Tuesday, February 14, 2012

Continuations in C++ with fork

[Update, Jan 2015: I've translated this code into Rust.]

While reading "Continuations in C" I came across an intriguing idea:

It is possible to simulate call/cc, or something like it, on Unix systems with system calls like fork() that literally duplicate the running process.

The author sets this idea aside, and instead discusses some code that uses setjmp/longjmp and stack copying. And there are several other continuation-like constructs available for C, such as POSIX getcontext. But the idea of implementing call/cc with fork stuck with me, if only for its amusement value. I'd seen fork used for computing with probability distributions, but I couldn't find an implementation of call/cc itself. So I decided to give it a shot, using my favorite esolang, C++.

Continuations are a famously mind-bending idea, and this article doesn't totally explain what they are or what they're good for. If you aren't familiar with continuations, you might catch on from the examples, or you might want to consult another source first (1, 2, 3, 4, 5, 6).

Small examples

I'll get to the implementation later, but right now let's see what these fork-based continuations can do. The interface looks like this.

template <typename T>
class cont {
public:
void operator()(const T &x);
};

template <typename T>
T call_cc( std::function< T (cont<T>) > f );

std::function is a wrapper that can hold function-like values, such as function objects or C-style function pointers. So call_cc<T> will accept any function-like value that takes an argument of type cont<T> and returns a value of type T. This wrapper is the first of several C++11 features we'll use.

call_cc stands for "call with current continuation", and that's exactly what it does. call_cc(f) will call f, and return whatever f returns. The interesting part is that it passes to f an instance of our cont class, which represents all the stuff that's going to happen in the program after f returns. That cont object overloads operator() and so can be called like a function. If it's called with some argument x, the program behaves as though f had returned x.

The types reflect this usage. The type parameter T in cont<T> is the return type of the function passed to call_cc. It's also the type of values accepted by cont<T>::operator().

Here's a small example.

int f(cont<int> k) {
std::cout << "f called" << std::endl;
k(1);
std::cout << "k returns" << std::endl;
return 0;
}

int main() {
std::cout << "f returns " << call_cc<int>(f) << std::endl;
}

When we run this code we get:

f called
f returns 1

We don't see the "k returns" message. Instead, calling k(1) bails out of f early, and forces it to return 1. This would happen even if we passed k to some deeply nested function call, and invoked it there.

This nonlocal return is kind of like throwing an exception, and is not that surprising. More exciting things happen if a continuation outlives the function call it came from.

boost::optional< cont<int> > global_k;

int g(cont<int> k) {
std::cout << "g called" << std::endl;
global_k = k;
return 0;
}

int main() {
std::cout << "g returns " << call_cc<int>(g) << std::endl;

if (global_k)
(*global_k)(1);
}

When we run this, we get:

g called
g returns 0
g returns 1

g is called once, and returns twice! When called, g saves the current continuation in a global variable. After g returns, main calls that continuation, and g returns again with a different value.

What value should global_k have before g is called? There's no such thing as a "default" or "uninitialized" cont<T>. We solve this problem by wrapping it with boost::optional. We use the resulting object much like a pointer, checking for "null" and then dereferencing. The difference is that boost::optional manages storage for the underlying value, if any.

Why isn't this code an infinite loop? Because invoking a cont<T> also resets global state to the values it had when the continuation was captured. The second time g returns, global_k has been reset to the "null" optional value. This is unlike Scheme's call/cc and most other continuation systems. It turns out to be a serious limitation, though it's sometimes convenient. The reason for this behavior is that invoking a continuation is implemented as a transfer of control to another process. More on that later.

Backtracking

We can use continuations to implement backtracking, as found in logic programming languages. Here is a suitable interface.

bool guess();
void fail();

We will use guess as though it has a magical ability to predict the future. We assume it will only return true if doing so results in a program that never calls fail. Here is the implementation.

boost::optional< cont<bool> > checkpoint;

bool guess() {
return call_cc<bool>( [](cont<bool> k) {
checkpoint = k;
return true;
} );
}

void fail() {
if (checkpoint) {
(*checkpoint)(false);
} else {
std::cerr << "Nothing to be done." << std::endl;
exit(1);
}
}

guess invokes call_cc on a lambda expression, which saves the current continuation and returns true. A subsequent call to fail will invoke this continuation, retrying execution in a world where guess had returned false instead. In Scheme et al, we would store a whole stack of continuations. But invoking our cont<bool> resets global state, including the checkpoint variable itself, so we only need to explicitly track the most recent continuation.

