I know it's possible to do this with GraalVM if you do a little dance and create an Isolate in your C program and pass it in as the first parameter in every function call. There is good sample code here.
The problem is that the system I'm writing for, Postgres, can load C libraries and call functions in them, but I would have to create a wrapper function in C that would wrap every function I wanted to expose. This really limits the value of being able to slap something together in Java and use it in Postgres directly. I'd have to do something like this:
int myPublicAddThreeFunction(int in) {
graal_isolatethread_t *thread = NULL;
if (graal_create_isolate(NULL, NULL, &thread) != 0) {
fprintf(stderr, "error on isolate creation or attach\n");
return 1;
}
return SomeClassName_addThree_big_random_string_here(thread, in);
}
Is there a way, in Java alone, to expose a simple C function? I'm thinking I could create the isolate in a static method that gets loaded once on startup, somehow set it as the current isolate, and have the Java method just use it. Haven't been able to figure it out, though.
Also, it would be real nice not to have to append a big random string to every function name.
for Mach kernel API emulation on Linux, I need for my kernel module to get called when a task has been just created or is being terminated.
In my kernel module, this could most nicely be done via Linux Security Modules, but a couple of years ago, they prevented external modules from acting as a LSM by unexporting the needed symbols.
The only other way I could find was to make my module act like a rootkit. Find the syscall table and hook it in there.
Patching the kernel is out of the question. I need my app to be installed easily. Is there any other way?
You can use Kprobes, which enables you to dynamically hook into code in the kernel. You will need to find the right function among the ones involves in creating and destroying processes that give you the information you need. For instance, for tasks created, do_fork() in fork.c would be a good place to start. For tasks destroyed, do_exit. You would want to write a retprobe, which is a kind of kprobe that additionally gives you control at the end of the execution of the function, before it returns. The reason you want control before the function returns is to check if it succeeded in creating the process by checking the return value. If there was an error, then the function will return a negative value or in some cases possibly 0.
You would do this by creating a kretprobe struct:
static struct kretprobe do_fork_probe = {
.entry_handler = (kprobe_opcode_t *) my_do_fork_entry,
.handler = (kprobe_opcode_t *) my_do_fork_ret,
.maxactive = 20,
.data_size = sizeof(struct do_fork_ctx)
};
my_do_fork_entry gets executed when control enters the hooked function, and my_do_fork_ret gets executed just before it returns. You would hook it in as follows:
do_fork_probe.kp.addr =
(kprobe_opcode_t *) kallsyms_lookup_name("do_fork");
if ((ret = register_kretprobe(&do_fork_probe)) <0) {
// handle error
}
In the implementation of your hooks, it's a bit unwieldy to get the arguments and return value. You get these via the saved registers pt_regs data structure. Let's look at the return hook, where on x86 you get the return value via regs->ax.
static int my_do_fork_ret(struct kretprobe_instance *ri, struct pt_regs *regs)
{
struct do_fork_ctx *ctx = (struct do_fork_ctx *) ri->data;
int ret = regs->ax; // This is on x86
if (ret > 0) {
// It's not an error, probably a valid process
}
}
In the entry point, you can get access to the arguments via the registers. e.g. on x86, regs->di is the first argument, regs->si is the second etc. You can google to get the full list. Note that you shouldn't rely on these registers for the arguments in the return hook as the registers may have been overwritten for other computations.
You will surely have to jump many hoops in getting this working, but hopefully this note should set you off in the right direction.