C language compilation process

Source: Internet
Author: User

C language compilation process

The concept of Compilation: compile a program to read the source program, analyze the lexical and syntax, and convert advanced language commands into functional equivalent assembly code, then, the assembler converts the program to the machine language and generates the executable program based on the requirements of the operating system for the executable file format.
Complete compilation process: C source program --> pre-compilation (. c) --> compile and optimize the program (. S ,. ASM) --> assembler (. OBJ ,. O ,. A ,. ko) --> chain program (.exe ,. elf ,. axf)

1. Compile preprocessing

Reads the C source program and processes the pseudocommands (commands starting with #) and special symbols.
Pseudoinstructions mainly include the following four aspects:
(1) macro definition commands, such as # define name tokenstring and # UNDEF.
For the previous pseudo command, all the names in the program must be replaced by tokenstring, but the name as a String constant is not replaced. For the latter, the definition of a macro will be canceled, so that the appearance of the string will not be replaced in the future.
(2) Conditional compilation commands, such as # ifdef, # ifndef, # else, # Elif, # endif.
The introduction of these pseudo commands allows programmers to define different macros to determine which code the program will process. The pre-compiled program filters out unnecessary code based on the relevant files.
(3) the header file contains commands, such as # include "FILENAME" or # include <FILENAME>.
In header files, a large number of macros (the most common is a character constant) are defined using a pseudo command # define, which also contains declarations of various external symbols.
The purpose of using header files is to make some definitions available for multiple different C source programs. In the C source program that needs to use these definitions, you only need to add a # include statement, instead of repeating these definitions in this file. The precompiled program adds all the definitions in the header file to the output file generated by the precompiled program for the Compilation Program to process it.
The header files contained in the C source program can be provided by the system. These header files are generally stored in the/usr/include directory. # Include them in the program using angle brackets (<> ). In addition, developers can also define their own header files. These files are generally placed in the same directory as the C source program. In this case, double quotation marks ("") are used in # include ("").
(4) special symbols. Pre-compiled programs can recognize some special symbols.
For example, the line mark in the source program will be interpreted as the current line number (in decimal number), and the file will be interpreted as the name of the currently compiled C source program. The pre-compiled program replaces these strings with appropriate values in the source program.
The pre-compiled program basically replaces the source program. After this replacement, an output file without macro definition, Conditional compilation instructions, and special symbols is generated. The meaning of this file is the same as that of the source file without preprocessing, but the content is different. Next, the output file will be translated into machine commands as the output of the Compilation Program.

2. Compilation and Optimization

The pre-compiled output file contains only constants, such as numbers, strings, variable definitions, and keywords in C language, such as main, if, else, for, while, {,}, +,-, *,/, and so on.
To compile a program, perform lexical analysis and syntax analysis. After confirming that all commands comply with the syntax rules, translate them into equivalent intermediate code representation or assembly code.
Optimization processing is a difficult technology in the compilation system. It involves not only the compilation technology itself, but also the hardware environment of the machine. The optimization part is the optimization of the intermediate code. This optimization does not depend on a specific computer. Another optimization is mainly for generating the target code.
For the previous optimization, the main work is to delete public expressions, loop optimization (out-of-code optimization, weak strength, changing cycle control conditions, merging of known quantities, etc.), and re-write propagation, and the deletion of useless values.
The Optimization of the latter type is closely related to the hardware structure of the machine. The main consideration is how to make full use of the values of relevant variables stored in each hardware register of the machine, to reduce the memory access times. In addition, how to make some adjustments to commands based on the features of machine hardware execution commands (such as pipelines, Proteus, CISC, and VLIW) to make the target code relatively short and the execution efficiency relatively high, it is also an important research topic.
The compiled code after optimization must be compiled by the assembler and converted into corresponding machine commands, which may be executed by machines.

3. Assembly Process

The assembly process refers to the process of translating the assembly language code into the target machine instructions. For each C language source program processed by the translation system, the corresponding target file will be obtained after this processing. What is stored in the target file is the machine language code equivalent to the source program.
The target file consists of segments. Generally, a target file contains at least two segments:
Code segment: This section mainly contains program instructions. This section is generally readable and executable, but generally cannot be written.
Data Segment: It mainly stores various global variables or static data used in the program. Generally, data segments are readable, writable, and executable.
There are three types of target files in UNIX:
(1) relocated files
It contains code and data suitable for creating an executable or shared target file by linking to other target files.
(2) shared target file
This type of file stores the code and data suitable for linking in two contexts.
The first is that the linking program can process it with other relocated files and shared target files to create another target file;
The second is that the dynamic link Program combines it with another executable file and other shared target files to create a process image.
(3) executable files
It contains a file that can be executed by a process created by the operating system.
The assembler generates the first type of target file. For the last two methods, some other processing is required. This is the work of The Link program.

4. Link Program


The target files generated by the assembler cannot be executed immediately, and many of them may not be resolved.
For example, a function in a source file may reference a symbol (such as a variable or function call) defined in another source file, or call a function in a library file in a program. All these problems must be handled by the linked program.
The main task of linking a program is to connect the target file to each other, or connect the symbols referenced in one file with the definition of the symbol in another file, this makes all these target files a unified whole that can be loaded and executed by the operating system.
Link processing can be divided into two types based on the connection methods specified by developers for functions of the same Library:
(1) Static Link
In this connection mode, the code of the function will be copied from its static Link Library to the final executable program. In this way, when the program is executed, the code will be loaded into the virtual address space of the process. The static Link Library is actually a collection of target files. Each file contains the code of one or more related functions in the library.
(2) Dynamic Link
In this way, the function code is put in a target file called a dynamic link library or shared object. What the linked program does at this time is to record the name of the shared object and a small amount of other registration information in the final executable program. When the executable file is executed, all content of the dynamic link library will be mapped to the virtual address space of the corresponding process at runtime. The dynamic link program finds the corresponding function code based on the information recorded in the executable program.
For function calls in executable files, dynamic or static links can be used respectively. Dynamic Links can make the final executable files relatively short, and save some memory when the shared object is used by multiple processes, because only one copy of the shared object code needs to be saved in the memory. However, dynamic links are superior to static links. In some cases, dynamic links may cause some performance damage.

The entire process of C language compilation is very complex. The Compiler knowledge, hardware knowledge, and tool chain knowledge involved in it are quite a lot, an in-depth understanding of the entire compilation process is of great help to engineers to understand the compilation of applications. I hope you can learn more about the compilation process and think more about and practice when you encounter problems.
In general, we only need to know the two phases of compilation and connection. In the compilation phase, the source program (*. c) convert to the target code (generally the OBJ file, and the specific process is the stages mentioned above). The connection phase refers to the target code (OBJ file) converted from the source program) connect the code corresponding to the library function called in your program to form the corresponding executable file (exe file, other things must be understood in practice.


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