Now we can implement the integer factoring example from "Continuations in C".

int integer(int m, int n) {
for (int i=m; i<=n; i++) {
if (guess())
return i;
}
fail();
}

void factor(int n) {
const int i = integer(2, 100);
const int j = integer(2, 100);

if (i*j != n)
fail();

std::cout << i << " * " << j << " = " << n << std::endl;
}

factor(n) will guess two integers, and fail if their product is not n. Calling factor(391) will produce the output

17 * 23 = 391

after a moment's delay. In fact, you might see this after your shell prompt has returned, because the output is produced by a thousand-generation descendant of the process your shell created.

Solving a maze

For a more substantial use of backtracking, let's solve a maze.

const int maze_size = 15;
char maze[] =
"X-------------+\n"
" | |\n"
"|--+ | | | |\n"
"| | | | --+ |\n"
"| | | |\n"
"|-+---+--+- | |\n"
"| | | |\n"
"| | | ---+-+- |\n"
"| | | |\n"
"| +-+-+--| |\n"
"| | | |--- |\n"
"| | |\n"
"|--- -+-------|\n"
"| \n"
"+------------- \n";

void solve_maze() {
int x=0, y=0;

while ((x != maze_size-1)
|| (y != maze_size-1)) {

if (guess()) x++;
else if (guess()) x--;
else if (guess()) y++;
else y--;

if ( (x < 0) || (x >= maze_size) ||
(y < 0) || (y >= maze_size) )
fail();

const int i = y*(maze_size+1) + x;
if (maze[i] != ' ')
fail();
maze[i] = 'X';
}

for (char c : maze) {
if (c == 'X')
std::cout << "\e[1;32mX\e[0m";
else
std::cout << c;
}
}

Whether code or prose, the algorithm is pretty simple. Start at the upper-left corner. As long as we haven't reached the lower-right corner, guess a direction to move. Fail if we go off the edge, run into a wall, or find ourselves on a square we already visited.

Once we've reached the goal, we iterate over the char array and print it out with some rad ANSI color codes.

Once again, we're making good use of the fact that our continuations reset global state. That's why we see 'X' marks not on the failed detours, but only on a successful path through the maze. Here's what it looks like.


X-------------+
XXXXXXXX|     |
|--+  |X|   | |
|  |  |X| --+ |
|     |XXXXX| |
|-+---+--+-X| |
| |XXX   | XXX|
| |X|X---+-+-X|
|XXX|XXXXXX|XX|
|X+-+-+--|XXX |
|X|   |  |--- |
|XXXX |       |
|---X-+-------|
|   XXXXXXXXXXX
+-------------X

Excess backtracking

We can run both examples in a single program.

int main() {
factor(391);
solve_maze();
}

If we change the maze to be unsolvable, we'll get:

17 * 23 = 391
23 * 17 = 391
Nothing to be done.

Factoring 391 a different way won't change the maze layout, but the program doesn't know that. We can add a cut primitive to eliminate unwanted backtracking.

void cut() {
checkpoint = boost::none;
}

int main() {
factor(391);
cut();
solve_maze();
}

The implementation

For such a crazy idea, the code to implement call_cc with fork is actually pretty reasonable. Here's the core of it.

template <typename T>
// static
T cont<T>::call_cc(call_cc_arg f) {
int fd[2];
pipe(fd);
int read_fd = fd[0];
int write_fd = fd[1];

if (fork()) {
// parent
close(read_fd);
return f( cont<T>(write_fd) );
} else {
// child
close(write_fd);
char buf[sizeof(T)];
if (read(read_fd, buf, sizeof(T)) < ssize_t(sizeof(T)))
exit(0);
close(read_fd);
return *reinterpret_cast<T*>(buf);
}
}

template <typename T>
void cont<T>::impl::invoke(const T &x) {
write(m_pipe, &x, sizeof(T));
exit(0);
}

To capture a continuation, we fork the process. The resulting processes share a pipe which was created before the fork. The parent process will call f immediately, passing a cont<T> object that holds onto the write end of this pipe. If that continuation is invoked with some argument x, the parent process will send x down the pipe and then exit. The child process wakes up from its read call, and returns x from call_cc.

There are a few more implementation details.

  • If the parent process exits, it will close the write end of the pipe, and the child's read will return 0, i.e. end-of-file. This prevents a buildup of unused continuation processes. But what if the parent deletes the last copy of some cont<T>, yet keeps running? We'd like to kill the corresponding child process immediately.

    This sounds like a use for a reference-counted smart pointer, but we want to hide this detail from the user. So we split off a private implementation class, cont<T>::impl, with a destructor that calls close. The user-facing class cont<T> holds a std::shared_ptr to a cont<T>::impl. And cont<T>::operator() simply calls cont<T>::impl::invoke through this pointer.

