What characters can a C user identifier contain?

C identifiers must start with letters or underscores, only contain letters, numbers and underscores, and cannot use control characters or special symbols.

C language user identifier? This question seems simple, but it actually has hidden mystery, and many beginners are prone to falling into the pit. Simply put, it is what characters can you use when you name variables, functions, and structures.

Don’t think that just typing a few alphanumeric numbers is done. C language has strict regulations on the composition of identifiers, which is not a joke. It's not as loose as some scripting languages. You have to remember that the compiler is not a roundworm in your belly. It has to follow the rules. If you write a wrong character, it may report an error to you and make you scratch your head.

So, what characters can be used? There are only two cores: letters (both upper and lower case, AZ, az) and numbers (0-9). But don't be too happy too early, it's just the basis. It must start with a letter or underscore '_', which is an iron rule! If you start with a number, the compiler will directly give you a big "error". Think about it, can the compiler recognize the name 123variable ? How does it know that you are a variable name, not some weird number?

For example, my_variable , counter1 , _private_data are all legal identifiers, while 123abc , my-variable , and my variable are all illegal. Note that the "-" sign in my-variable and the spaces in my variable are not allowed.

You might ask, what is an underscore '_'? It is not a decoration, it is widely used in C language, especially in naming conventions. It is often used to represent private variables or internal functions, such as _internal_function . You can see at a glance that this is an internal function, avoiding direct calls from external code. This is a good programming habit that can improve the readability and maintainability of the code.

To go a little further, some characters are in the ASCII code table, but they cannot be used in identifiers. For example, some control characters or some special symbols, these character compilers do not recognize them at all. Try to stuff a newline into the identifier? The consequences are at your own risk.

Therefore, when writing C code, you must pay attention to the naming specifications of identifiers. This is not only a compiler requirement, but also to improve the readability and maintainability of the code. A good identifier, like a good variable name, can let you understand the meaning of the code at a glance, reduce the possibility of errors, and facilitate future maintenance and modification. A bad identifier, like a mine in the code, can explode at any time. Don't underestimate these details, they are often the culprits that cause program bugs.

Finally, I will give you a piece of code and experience it:

 <code class="c">#include <stdio.h> int my_variable = 10; // 合法的标识符int _private_data = 20; // 合法的标识符，通常表示私有变量int counter1 = 30; // 合法的标识符//int 123abc = 40; // 非法的标识符，数字开头//int my-variable = 50; // 非法的标识符，包含'-' //int my variable = 60; // 非法的标识符，包含空格int main() { printf("my_variable = %d\n", my_variable); printf("_private_data = %d\n", _private_data); printf("counter1 = %d\n", counter1); return 0; }</stdio.h></code>

This code shows both legal and illegal identifiers, and you can understand them more intuitively after compiling and running them. Remember, standardized code is a compulsory course for programmers!

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