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

C has a variety of integer types, such as short, int, and long. A C programmer needs to decide on the most appropriate type for each piece of information in their program. Smaller types use less memory but can only store a narrower range of values. On a typical computer, the maximum short value is about 32 thousand and the maximum int value is about 2 billion. A good practice is to think critically about how large a particular value might become and pick the smallest data type that accommodates that range.

The YouTube programmers faced this issue when they implemented the view counter for YouTube videos. They had to think through: what is the maximum number of views that a video is likely to garner? Two billion seemed like a safe choice, so they chose int.

This decision turned out to be misguided. In 2014, the viral hit music video “Gangnam Style” by the Korean artist Psy accumulated more than two billion views, and the view counter broke. The int variable storing the number of views of “Gangnam Style” overflowed and wrapped around to a negative number. This proved to be an embarrassment for YouTube, who had to quickly change their code to use long instead.

What is the most appropriate integer data type (short, int, or long) for the following quantities?

1. The number of people on an airplane

2. The number of people on Earth

3. The number of people in a household

4. The number of dollars in a bank account

High-level languages usually check array indices at runtime, which makes out-of-range bugs easy to identify and fix, but slows down array subscripts slightly. As a middle-level language, C does not check array indices. An array is a storage space where the elements are stored in contiguous memory cells. They are indexed from 0 (the first cell) to n−1 (last cell).

In C, an invalid array subscript will access memory outside of the array variable. If the subscript is only out of range by a little bit, this will access nearby variables, which is a subtle bug that may go unnoticed. A segmentation fault (“segfault” for short) occurs if the subscript is very far out of range. When this occurs, it will access a memory address that is off-limits to the program, and your operating system will forcibly shut down the program in response. This kind of runtime error can be notoriously difficult to remedy. Out-of-bounds array subscripts are a common source of segmentation fault errors.

Every value in a program is stored at a specific memory address. A pointer is a value that contains a memory address. Technically, a pointer should contain the location of a valid data value. However, many memory locations do not contain valid data values, so it is possible to have an invalid pointer that does not hold a valid location. The pointee is the value that a pointer points at. A pointer is analogous to a street address such as “123 Main Street,” because it refers to a specific location. In that analogy, each building is a pointee. Usually, an address is valid and refers to a place you can visit. However, it is possible to have an invalid address that is not a place that can be visited; for example, if the building at that location was demolished.

One of the differences between middle-level and high-level languages is that high-level languages either prohibit invalid pointers entirely, or provide mechanisms to handle them safely. As a middle-level language, C gives programmers the freedom to create null/invalid pointers, which can be helpful when writing code that interfaces with hardware devices. Since all hardware devices do not support the same functionality, the support of individual features by a given device may be indicated as a null/uninitialized pointer, which is fine as long as the program checks for un-initialized pointers to determine if a given functionality is available. However, in general, the freedom of using null/invalid pointers comes with a responsibility to ensure that pointers are always used properly. This has proven to be difficult; invalid pointers are a common source of bugs in C programs.

In C, the programmer is responsible for making sure that character arrays are actually big enough to fit strings, and that strings include the null terminator character. A character array is a string of characters sometimes terminated using a null. An example might be something like: char *arr= "string\0". Overlooking either of these results in bugs. This is a prime example of how middle-level languages such as C expect programmers to deal with more details than do high-level languages.