Merge pull request #25915 from lkxed/20220603-How-static-linking-works-on-Linux

[手动选题][tech]: 20220603 How static linking works on Linux.md

sources/tech/20220603 How static linking works on Linux.md

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+[#]: subject: "How static linking works on Linux"
+[#]: via: "https://opensource.com/article/22/6/static-linking-linux"
+[#]: author: "Jayashree Huttanagoudar https://opensource.com/users/jayashree-huttanagoudar"
+[#]: collector: "lkxed"
+[#]: translator: " "
+[#]: reviewer: " "
+[#]: publisher: " "
+[#]: url: " "
+
+How static linking works on Linux
+======
+Learn how to combine multiple C object files into a single executable with static libraries.
+
+![Woman using laptop concentrating][1]
+
+Image by Mapbox Uncharted ERG, [CC-BY 3.0 US][2]
+
+Code for applications written using C usually has multiple source files, but ultimately you will need to compile them into a single executable.
+
+You can do this in two ways: by creating a static library or a dynamic library (also called a shared library). These two types of libraries vary in terms of how they are created and linked. Your choice of which to use depends on your use case.
+
+In a [previous article][3], I demonstrated how to create a dynamically linked executable, which is the more commonly used method. In this article, I explain how to create a statically linked executable.
+
+### Using a linker with static libraries
+
+A linker is a command that combines several pieces of a program together and reorganizes the memory allocation for them.
+
+The functions of a linker include:
+
+* Integrating all the pieces of a program
+* Figuring out a new memory organization so that all the pieces fit together
+* Reviving addresses so that the program can run under the new memory organization
+* Resolving symbolic references
+
+As a result of all these linker functionalities, a runnable program called an executable is created.
+
+Static libraries are created by copying all necessary library modules used in a program into the final executable image. The linker links static libraries as a last step in the compilation process. An executable is created by resolving external references, combining the library routines with program code.
+
+### Create the object files
+
+Here's an example of a static library, along with the linking process. First, create the header file `mymath.h` with these function signatures:
+
+```
+int add(int a, int b);
+int sub(int a, int b);
+int mult(int a, int b);
+int divi(int a, int b);
+```
+
+Create `add.c`, `sub.c` , `mult.c` and `divi.c` with these function definitions:
+
+```
+// add.c
+int add(int a, int b){
+return (a+b);
+}
+
+//sub.c
+int sub(int a, int b){
+return (a-b);
+}
+
+//mult.c
+int mult(int a, int b){
+return (a*b);
+}
+
+//divi.c
+int divi(int a, int b){
+return (a/b);
+}
+```
+
+Now generate object files `add.o`, `sub.o`, `mult.o`, and `divi.o` using GCC:
+
+```
+$ gcc -c add.c sub.c mult.c divi.c
+```
+
+The `-c` option skips the linking step and creates only object files.
+
+Create a static library called `libmymath.a`, then remove the object files, as they're no longer required. (Note that using a `trash` [command][4] is safer than `rm`.)
+
+```
+$ ar rs libmymath.a add.o sub.o mult.o divi.o
+$ trash *.o
+$ ls
+add.c  divi.c  libmymath.a  mult.c  mymath.h  sub.c
+```
+
+You have now created a simple example math library called `libmymath`, which you can use in C code. There are, of course, very complex C libraries out there, and this is the process their developers use to generate the final product that you and I install for use in C code.
+
+Next, use your math library in some custom code and then link it.
+
+### Create a statically linked application
+
+Suppose you've written a command for mathematics. Create a file called `mathDemo.c` and paste this code into it:
+
+```
+#include <mymath.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+  int x, y;
+  printf("Enter two numbers\n");
+  scanf("%d%d",&x,&y);
+ 
+  printf("\n%d + %d = %d", x, y, add(x, y));
+  printf("\n%d - %d = %d", x, y, sub(x, y));
+  printf("\n%d * %d = %d", x, y, mult(x, y));
+
+  if(y==0){
+    printf("\nDenominator is zero so can't perform division\n");
+      exit(0);
+  }else{
+      printf("\n%d / %d = %d\n", x, y, divi(x, y));
+      return 0;
+  }
+}
+```
+
+Notice that the first line is an `include` statement referencing, by name, your own `libmymath` library.
+
+Create an object file called `mathDemo.o` for `mathDemo.c` :
+
+```
+$ gcc -I . -c mathDemo.c
+```
+
+The `-I` option tells GCC to search for header files listed after it. In this case, you're specifying the current directory, represented by a single dot (`.` ).
+
+Link `mathDemo.o` with `libmymath.a` to create the final executable. There are two ways to express this to GCC.
+
+You can point to the files:
+
+```
+$ gcc -static -o mathDemo mathDemo.o libmymath.a
+```
+
+Alternately, you can specify the library path along with the library name:
+
+```
+$ gcc -static -o mathDemo -L . mathDemo.o -lmymath
+```
+
+In the latter example, the `-lmymath` option tells the linker to link the object files present in the `libmymath.a` with the object file `mathDemo.o` to create the final executable. The `-L` option directs the linker to look for libraries in the following argument (similar to what you would do with `-I` ).
+
+### Analyzing the result
+
+Confirm that it's statically linked using the `file` command:
+
+```
+$ file mathDemo
+mathDemo: ELF 64-bit LSB executable, x86-64...
+statically linked, with debug_info, not stripped
+```
+
+Using the `ldd` command, you can see that the executable is not dynamically linked:
+
+```
+$ ldd ./mathDemo
+        not a dynamic executable
+```
+
+You can also check the size of the `mathDemo` executable:
+
+```
+$ du -h ./mathDemo
+932K    ./mathDemo
+```
+
+In the example from my [previous article][5], the dynamic executable took up just 24K.
+
+Run the command to see it work:
+
+```
+$ ./mathDemo
+Enter two numbers
+10
+5
+
+10 + 5 = 15
+10 - 5 = 5
+10 * 5 = 50
+10 / 5 = 2
+```
+
+Looks good!
+
+### When to use static linking
+
+Dynamically linked executables are generally preferred over statically linked executables because dynamic linking keeps an application's components modular. Should a library receive a critical security update, it can be easily patched because it exists outside of the applications that use it.
+
+When you use static linking, a library's code gets "hidden" within the executable you create, meaning the only way to patch it is to re-compile and re-release a new executable every time a library gets an update—and you have better things to do with your time, trust me.
+
+However, static linking is a reasonable option if the code of a library exists either in the same code base as the executable using it or in specialized embedded devices that are expected to receive no updates.
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/22/6/static-linking-linux
+
+作者：[Jayashree Huttanagoudar][a]
+选题：[lkxed][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/jayashree-huttanagoudar
+[b]: https://github.com/lkxed
+[1]: https://opensource.com/sites/default/files/lead-images/lenovo-thinkpad-laptop-concentration-focus-windows-office.png
+[2]: https://creativecommons.org/licenses/by/3.0/us/
+[3]: https://opensource.com/article/22/5/dynamic-linking-modular-libraries-linux
+[4]: https://www.redhat.com/sysadmin/recover-file-deletion-linux
+[5]: https://opensource.com/article/22/5/dynamic-linking-modular-libraries-linux