Tech Archives - Page 32 of 33

Decorators In C

Turns out you can also do decorators in C, this came in handy for something else I had to work on recently. It’s not quite as nice, because functions aren’t first-class, but handy nonetheless.

Here’s what that looks like:

#include <stdio.h>

typedef int (* const decorableFnType)(int paramOne, int paramTwo);

int functionOne(int paramOne, int paramTwo)
{
	printf("functionOne: %d, %dn", paramOne, paramTwo);
	return 0;
}

int functionTwo(int paramOne, int paramTwo)
{
	printf("functionTwo: %d, %dn", paramOne, paramTwo);
	return 0;
}

int functionThree(int paramOne, int paramTwo)
{
	printf("functionThree: %d, %dn", paramOne, paramTwo);
	return 0;
}

int decorator(int paramOne, int paramTwo, decorableFnType originalFn)
{
	printf("decorator precondition!n");
	originalFn(paramOne, paramTwo);
	return 0;
}

int main(int argc, char **argv)
{
	printf("hello, C decorators!n");
	
	functionOne(1, 2);
	
	decorator(1, 2, functionOne);
	decorator(1, 2, functionTwo);
	decorator(1, 2, functionThree);
	
	return 0;
}

#include <stdio.h>

typedef int (* const decorableFnType)(int paramOne, int paramTwo);

int functionOne(int paramOne, int paramTwo)

{

printf("functionOne: %d, %dn", paramOne, paramTwo);

return 0;

}

int functionTwo(int paramOne, int paramTwo)

{

printf("functionTwo: %d, %dn", paramOne, paramTwo);

return 0;

}

int functionThree(int paramOne, int paramTwo)

{

printf("functionThree: %d, %dn", paramOne, paramTwo);

return 0;

}

int decorator(int paramOne, int paramTwo, decorableFnType originalFn)

{

printf("decorator precondition!n");

originalFn(paramOne, paramTwo);

return 0;

}

int main(int argc, char **argv)

{

printf("hello, C decorators!n");

functionOne(1, 2);

decorator(1, 2, functionOne);

decorator(1, 2, functionTwo);

decorator(1, 2, functionThree);

return 0;

}

The output of which is:

vilimpoc@funky:~$ ./c-decorators
hello, C decorators!
functionOne: 1, 2
decorator precondition!
functionOne: 1, 2
decorator precondition!
functionTwo: 1, 2
decorator precondition!
functionThree: 1, 2
vilimpoc@funky:~$

This might be a bit simplistic, in reality, you’d probably want to decorate a function with a signature like the following, so that you can just change the structure and not bother w/a bajillion function declarations:

typedef int (* const decorableFnType)(SomeStruct * data);

1	typedef int (* const decorableFnType)(SomeStruct * data);

Again, not super elegant, but handy.

Javascript Decorators

One reason why Javascript rocks:

As a use case, imagine you want to restrict a certain set of functions to only run if you are logged in. Doing stuff like this is ridiculously easy with first-class functions.

Here’s a generic decoration example:

(function()
{
	var original = function(paramOne, paramTwo) {
		console.log('original: ' + paramOne + ' ' + paramTwo);
	};

	var decorator = function(originalFn) {
		return function(paramOne, paramTwo) {
			console.log('decorator: ' + paramOne + ' ' + paramTwo);
			originalFn(paramOne, paramTwo);
		};
	};

	var fnTable = {};
	var registerCallback = function(url, callback) {
		fnTable[url] = callback;
	};

	registerCallback('/abc', original);
	registerCallback('/def', decorator(original));

	fnTable['/abc']('one', 'two');
	fnTable['/def']('three', 'four');
})();

(function()

{

var original = function(paramOne, paramTwo) {

console.log('original: ' + paramOne + ' ' + paramTwo);

};

var decorator = function(originalFn) {

return function(paramOne, paramTwo) {

console.log('decorator: ' + paramOne + ' ' + paramTwo);

originalFn(paramOne, paramTwo);

};

var fnTable = {};

var registerCallback = function(url, callback) {

fnTable[url] = callback;

};

registerCallback('/abc', original);

registerCallback('/def', decorator(original));

fnTable['/abc']('one', 'two');

fnTable['/def']('three', 'four');

})();

Update: Here’s the specific way you’d do this with node.js/Express:

Since all of the URL handlers you register have the same signature, it’s easy to add precondition checks to the handlers via decorators.

var decorator = function(originalFn) {
    return function(req, res) {
        if (req.session.userLoggedIn) {
            originalFn(req, res);
        } else {
            redirectSomewhere(res);
        }
    }
}

app.get('/url', decorator(original));
app.post('/other', decorator(original));

var decorator = function(originalFn) {

return function(req, res) {

if (req.session.userLoggedIn) {

originalFn(req, res);

} else {

redirectSomewhere(res);

}

app.get('/url', decorator(original));

app.post('/other', decorator(original));

You get preconditions essentially for free, which is a damn sight better than adding the if() block to each and every handler function. Also, if you need more preconditions in the future, you can just stack them.

Starting Copies of Firefox with Different Profiles on Ubuntu

Just a quick note to self, how to setup Firefox to a) start separate instances of itself, b) load a different profile, c) use a particular locale.

Useful when developing localized web apps.

Installing Mercurial Without Admin Rights And No C Compiler

Cheap webhosters can be pretty limiting sometimes in the platforms they run and the software packages on offer, even when you do have shell access. Here’s a quick guide to setting Mercurial up on a shared webhost for use when:

there is no Python.h available, so you cannot build the C extensions
you can build only the pure Python modules, which is slower but more compatible
you have to do all of this in your home directory somewhere

Continue reading Installing Mercurial Without Admin Rights And No C Compiler

Website Backup With rsync

One thing that kept me from getting my blog back up for a while was the lack of a consistent backup strategy for the WordPress content and the website as a whole. Website backup should be easy, with these commands and the following script, it’s very easy.

This is just a starting point though, I have some ideas about backup that I plan to explore in future experiments.

Manual Runs

Quick backup command lines:

ssh user@host mysqldump --all-databases -p --result-file=db-backup-`date +%Y%m%d`.sql rsync -av --delete user@host:/remoteSource localDestination

1

2

ssh user@host mysqldump --all-databases -p --result-file=db-backup-`date +%Y%m%d`.sql

rsync -av --delete user@host:/remoteSource localDestination

Continue reading Website Backup With rsync