Programming

By default, the alpine Docker image which is mostly used for Golang programs does not contain /etc/nsswitch.conf. Golang’s net package used to do a sane thing when that file did not exist back in the day (before April 30, 2015) i.e. they checked the /etc/hosts file first, and then moved on to trying to query the DNS servers. However, it was changed later - now Go’s resolver tries to query the local machine as per the manual page of nsswitch.conf(5). You can find the commit here. ...

Out there, in the wild exists a lot of different authentication schemes or methods however one of them which is relatively popular because it’s implemented in most popular browsers at the moment, has this one peculiar “bug” in its specification - you cannot use a colon (’:’) in the username field. If you have ever seen a window such as this: Then that website is probably using HTTP basic auth and you must not use a colon in your username on that site because simply you would not be able to do that. ...

If you have not known before, scanf(3) and fgets(3) are both functions intended for reading something from standard input and doing something with the result - in the former case it is interpreted and the results might possibly be stored in specified arguments and in the latter case the result is simply put into a buffer. The first one is commonly recommended to beginner C programmers for reading something from the user and parsing it. However, it should be avoided mainly because it is very error-prone and it is difficult to understand how it actually works for new people. Instead of scanf(3), sscanf(3) should be used in combination with fgets(3). That way, you can easily guess in what state your standard input stream is after using those functions and you get other benefits. Let me give some examples and explain more. ...

I see time and time again this myth that programming languages have varying speeds perpetually peddled in various discussions between programmers in different IRC channels, forums, and so on. Some people think that, for example, C is much, much faster than Python. And a few of them go even further - they argue that C is a language that is “close to the metal”. Well, let me tell you. The code that you write in C is actually dedicated for a virtual machine that can eat that code and spit out assembly which roughly does the things that it is supposed to according to the C language standard. Why roughly? Simply because that virtual machine is poorly defined and it has a lot of undefined behaviour. This opposes our original argument that C is “close to the metal”. How could real hardware be poorly defined? Could it be that if you executed a certain instruction one time it would do something completely different, that was not mentioned at all in the CPU manual? Well, the answer is obviously no. Nobody would use such computers. ...

You could probably find “Code Complete” by Steve McConnell in any of the top 10 lists of books recommended to read for all programmers. I recently finished reading that book too. I have to confess that at times the tips and knowledge in there was a bit too basic and fundamental but, on the other hand, I also think that it has a lot of golden nuggets of knowledge. While reading it, I was writing down notes after every section. Essentially, they were three/four ideas that I thought were the most important in each section. Sometimes, they were paraphrased from the conclusions of each section written by the author himself. In other cases, I summarised them myself, in my own words. ...

Recently I ran into an issue where this, for example, code fails: import os os.rename('/foo/a.txt', '/bar/b.txt') Traceback (most recent call last): File "", line 1, in OSError: [Errno 18] Invalid cross-device link The documentation for the os module says that sometimes it might fail when the source and the destination are on different file-systems: os.rename( src, dst, *, src_dir_fd=None, dst_dir_fd=None) Rename the file or directory src to dst. If dst is a directory, OSError will be raised. On Unix, if dst exists and is a file, it will be replaced silently if the user has permission. The operation may fail on some Unix flavors if src and dst are on different filesystems. If successful, the renaming will be an atomic operation (this is a POSIX requirement). On Windows, if dst already exists, OSError will be raised even if it is a file. ...

Intro Did you know that in Python 2.x you can do the following? $ python2 Python 2.7.14 (default, Sep 20 2017, 01:25:59) [GCC 7.2.0] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> True = False >>> False False >>> True False >>> not True True >>> True == False True >>> True != False False How can it be that not True is True and True is equal to False? Why is it even possible to do this? Isn’t what is True and False in the language defined to be constant and unchangeable? What sense does it make to change the meaning of what is True and what is False? In any way, to fix this bug in the matrix, do this: ...

Intro Today I wanted to talk about unwinding and releasing resources in C functions. Let’s begin by stating that there are three main techniques for handling errors in the C programming language. Sometimes more than one technique may be used. Here is a list of them: You must test the value functions return. Abnormal value indicates that some kind of error has happened and a normal value indicates that it was successful; There is an external variable whose value you must check. For example, the POSIX variant of this is to have an variable called errno that changes to 0 when nothing bad happened and it has some kind of other value when an error occurs; You pass a pointer to a function. The function changes the value of the variable it points to or even calls it with certain arguments if it is a function pointer depending on the result. I have not mentioned one method but some people use atexit(3) to register functions that will be called at the end of a program which will release resources. However, this is unusual so I have not included it in the list.This is very much related to our topic because when an error occurs, you will have to handle it. That process includes releasing the resources which were acquired before in the function. Especially if you are deep down in your function and then an error occurred, the choice that you make in how to release the resources will matter a lot so it is important to make the correct decision. In C++ you have the destructors and so on but how are you going to do that in C? Are you going to sprinkle all of your error paths with: ...

Time and time I see people who follow all these random online tutorials and then when something does not work they become dazed and confused. “Why this does not work? But this tutorial shows that it should work” – I see similar questions occasionally in various forms on forums and IRC. I think people do not realize that there is some kind of hierarchy of trustworthiness of information sources. We should be conscious of that hierarchy when looking for information and remember it when we notice that something is not correct or up-to-date. ...

Nowadays it seems like we are on an information highway and it is very hard to keep up with all the knowledge. Especially this is acute when considering how many websites there are on the Internet and how long it would take you to read and understand them all. However, there is a solution – books offer a way to get condensed information from the field experts and learn a lot of things. Many people think that this is only true for other fields except programming. But I think that is false. In software development, a lot of things are timeless as well and they do not change so often. Let me present this and other arguments in more detail, and try to convince you to pick up a programming book and read it. ...