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【C Language】Gcc compiles dynamic library static library and usage instructions

Table of contents

(1) The suffix name of the linux and win library

(2) Encapsulation and use of dynamic libraries

Example 1

Example 2

(3) Instructions for finding libraries when compiling gcc program

(4) Encapsulation and use of static libraries

Example 1

Example 2

(5) The difference between using dynamic libraries and static libraries

(6) Specify dynamic library or static library compilation during compilation

（1）linuxand win library suffix name

Encapsulated as .dll (moving) and .lib (static) on WIN.

Encapsulated as .so (moving), .a (static) on LINUX

(2) Encapsulation and use of dynamic libraries

Example 1

source codedocument:,

1. Generate dynamic library (the dynamic library will be generated)

    

  1. 

    

    #generate
    

    2. 

  2. 

    

    # Parameter -fPIC means to generate location-independent code, and after execution, a series of .o files are generated
    

    3. 

  3. 

    

    gcc  -o  -c -fPIC
    

    4. 

  4. 

    

     
    

    5. 

  5. 

    

    #Generate (windows dynamic library)
    

    6. 

  6. 

    

    gcc  -shared -o  
    

    7. 

  7. 

    

     
    

    8. 

  8. 

    

    #Generate (linux dynamic library)
    

    9. 

  9. 

    

    gcc  -shared -o 
    

    10.

2. Compile executable files (compile only with func dynamic libraries)

    

  1. 

    

    # Assume that the func dynamic library is in the current directory
    

    2. 

  2. 

    

    # Generate executable file main
    

    3. 

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    gcc  -o main -L . -l func 
    

    4.

3. Run the executable file

    

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    First put the dynamic library in the same directory as the executable file
    

    2. 

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    Or execute the following command (in linux environment)
    

    3. 

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export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:Directional library directory path
    

    4. 

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    5. 

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    Then execute the executable file
    

    6. 

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    ./main
    

    7.

Example 2

1. Generate a "location-independent" target file

gcc -fPIC    -c -I ../include

Parameter -fPIC means to generate location-independent code, and after execution, a series of .o files are generated

2. Create dynamic library

    

  1. 

    

    # linux
    

    2. 

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    gcc -shared -o    
    

    3. 

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    4. 

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    # windows
    

    5. 

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    gcc -shared -o    
    

    6.

Dynamic library usage

Example

    

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    [root@CentOS2 xxx]# ls

    

    2. 

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            temp  
    

    3. 

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    [root@CentOS2 xxx]# gcc  -L./ -ladd -I./ -o test_so

    

    4. 

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    [root@CentOS2 xxx]# ls

    

    5. 

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            temp  test_so  
    

    6. 

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    [root@CentOS2 xxx]# pwd

    

    7. 

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    /root/xxx
    

    8. 

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    [root@CentOS2 xxx]# export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/root/xxx

    

    9. 

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    [root@CentOS2 xxx]# ./test_so

    

    10. 

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    1+2=3
    

    11. 

  11. 

    

    [root@CentOS2 xxx]#

    

    12.

（3）gcc compilationInstructions for searching for libraries during program

When we use gcc to compile programs, we often use parameters such as "-I" (uppercase i), "-L" (uppercase l), "-l" (lowercase l), and make a record below: Example:

gcc -o hello    -I /home/hello/include -L /home/hello/lib -l world

After this command is run, a hello file will be generated in the current folder, which is an executable file. Run the file by executing the command ./hello in the current file.

  • -I /home/hello/include means that the /home/hello/include directory is the first directory to find the header file introduced in the source code. The search order is: /home/hello/include –> /usr/include –> /usr/local/include, which is the directory that specifies the priority search. If it cannot be found, it will look for the default directory. If there is no -I related specification, then the header file location is omitted by default to the current folder.
  • -L /home/hello/lib means to use the /home/hello/lib directory as the first directory to search for library files. The order of search is: /home/hello/lib –> /lib –> /usr/lib –> /usr/local/lib, the same as above, is also the directory specified for priority search. If omitted, it means that the current folder is preferred. "-L . " means to find it in the current directory.
  • -l word, means to find a specific dynamic link library file or, if the compilation option static is added at the end, means to find a static link library file, that is, or.

(4) Encapsulation and use of static libraries

Example 1

Source code file:

1. Generate static library (the static library will be generated)

    

  1. 

    

    #generate
    

    2. 

