Converting a source program written in a high-level language into an executable program requires "compilation and linking". Source programs written in high-level languages cannot be directly executed on the machine and must be compiled and linked.
To run a program, it must go through four steps: preprocessing, compilation, assembly and linking. Next, we will explain these processes in detail through a few simple examples.
We need to explain some of the options used above.
If you use the gcc command without any options, the entire process of preprocessing, compilation, assembly, and linking will be performed by default. If the program is correct, you will get an executable file, which defaults to a.out
-E option: prompts the compiler to stop after performing preprocessing, and subsequent compilation, assembly, and linking will not be executed.
-S option: prompts the compiler to stop after compilation and not to perform assembly and linking.
-c option: prompts the compiler to stop after executing assembly.
So, these three options are equivalent to limiting the stop time of the compiler execution operation, rather than taking out a certain step separately for execution.
#Everyone should be familiar with the execution process of the above program, so I won’t waste any time.
1. Preprocessing:
Use the -E option to indicate that only precompilation will be performed, and a .i file will be generated accordingly.
Operations performed during the preprocessing process:
Use a simple program to verify whether the facts are as mentioned above
Write a simple program, and then use the -E option to perform the preprocessing process and open the generated Compare the .i file with the source file, and the result is clear at a glance
Adding line numbers to the code will not be demonstrated here. We will not do it manually when writing code When adding line numbers, the line numbers we see are automatically added by the editing tools we use, and these line numbers cannot be seen by the compilation system. However, we find that if there is a problem with any line of our code, When compiling, a prompt will be given to tell which line of code has a problem. This has proven that the compiler will automatically add line numbers.
2. Compilation:
Use the -S option to indicate that the compilation operation will end after execution. A .s file is generated accordingly.
The compilation process is the core part of the entire program construction. If the compilation is successful, the source code will be converted from text form into machine language. The compilation process is to perform a series of lexical analysis, syntax analysis, and semantic analysis on the preprocessed files. After analysis and optimization, the corresponding assembly code file is generated.
Lexical analysis uses a program called lex to implement lexical scanning. It will analyze the input string according to the lexical rules previously described by the user. Divide it into individual tokens. The generated tokens are generally divided into: keywords, identifiers, literals (including numbers, strings, etc.) and special symbols (operators, equal signs, etc.), and then they are placed in the corresponding tables.
Grammatical analysis completes the analysis of the syntax level of the expression, but it does not understand whether the statement is truly meaningful. Some statements are grammatically legal, but have no practical meaning. For example, when two pointers are multiplied, semantic analysis is required. However, the only semantics that the compiler can analyze are static semantics.
Static semantics: Semantics that can be determined at compile time. Usually includes declaration and type matching and type conversion. For example, when a floating-point expression is assigned to an integer expression, it implies a conversion from floating-point to integer, and semantic analysis needs to complete this conversion. For another example, converting a floating-point type into Assigning an expression to a pointer is definitely not possible. During semantic analysis, it will be found that the two types do not match, and the compiler will report an error.
Dynamic semantics: Semantics that can only be determined at runtime. For example, if you divide two integers, there is no problem with the syntax and the types match. It sounds like there is nothing wrong with it. However, if the divisor is 0, there will be a problem. This problem is not known in advance and can only be done during operation. Only when the time comes can we find out that there is something wrong with him. This is dynamic semantics.
Our code can be optimized. For some values that can be determined during compilation, they will be optimized, such as Speaking of 2 6 in the above example, its value can be determined to be 8 during compilation, but it is more difficult to directly optimize the syntax. In this case, the optimizer will first convert the syntax tree into intermediate code. Intermediate code is generally independent of the target machine and operating environment. (Does not include data size, variable address, register name, etc.). Intermediate codes have different forms in different compilers. The more common ones are three-address code and P-code.
The intermediate code allows the compiler to be divided into front-end and back-end. The compiler front-end is responsible for generating machine-independent intermediate code, and the compiler back-end converts the intermediate code into machine code.
The code generator converts the intermediate code into machine code. This process depends on the target machine, because different machines have different Word length, register, data type, etc.
Finally, the target code optimizer optimizes the target code, such as selecting appropriate addressing methods, using unique ones to replace multiplication and division, and deleting redundant instructions.
3. Assembly
The assembly process is completed by calling the assembler as, which is used to convert the assembly code into instructions that the machine can execute. Almost every assembly statement Corresponds to a machine instruction.
Use the command as hello.s -o hello.o or use gcc -c hello.s -o hello.o to execute until the end of the assembly process, and the corresponding generated file is an .o file.
4. Links
The main content of the link is to correctly connect the parts that reference each other between the modules. Its job is to correct the references of some instructions to other symbol addresses. The linking process mainly includes address and space allocation, symbol resolution and redirection
Symbol resolution: sometimes also called symbol binding, name binding, name resolution, or address binding, it actually refers to the use of symbols Come and go to identify an address.
For example, Int A = 6; such a code, use A to identify a 4 -byte size space in the space. The content stored in the space is 4.
The process of addressing each target is called relocation.
The most basic link is called static linking, which is to compile the source code file of each module into a target file (Linux: .o Windows: .obj), and then link the target file and library together to form the final executable file. A library is actually a package of a set of target files. Some of the most commonly used codes are mutated into target files and then packaged and stored. The most common library is the runtime library, which is a collection of basic functions that support program running.
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