  • It would be nice to tell the compiler that cont<T>::operator() won't return, to avoid warnings like "control reaches end of non-void function". GCC provides the noreturn attribute for this purpose.

  • We want the cont<T> constructor to be private, so we had to make call_cc a static member function of that class. But the examples above use a free function call_cc<T>. It's easiest to implement the latter as a 1-line function that calls the former. The alternative is to make it a friend function of cont<T>, which requires some forward declarations and other noise.

There are a number of limitations too.

  • As noted, the forked child process doesn't see changes to the parent's global state. This precludes some interesting uses of continuations, like implementing coroutines. In fact, I had trouble coming up with any application other than backtracking. You could work around this limitation with shared memory, but it seemed like too much hassle.

  • Each captured continuation can only be invoked once. This is easiest to observe if the code using continuations also invokes fork directly. It could possibly be fixed with additional forking inside call_cc.

  • Calling a continuation sends the argument through a pipe using a naive byte-for-byte copy. So the argument needs to be Plain Old Data, and had better not contain pointers to anything not shared by the two processes. This means we can't send continuations through other continuations, sad to say.

  • I left out the error handling you would expect in serious code, because this is anything but.

  • Likewise, I'm assuming that a single write and read will suffice to send the value. Robust code will need to loop until completion, handle EINTR, etc. Or use some higher-level IPC mechanism.

  • At some size, stack-allocating the receive buffer will become a problem.

  • It's slow. Well, actually, I'm impressed with the speed of fork on Linux. My machine solves both backtracking problems in about a second, forking about 2000 processes along the way. You can speed it up more with static linking. But it's still far more overhead than the alternatives.

As usual, you can get the code from GitHub.

44 comments:

  1. you haven't stated the largest problem with this approach, it breaks the C++ RAII. When you call the continuation it goes up the stack without unwinding the stack during the process. Since RAII is not only about memory(*) this could lead to leaks in resources.

    sample without formatting because the commentary cannot have pre or code tags :

    boost::optional< cont > global_k;

    int global_int = 5;

    int g(cont k) {
    std::cout << "g called" << std::endl;
    global_k = k;
    global_int++;
    return 0;
    }

    class raii
    {
    public:
    raii() { std::cout << "build " << this << "\n"; }
    ~raii() { std::cout << "destroy " << this << "\n"; }
    };

    int foo()
    {
    raii a;

    if (global_k)
    (*global_k)(1);
    }

    void example_g() {
    std::cout << "g returns " << call_cc(g) << std::endl;

    std::cout << "global_int = " << global_int << std::endl;

    foo();
    }

    *) Memory and some other kind of resources are not really a problem, since the
    process exiting will free those.

    ReplyDelete
  2. Glad to find this blog. There are not much available on C++ . keep it up and thanks for sharing information.

    ReplyDelete
  3. This helped me lot in learning about c++.I must say this is an excellent article and this blog is very informative for c++ learners.

    ReplyDelete
  4. @e commerce web development This is not exactly a C++. This is code mess. Unhappily such programs brings up the myth about "hard to use" C++. There are always clean and easy ways to do any job.

    ReplyDelete
  5. Thanks for writing this! It's creative and pretty entertaining :-).

    ReplyDelete
  6. Nice article, thanks for the information. It's very complete information. I will bookmark for next reference
    jaring futsal | jaring golf | jaring pengaman proyek |
    jaring pengaman bangunan | jaring pengaman gedung
    http://www.jual-jaring.blogspot.com/
    http://www.agen-jaring.blogspot.com/
    http://www.pancasamudera-safetynet.blogspot.com/
    http://www.toko-jaring.blogspot.com/
    http://www.pusat-jaring.blogspot.com/
    http://jualjaringpengaman.blogspot.com/
    https://pancasamudera.wordpress.com/
    https://pasangjaringfutsal.wordpress.com/
    https://jualtambangmurah.wordpress.com/
    https://tokojaring.wordpress.com/
    https://jualjaringfutsal.wordpress.com/
    https://jaringfutsal.wordpress.com/


    ReplyDelete
  7. I always appreciate learning your material – they are always humorous and amazing. I am a china tour lover,You can learn more: China tours | China travels | China Trip

    ReplyDelete
  8. Awesome to see your site. Keep up the great work. Thanks for all you could do.Lets stay in touch
    Mini IPL Season 1
    EPL 2016
    English Premier League Schedule
    US Open 2016 Telecast

    ReplyDelete
  9. Nice to be visiting your blog again, it has been months for me. Well this article that i’ve been waited for so long. I need this article to complete my assignment in the college, and it has same topic with your article. Thanks, great share :

    pengobatan cacar alami
    manfaat sari kurma untuk kesehatan
    cara mengobati batu empedu
    obat hidung mimisan disertai sakit kepala
    obat hernia alami
    obat penghilang benjolan di pipi
    pengobatan radang empedu