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    gcc  -o  -c
    

    3. 

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    4. 

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    #Generate (linux static library)
    

    5. 

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    ar -crv    
    

    6. 

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    7. 

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    #Generate (windows static library
    

    8. 

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    ar -crv    
    

    9.

2. Compile executable files (compile only with func static libraries)

    

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    # Assuming that the func static library is in the current directory and there is no func dynamic library
    

    2. 

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    # Generate executable file main
    

    3. 

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    gcc  -o main -L . -l func 
    

    4.

3. Run the executable file

    

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    # As long as the executable file is linked to the static library generated
    

    2. 

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    # No matter whether there is a static library in the current directory, just run the executable file directly.
    

    3. 

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    ./main
    

    4.

Example 2

1. Generate .o file

Here, files will be generated

gcc    -c -I ../include

2. Package the .o file to get a static library file

    

  1. 

    

    # is a process of packaging .o files, as an example
    

    2. 

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    3. 

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    # linux
    

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    ar rcs     
    

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    6. 

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    # windows
    

    7. 

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    ar rcs     
    

    8.

illustrate:

  • ar tool is not included in gcc
  • r --> Insert the file into the static library
  • c --> Create a static library regardless of whether the library exists or not
  • s --> Write an object file index into the library, or update an existing object file index.

Static library usage

gcc + source file + -L static library path + -l static library name + -I header file directory + -o executable file name

    

  1. 

    

    #gcc + source file + -L static library path + -l static library name + -I header file directory + -o executable file name
    

    2. 

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    gcc  -L./ -lmytest -I./ -o test

    

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    or
    

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    6. 

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    #gcc + source file + -I header file directory + + -o executable file name
    

    7. 

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    gcc  -I./  -o test

    

    8.

(5) The difference between using dynamic libraries and static libraries

Static libraries must be linked to the execution file, while dynamic libraries do not need to be linked to the last execution file. That is to say, for the last execution file, if it is an executable file generated by linking the static library, it doesn't matter whether you delete the static library when running the executable file. However, if it is an executable file generated by linking the dynamic library, once you delete the dynamic library, the final executable file will no longer be able to play with it.

Features of compiling with dynamic libraries：

1. The library code will not be compiled into the program, so the program compiled by the dynamic library is relatively small.

2. The program compiled by the dynamic library depends on the system's environment variables. If there is any library file, it cannot run.

Features of compiling with static libraries：

1. The library code will be compiled into the program, so the static library compiled programs are relatively large.

2. Programs compiled by static libraries do not need to rely on the system's environment variables, so if the environment variable has this library file, it can also run.

Advantages and disadvantages of static library use

  • Advantages: The library is packaged into an executable program, and it can be used directly by publishing the executable program.
  • Disadvantages: The code of the static library has been loaded into the executable program during compilation, so it is relatively large. If the static function library changes, your program must be recompiled.
  • Use occasions: Use on core programs to ensure speed and ignore space. Mainstream applications were used in the 1980s and 1990s and are rarely used now.

Advantages and disadvantages of dynamic library use

  • Mechanism: The code of the shared library is loaded into memory when the executable program is run. It is only a simple reference during the compilation process, so the code is small.
  • Advantages: Memory saving (sharing), easy to update (dynamic link): Stop running the program → Overwrite the old library with the new library (ensure that the names of the new and old libraries are consistent and the interface is consistent) "Interface" → Restart the program
  • Disadvantages: Delay binding, slightly slower
  • Use occasions: Dynamic libraries should be used in places where speed requirements are not very strong
  • Note: Whether the dynamic library is loaded into memory depends on whether the program is running.

(6) Specify dynamic library or static library compilation during compilation

If there are both dynamic libraries and static libraries in the current folder. If the static library and dynamic libraries have the same name (such as both exist and), when compiling, the dynamic library will be preferred by default for compilation.

gcc  -o main -L. -l ToolFunc

-L. -l ToolFunc will look for libraries in the current directory and give priority to finding dynamic libraries. If found, use dynamic libraries to compile directly, rather than static libraries to compile; if the dynamic libraries are not found, look for static libraries to compile; if the static libraries are not found, an error is reported.

How to force static library compilation: if at the endCompile options-static, means to find static link library files. You can force the use of static libraries during compilation. If this parameter is not used, and the static library is the same name as the dynamic library, the dynamic library will be used first.

gcc  -o main2  -L . -l ToolFunc -static

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