第4章アルゴリズムの言語による実現：低水準プログラミング言語

By the end of this section, you will be able to:

• Write C code using fundamental elements of the language

• Summarize the steps to develop a C program

• Understand the process to compile and run a C program

• Describe how linking is used in a C program

• Understand how to apply version control management

As discussed, a programming language is a kind of computational model that is used to write programs. C is a popular middle-level language that is widely used to create systems software. This section is a crash course in the essentials of C.

The C programming language was invented in 1972 by Dennis Ritchie of Bell Labs (Figure 4.15) and popularized by the book The C Programming Language by Brian Kernighan and Dennis Ritchie. C’s peculiar name—a single letter—was a pragmatic choice, since C replaced an earlier language named B. C is a procedural, middle-level language that gives programs low-level access to memory. It is a relatively simple language, which makes learning C, and creating a C compiler, easier than for more complex languages. This combination of features made C an instant hit, and it has maintained great popularity and import to this day. C has influenced other programming languages, too. C++ is a newer language that adds the object-oriented paradigm to C.

Why is C so popular? Mainly because its designers managed to strike a balance between low-level and middle-level features that allows C code to execute at practically the same speed as assembly language, while allowing programmers to be productive enough to create large, dependable, programs. C is the programming language behind much of the lower-level software that we depend on, including operating systems, language compilers, assemblers, text editors, print servers, network drivers, language interpreters, and command-line utilities. Here are some specific software products that are written in C:

• The Java virtual machine (ANSI C)

• Linux, an open-source operating system (C, and some assembly)

• Python (C)

• macOS X kernel (C)

• Windows (C, C++)

• The Oracle database (C, C++)

• Cisco routers (C)

C is used in a variety of industries. One example is astrophysics, where scientists write programs that simulate the motion of stellar bodies, and control instruments such as telescopes. Owing to the large size of the universe, these simulations involve performing calculations on very large arrays of numbers. C’s ability to execute fast, and control the layout of large arrays in memory, is advantageous for this application. As a relatively simple language, C is approachable to physicists who are not necessarily expert in computer science. Scientific experiments need to be reproducible, which means that code involved in science needs to work even decades in the future. The fact that C has been a stable, popular language for so long means that it is very likely to endure, which cannot be said of newer niche languages.

One notable feature of C is the way it handle memory. In a program, we have variables and values. For example, in x = 10, x is a variable and 10 is the value. Every value in a program is stored in memory. Memory regions are divided into four blocks: stack, heap, static, and code blocks. These regions store various parts of a running program. Running programs create and destroy values extremely rapidly (perhaps millions or billions per second), and memory is finite, so memory locations must be reused, or else would run out quickly. When a value is created, memory is set aside as allocated memory to hold that value. Eventually, when the value is no longer needed, that memory becomes freed memory, meaning it is given back so that it can be reused. The process of allocating and freeing memory is called memory management. A memory leak happens when some memory is allocated but never freed. A memory leak is a bug that causes a program to waste resources; severe leaks can waste all the memory on the computer, causing it to become unresponsive or crash. As a middle-level programming language, C requires programmers to handle memory management manually. This type of flexibility must be used with caution as it may result in creating programs that are not reliable and secure. In high-level languages, memory management is automated.

Here are some other notable features of C:

1. Efficient execution: C is lower in expressive power than some other middle-level languages like C++ and yet simple enough that compilers can generate machine code that is comparable in speed to hand-written assembly code. A lot of research and development have focused on creating performance-oriented C compilers.

2. Portability: C can run in multiple computing environments, also known as having the property of portability. Unix was designed to work on various hardware architectures, so the C language is not hardware-dependent. The same C code can be compiled and executed on different hardware architectures and operating systems.

3. Modularity: Modular programming refers to the process of dividing computer programs into separate sub-programs. A module is a separate software component, such as an error handler, that may be used by a variety of applications and functions within a system. C has language support for modularity.

4. Procedural and structured programming support: C adheres to the procedural and structured paradigms.

5. Data types and operators: Every variable in a C program has a data type. Data types dictate how much memory is used to store the variable, and which kinds of operators can be used with the variable.

6. Recursion support: Recursion is the phenomenon of a system being defined in terms of itself. In code, this means a function may call itself again and again. C supports recursion. However, it does not provide a feature called “tail-calling” that makes recursion efficient, so recursion is not used in C as much as in languages that provide tail-calling. A tail call is a function call performed as the final action of a function. If the target function of a tail is the same function, the function is said to be tail recursive, which is a special case of recursion. Tail recursion (also called “tail-end recursion”) is useful and helps with code optimizations.

7. Pointers: A pointer is a variable that holds the memory address of another variable and points to that variable (Figure 4.16). Pointers play a crucial role in the C language. They are used to store and manage addresses of dynamically allocated blocks in memory in the underlying computer system. Managing hardware devices involves manipulating certain memory locations, and C’s support for pointers is one of the reasons that it is used to implement kernels and device drivers.