    ReplyDelete
  10. Nice to be visiting your blog again, it has been months for me. Well this article that i’ve been waited for so long. I need this article to complete my assignment in the college, and it has same topic with your article. Thanks, great share :

    cara mengobati kencing manis
    cara mengobati nyeri testis
    cara mengobati gondok secara alami
    cara mengobati radang tenggorokan

    ReplyDelete
  11. It is truly a nice and helpful piece of information. I'm happy that you shared this helpful info with us. Please keep us informed like this. Thanks for sharing. www.gumawawebsite.blogspot.com

    ReplyDelete
  12. Thanks for sharing this helpful post, hopefully you share some more it related post.
    Get the best social media optimization uk that will help you to promote your brand, business or blogs on an online platforms.

    ReplyDelete


  13. Excellent Blog! I would like to thank for the efforts you have made in writing this post. I am hoping the same best work from you in the future as well.
    I wanted to thank you for this websites! Thanks for sharing. Great websites!

    Pubg APK

    Pubg mobile APK

    ReplyDelete
  14. Excellent Blog! I would like to thank for the efforts you have made in writing this post. I am hoping the same best work from you in the future as well.
    I wanted to thank you for this websites! Thanks for sharing. Great websites!
    APK Apps for fire stick
    whatsapp war apk
    whatsapp yo apk
    whatsapp faud apk
    whatsapp latest gb apk

    ReplyDelete
  15. We are situated in Rugby, North Dakota, not a long way from Minot International Airport. We offer conveyance anyplace in the United States. You can likewise get your furbaby from our cattery or airport..our favored decision. During the time we will have an assortment of excellent Standard Munchkin Kitten for sale accessible including silvers.
    https://munchkinskitty.company.com
    Common tags: munchkin cat breeder, munchkin cat breeders near me, munchkin cats for sale near me, munchkin cat adoption, munchkin cat price, munchkin kitty for sale, buy munchkin kitty, buy munchkin kitty online, buy munchkin kitty for charismas, best kitty for charismas gifts, munchkin kitty, munchkin cat for sale

    munchkin cat breeder
    munchkin cat breeders near me
    munchkin cats for sale near me
    munchkin cat adoption
    munchkin cat for sale
    munchkin kittens for sale
    munchkin cats for sale
    standard munchkin kittens for sale
    munchkin cat price
    munchkin breeders
    munchkin cats kittens
    munchkin kitty for sale
    munchkin cat breeder
    munchkin cat for sale ohio
    https://munchkinskitty.company.com
    munchkin cat near me
    munchkin cat forsale
    munchkin cat for sale

    ReplyDelete
  16. Hi, my names is Scott am new here and I would be needing help from you guys. My Late Dad was a munchkin breeder, he started breeding Munchkin kittens �� when I was 15 years old and he named the small cattery after me, "Scott Munchkin Cattery". He has been breeding for 13 years before he passed away and before he died he asked me to continue running the cattery. So I contact some of the registered Munchkin breeder for advice, they told me I need to build a website to advertise the Liter I have available so I did that. Here is the website I built https://munchkins.company.com please you guy should review and advise me on what to do please �� I need more advice from you on what to do
    Thanks

    ReplyDelete
  17. Karnataka board will release the 2nd PUC result 2021 one month after the conclusion of examinations. Students can check their Karnataka 2nd PUC result PUC Result 2021 Students can also make use of the direct link given on this page to download their PUC results 2021.

    ReplyDelete
  18. Hii I Am Form
    WhatsApp Group Links
    Your Content Was Very Nice Thanks For You

    ReplyDelete
  19. Wonderful Information. This post is written in a very ideal manner. Keep sharing such a post.
    Get in touch with our Qwik Aid email technical support staff, who is available 24*7 to help you fix your problems as quickly as possible. If you need help Recover Hushmail Account Password or with other problems, call our toll-free numbers. Our professionals are always available to answer your email, whether day or night.If you contact our Qwik Aid support team, we guarantee that our support team will be ready to assist you in any way possible.

    ReplyDelete
  20. Such a informative post. Thanks for sharing with us.
    The Gardens Care Homes is one of the best residential firms in Lakewood. Our Assisted Living community offers you Lakewood Senior Living with separate & secure residents with utmost facilities. To know more about Lakewood Senior Living Homes, visit our website.

    ReplyDelete
  21. Getting Solution Manual For Accounting Information Systems 11 E is easier than ever with our exclusive collection of testbanks and solution manuals online.

    ReplyDelete