Non Local Jumps with setjmp and longjmp

Posted on 2023-11-01 :: Tags: C, Low-Level

While going through Postgres source code, I found a really cool way technique they were using to do non local jumps(acting sort of as an exception handling mechanism). Postgres is written in C and C doesn't really have a construct for exceptions, so how would they do something like this?

One very common functionality we observe in any interactive shell is when we press Ctrl-c, that operation gets cancelled. You can take python repl as an example. When I pressed Ctrl-c it aborted and printed KeyboardInterrupt.

λ python
Python 3.11.5 (main, Sep  2 2023, 14:16:33) [GCC 13.2.1 20230801] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print(
KeyboardInterrupt
>>>

Postgres also offers a similar interactive environment in the form of psql binary which lets us run db operations interactively. They also obviously offer similar functionality of aborting when you press Ctrl-c.

To understand how this functionality would be implemented, we need to know a few things. When we press Ctrl-c, we are sending something called an interrupt. That interrupt is handled by the kernel by sending a signal to the running process(1). Our Ctrl-c keypress will trigger a SIGINT signal. And we have mechanism to handle signals(2) ourselves in user code. Using that, we can use that to do a lot of powerful things.

Let's take a look into the postgres(psql to be specific) code to figure out how they do this.

The directory for psql code lies at src/bin/psql. The entrypoint(main function) is inside startup.c, which internally calls MainLoop function in mainloop.c(aptly named).

In startup.c, there's a call to psql_setup_cancel_handler(which is a oneliner setup_cancel_handler(psql_cancel_callback)) which eventually ends up calling setup_cancel_handler() in src/fe_utils/cancel.c(there signal handler for SIGINT is registered, and also a callback function is registered). The handle_sigint function is registered for handling SIGINT, inside which it calls cancel_callback function if it's not NULL(which it won't be for our particular codepath).

setup_cancel_handler handle_sigint

So, basically psql_cancel_callback function is run as part of the signal handler (handle_sigint).

Now, let's look at the main loop. The MainLoop function does a bunch of stuff at the start(which I have no clue about) but the actual loop is mainly the following code. It's a huge function which does a lot of things(expected since it's what drives the whole program). If you scroll through it you'll find that it's getting line and executing it(handling exits,clearing,etc. too), it's quite involved really.

I came across this particular code snippet(where I have my cursor in the screenshot). I had heard about something called setjmp/longjmp before but hadn't really encountered them in real world code(Could be that I haven't seen a lot of real world code).

MainLoop

Short primer on non local jump construct in C

setjmp : Marks the point where this was called as somewhat of a checkpoint by saving the execution state
longjmp: We jump to the checkpoint from wherever we are. This can be across functions (of course locally too).

An example (taken straight from Wikipedia) will make it a bit more clearer. The wikipedia page does a great job of showing how it can be used.

#include <stdio.h>
#include <setjmp.h>

static jmp_buf buf;

void second() {
    printf("second\n");         // prints
    longjmp(buf,1);             // jumps back to where setjmp was called - making setjmp now return 1
}

void first() {
    second();
    printf("first\n");          // does not print
}

int main() {
    if (!setjmp(buf))
        first();                // when executed, setjmp returned 0
    else                        // when longjmp jumps back, setjmp returns 1
        printf("main\n");       // prints

    return 0;
}

Postgres doesn't use setjmp/longjmp but uses sigsetjmp/siglongjmp because they're supposed to be used if you're using them in signal handling context it seems(again a piece of knowledge from the linked wikipedia page).

Something fun to do: Look into if setjmp/longjmp allow us to implement delimited continuations(TODO: I need to understand them better)

Tying together all the pieces

We saw the sigsetjmp call to establish a checkpoint(here). Now we need to find where siglongjmp call is coming from. Well, that's easy, it's probably coming somewhere from the signal handler.

Seeing setup_cancel_handler, we see a callback function(i.e query_cancel_callback, which is basically psql_cancel_callback) being assigned to cancel_callback variable and that is called in the handle_sigint function. I had already mentioned this before).

psql_cancel_callback