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Showing posts from September, 2019

Simplest Exceptions Handler Macros for C

More adventures in C, I revisited my exception handler implementation libex. I realized there's an even simpler version that's more efficient if we give up the requirement that exceptions can be propagated to callers:#define TRY #define THROW(E) goto E #define CATCH(E) goto FINALLY_H; E: #define FINALLY FINALLY_H: This implementation requires only a single exception handling block per function, which is probably good idea as a general rule. Usage looks like:static void foo(size_t sz) { char* x; TRY { x = (char*)malloc(sz); if (x == NULL) THROW(ENoMem); // do something with x } CATCH(ENoMem) { // handle out of memory } FINALLY { if (x != NULL) free(x); } } Unlike libex, this version is actually compatible with break and continue, although using them runs the risk of skipping the FINALLY handler. An almost equally simple version of exception handlers which ensures that break/continue cannot skip the FINALLY block wraps everything in a loop:#define T…

async.h - asynchronous, stackless subroutines in C

The async/await idiom is becoming increasingly popular. The first widely used language to include it was C#, and it has now spread into JavaScript and Rust. Now C/C++ programmers don't have to feel left out, because async.h is a header-only library that brings async/await to C!Features:It's 100% portable C.It requires very little state (2 bytes).It's not dependent on an OS.It's a bit simpler to understand than protothreads because the async state is caller-saved rather than callee-saved.#include "async.h" struct async pt; struct timer timer; async example(struct async *pt) { async_begin(pt); while(1) { if(initiate_io()) { timer_start(&timer); await(io_completed() || timer_expired(&timer)); read_data(); } } async_end; } This library is basically a modified version of the idioms found in the Protothreads library by Adam Dunkels, so it's not truly ground breaking. I've mad…

Building a Query DSL in C#

I recently built a REST API prototype where one of the endpoints accepted a string representing a filter to apply to a set of results. For instance, for entities with named properties "Foo" and "Bar", a string like "(Foo = 'some string') or (Bar > 99)" would filter out the results where either Bar is less than or equal to 99, or Foo is not "some string".This would translate pretty straightforwardly into a SQL query, but as a masochist I was set on using Google Datastore as the backend, which unfortunately has a limited filtering API:It does not support disjunctions, ie. "OR" clauses.It does not support filtering using inequalities on more than one property.It does not support a not-equal operation.So in this post, I will describe the design which achieves the following goals: A backend-agnostic querying API supporting arbitrary clauses, conjunctions ("AND"), and disjunctions ("OR").Implementations of this…