What is the history behind C Programming and Unix?

Multics OS and Unix

Even before having Unix, we had the Multics OS. It was a joint project launched in 1964 as a cooperative project led by MIT, General Electric, and Bell Labs. Multics OS was a huge success because it could introduce the world to a real working and secure operating system. Multics was installed everywhere from universities to government sites. Fast-forward to 2019, and every operating system today is borrowing some ideas from Multics indirectly through Unix.

In 1969, because of the various reasons that we will talk about shortly, some people at Bell Labs, especially the pioneers of Unix, such as Ken Thompson and Dennis Ritchie, gave up on Multics and, subsequently, Bell Labs quit the Multics project. But this was not the end for Bell Labs; they had designed their simpler and more efficient operating system, which was called Unix.

It is worthwhile to compare the Multics and Unix operating systems. In the following list, you will see similarities and differences found while comparing Multics and Unix:

• Both follow the onion architecture as their internal structure. We mean that they both have the same rings in their onion architecture, especially kernel and shell rings. Therefore, programmers could write their own programs on top of the shell ring. Also, Unix and Multics expose a list of utility programs, and there are lots of utility programs such as ls and pwd. In the following sections, we will explain the various rings found in the Unix architecture.
• Multics needed expensive resources and machines to be able to work. It was not possible to install it on ordinary commodity machines, and that was one of the main drawbacks that let Unix thrive and finally made Multics obsolete after about 30 years.
• Multics was complex by design. This was the reason behind the frustration of Bell Labs employees and, as we said earlier, the reason why they left the project. But Unix tried to remain simple. In the first version, it was not even multitasking or multi-user!

You can read more about Unix and Multics online, and follow the events that happened in that era. Both were successful projects, but Unix has been able to thrive and survive to this day.

It is worth sharing that Bell Labs has been working on a new distributed operating system called Plan 9, which is based on the Unix project. 

 Figure 1-1: Plan 9 from Bell Labs

Suffice to say that Unix was a simplification of the ideas and innovations that Multics presented; it was not something new, and so, I can quit talking about Unix and Multics history at this point.

So far, there are no traces of C in the history because it has not been invented yet. The first versions of Unix were purely written using assembly language. Only in 1973 was Unix version 4 written using C.

Now, we are getting close to discussing C itself, but before that, we must talk about BCPL and B because they have been the gateway to C.

About BCPL and B

BCPL was created by Martin Richards as a programming language invented for the purpose of writing compilers. The people from Bell Labs were introduced to the language when they were working as part of the Multics project. After quitting the Multics project, Bell Labs first started to write Unix using assembly programming language. That’s because, back then, it was an anti-pattern to develop an operating system using a programming language other than assembly.

For instance, it was strange that the people at the Multics project were using PL/1 to develop Multics but, by doing that, they showed that operating systems could be successfully written using a higher-level programming language other than assembly. As a result, Multics became the main inspiration for using another language for developing Unix.

The attempt to write operating system modules using a programming language other than assembly remained with Ken Thompson and Dennis Ritchie at Bell Labs. They tried to use BCPL, but it turned out that they needed to apply some modifications to the language to be able to use it in minicomputers such as the DEC PDP-7. These changes led to the B programming language.

While we won’t go too deep into the properties of the B language here you can read more about it and the way it was developed at the following links:

• The B Programming Language 
• The Development of the C Language
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Dennis Ritchie authored the latter article himself, and it is a good way to explain the development of the C programming language while still sharing valuable information about B and its characteristics.

B also had its shortcomings in terms of being a system programming language. B was typeless, which meant that it was only possible to work with a word (not a byte) in each operation. This made it hard to use the language on machines with a different word length.

Therefore, over time, further modifications were made to the language until it led to developing the NB (New B) language, which later derived the structures from the B language. These structures were typeless in B, but they became typed in C. And finally, in 1973, the fourth version of Unix could be developed using C, which still had many assembly codes.

In the next section, we talk about the differences between B and C, and why C is a top-notch modern system programming language for writing an operating system.

The way to C programming and Unix

I do not think we can find anyone better than Dennis Ritchie himself to explain why C was invented after the difficulties met with B. In this section, we’re going to list the causes that prompted Dennis Ritchie, Ken Thompson, and others create a new programming language instead of using B for writing Unix.

Limitations of the B programming language:

• B could only work with words in memory: Every single operation should have been performed in terms of words. Back then, having a programming language that was able to work with bytes was a dream. This was because of the available hardware at the time, which addressed the memory in a word-based scheme.
• B was typeless: More accurately, B was a single-type language. All variables were from the same type: word. So, if you had a string with 20 characters (21 plus the null character at the end), you had to divide it up by words and store it in more than one variable. For example, if a word was 4 bytes, you would have 6 variables to store 21 characters of the string.
• Being typeless meant that multiple byte-oriented algorithms, such as string manipulation algorithms, were not efficiently written with B: This was because B was using the memory words not bytes, and they could not be used efficiently to manage multi-byte data types such as integers and characters.
• B didn’t support floating-point operations: At the time, these operations were becoming increasingly available on the new hardware, but there was no support for that in the B language.
• Through the availability of machines such as PDP-1, which could address memory on a byte basis, B showed that it could be inefficient in addressing bytes of memory: This became even clearer with B pointers, which could only address the words in the memory, and not the bytes. In other words, for a program wanting to access a specific byte or a byte range in the memory, more computations had to be done to calculate the corresponding word index.

The difficulties with B, particularly its slow development and execution on machines that were available at the time, forced Dennis Ritchie to develop a new language. This new language was called NB, or New B at first, but it eventually turned out to be C.

This newly developed language, C, tried to cover the difficulties and flaws of B and became a de facto programming language for system development, instead of the assembly language. In less than 10 years, newer versions of Unix were completely written in C, and all newer operating systems that were based on Unix got tied with C and its crucial presence in the system.

As you can see, C was not born as an ordinary programming language, but instead, it was designed by having a complete set of requirements in mind. You may consider languages such as Java, Python, and Ruby to be higher-level languages, but they cannot be considered as direct competitors as they are different and serve different purposes. For instance, you cannot write a device driver or a kernel module with Java or Python, and they themselves have been built on top of a layer written in C.

Unlike some programming languages, C is standardized by ISO, and if it is required to have a certain feature in the future, then the standard can be modified to support the new feature.

To summarize

In this article, we began with the relationship between Unix and C. Even in non-Unix operating systems, you see some traces of a similar design to Unix systems. We also looked at the history of C and explained how Unix appeared from Multics OS and how C was derived from the B programming language.

The book Extreme C, written by Kamran Amini will help you make the most of C's low-level control, flexibility, and high performance.

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