Making and using of dynamic library (shared library)

add.c div.c mult.c sub.c

Making dynamic library

        1. First, all the files that need to generate the library file “.c” File compiled into “.o” file

gcc -c -fpic/fPIC add.c div.c sub.c mult.c

        2. Use gcc The command will all the files compiled in the first step “.o” File generation dynamic library

gcc -shared add.o div.o mult.o sub.o -o ../lib/libcalc.so

The use of dynamic libraries

        1. Generate executable files , among “-L” Specify the storage path of the library ,“-l” Specify the name of the library （ Unwanted “lib” And extension “.so”）, If there are dynamic and static libraries with the same name in the storage path of the library ,gcc Dynamic library is used by default

gcc main.c -o main -I ./include -L ./lib -l clac

        2. Execution method ：

After the program starts , The dynamic library will be dynamically loaded into memory , adopt ldd（list dynamic dependencies） Command to check dynamic library dependencies

  You can see that the path of the shared library is not specified

When the system loads executable code , Be able to know the name of the library it depends on , But we still need to know the absolute way
path . At this time, the dynamic loader of the system is required to obtain the absolute path . about elf Format executable program , yes
from ld-linux.so To complete , It searches for elf Of documents DT_RPATH paragraph ——> environment variable
LD_LIBRARY_PATH ——> /etc/ld.so.cache File list ——> /lib/, /usr/lib
The directory finds the library file and loads it into memory .

There are five ways to specify the path to a shared library

（1） Temporary modification Linux environment variable LD_LIBRARY_PATH

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/ The directory where the dynamic library is located （ Replace with your own ）

（2） Changes at the user level Linux environment variable

There are... In the user directory .bashrc（ This file is hidden ） file , Add the above statement on the last line

vim /home/ User name /.bashrc

or

vim ~/.bashrc

After that, you need to update the environment variables

source ~/.bashrc

（3） Changes at the system level Linux environment variable

sudo vim /etc/profile

After opening the document , Fill in the above sentence on the last line , Update environment variables at the same time

source /etc/profile

（4） By modifying the /etc/ld.so.cache File list To use dynamic libraries

sudo vim /etc/ld.so.conf

  Load the path into the document

  to update

sudo ldconfig

（5） Put dynamic inventory in /lib/ perhaps /usr/lib Under the table of contents （ Not recommended , It may be confused with the library file of the system itself , There are some unnecessary problems ）

Static library and dynamic library comparison

         The methods used by the static library when linking are included in the final generated executable program , The shared library does not contain , Only do
Mark , When running a program , To dynamically load .

        1. Advantages and disadvantages of static library ：

• advantage ： It is packaged into the application and loaded quickly ; The publisher does not need to provide a static library , Transplant convenience
• shortcoming ： Consume system resources , Waste of memory ; to update 、 Deploy 、 Publishing trouble

        2. Advantages and disadvantages of dynamic library ：

• advantage ： Inter process resource sharing can be realized （ Shared library ）; to update 、 Deploy 、 Easy to release ; You can control when dynamic libraries are loaded
• shortcoming ： Loading speed is slower than static library ; When publishing programs, you need to provide dependent dynamic libraries