What is the definition of user identifiers by the C language standard?

The C standard defines the identifier's composition rules: it starts with letters or underscores, can contain letters, numbers or underscores, is case sensitive, keywords cannot be used, length is limited, and the effective range is affected by the declared position.

What is the definition of user identifiers in C language standards? This question seems simple, but in fact it has hidden mystery, and many beginners are prone to falling into the pit. Simply put, it is the names you use to name variables, functions, structures, etc. But the standard regulations are much more complicated than this, so we have to break them apart and think about it carefully.

The C standard definition of identifiers is mainly reflected in its composition rules. It is not something you can use by just writing it randomly. The identifier must begin with a letter or an underscore, followed by a letter, number or underscore. This looks quite straightforward, but there are some considerations in it.

First, case sensitive. myVar and myvar are two completely different identifiers. This is not a joke, and many bugs come from this, especially when you stay up late to write code, it is easy to ignore the case difference. It is recommended to develop good naming habits to avoid unnecessary trouble. I personally prefer all lowercase underscores, such as my_variable , which is clear and easy to read and easy to search.

Secondly, the keyword is the restricted area. int , float , for , while these keywords are built in C languages, and you must not use them as identifiers. The compiler will directly report an error, which will drive you crazy. Therefore, before writing code, it is best to be familiar with the keyword list in C language to avoid in vain.

Furthermore, length limit. Although the standard does not specify the maximum length of the identifier, compilers usually have their own limitations. Although compilers generally support long identifiers nowadays, it is recommended to keep the identifiers concise for the readability and maintainability of the code. Too long identifiers are not only difficult to read, but also prone to errors.

Finally, there is another point that is easily overlooked: the valid range of identifiers. The scope of an identifier depends on its declared location. Global variables, local variables, function parameters, etc., have different effective ranges. If you don’t understand the scope, you can easily make unexpected mistakes. This requires careful understanding of the scope rules of variables in the process of learning C language.

For example, take a look at the following code:

 <code class="c">#include <stdio.h> int my_global_variable = 10; // 全局变量int main() { int my_local_variable = 20; // 局部变量int _another_variable = 30; // 以下划线开头的变量printf("Global: %d, Local: %d, Another: %d\n", my_global_variable, my_local_variable, _another_variable); return 0; }</stdio.h></code>

In this code, my_global_variable is a global variable that is valid throughout the program; my_local_variable and _another_variable are local variables that are valid only within main function.

Regarding the naming of identifiers, I personally have some experience: Try to use meaningful names to clearly express the functions of variables or functions; maintain the consistency of the naming style, do not use the camel nomenclature for a while, and then use the underlined nomenclature; avoid using abbreviations unless the abbreviation is conventional.

In short, the definition of identifiers in C language seems simple, but there are many details that need attention in actual applications. Only by mastering these details can you write high-quality and easy-to-maintain C code. Remember, the code should not only be able to run, but also be easy to understand. This is the highest level of programming.

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