Reading notes series "modern methods of C language programming" -- Chapter 2

Chapter two C Basic concepts of language

2.1 Write a simple one C Program

2.1.1 Compile and link

• Preprocessing ： First, the program will be submitted to The preprocessor . The preprocessor executes to # The instructions at the beginning （ It is often called an instruction ）. The preprocessor is a bit like an editor , It can add content to the program , You can also modify the program （ For example, modify the name of the main function ）.
• compile . The modified program can now enter the compiler . The compiler will translate the program into machine instructions （ I.e. target code ）. However , Such a program still cannot run .
• link . In the last step , The linker integrates the object code generated by the compiler with other additional code required , Only in this way can we finally produce a fully executable program . These additional codes include library functions used in the program .

2.2.1 Instructions

Instructions ： We call the commands executed by the preprocessor instructions . All instructions are written in characters # At the beginning . By default, the instruction occupies only one line , There is no semicolon or other special mark at the end of each instruction .

2.2.2 function

One C A program can contain multiple functions , But only main() Functions are required , When executing the program, the system will automatically call main() function .

return 0; The role of ：

1. send main() Function termination （ This ends the program ）.
2. Pointed out that main The return value of the function is 0.

Be careful ： If main() There is no at the end of the function return sentence , The program can still terminate . But many compilers produce a warning message （ Because the function should return an integer, but it doesn't ）.

2.4.3 assignment

When we assign a constant containing a decimal point to float Type variable , It is best to add a character after the constant f（ representative float）.

2.7 identifier

Some compilers put specific identifiers （ Such as asm） Treat as additional keywords . Identifiers belonging to the standard library are also restricted . Misuse of these names may lead to compilation or link errors . Identifiers that begin with underscores are also restricted .

Q & A

1. ask ： In some C In the language book ,main() The function ends with exit(0) instead of return 0, Are they the same ？

   answer ： When it appears in main() Function to make , These two statements make completely equivalent , Both terminate program execution , And return to the operating system 0 value . Which statement to use depends entirely on your personal preferences .

2. ask ： If main() There is no... At the end of the function return What are the consequences of statements ？

   answer ：return Statement is not necessary ; without return sentence , The program will also terminate . stay C89 in , The value returned to the operating system is undefined . stay C99 in , If main() The return type in the function declaration is int, The program will return to the operating system 0; Otherwise, an indefinite value will be returned .

3. ask ： Does the compiler remove comments completely or replace them with spaces ？

   answer ： Some early compilers deleted all fields in each comment , Make the statement

   a/**/b = 0;

   Comprehend

   ab = 0;

   However , basis C standard , The compiler must replace each comment statement with a space character , Therefore, the techniques mentioned above are not feasible .

   We'll actually get the following statement ：

   a b = 0;

4. ask ： Is there really no limit on the length of identifiers ？

   answer ： yes , Not yet .C89 The standard declaration identifier can be any length , But it is worth asking the compiler to remember before 31 Characters （C99 Medium is 63 Characters ）. therefore , If the first two names 31 All characters are the same , The compiler may not be able to distinguish them .

   More complicated is ,C The standard has special provisions for the identifier of external links , Most function names are such identifiers . Because the linker must be able to recognize these names , Some early linkers could only handle short names , So in C99 Only before 6 Characters are valid . Besides , Not case sensitive yet . therefore ABCDEFG and abcdefg May be treated as the same name .（C99 in , front 31 Characters are valid , And the letters are case sensitive .）

   Most compilers and linkers are looser than the standard requires , So these rules are not a problem in practical use , Don't worry about identifiers being too long , Don't define them too short .