Brief knowledge of Linux GCC**

Catalog

• Cross compilation
• Brief knowledge points **
  • Some of the concepts
• GCC compiler
  • GCC Briefly use
    • GCC The build process **
    • Common compilation options
    • Compile multiple files
    • Make 、 Using dynamic libraries
    • Make 、 Use static libraries
    • A very useful option
• Reference resources

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Cross compilation

Use different cross compiler toolchains to compile source code , Can run on processors of different frameworks .
Such as ：

• X86

gcc -o hello hello.c

The file compiled by the above command can be in X86 Up operation , because gcc The compiler tool chain is for PC Compilation of .

• ARM

arm-linux-gnueabihf-gcc -o hello hello.c

The file compiled by the above command can be in ARM Up operation .

Brief knowledge points **

1. The role of header file ：
   1. Statement （declare）
2. c Role of documents
   1. Definition 、 Realization （define）
3. Header file addressing
   1. The default path ： In the compiler include Catalog
   2. Can be specified ：
      1. #include "xx/xx.h" ： From the current file path
      2. Compile with "-I" Option assignment
4. c Where are the external functions in the file , Such as printf function ：
   1. library ：
      1. The default path ： In the compiler lib Catalog
      2. Can be specified ：
         1. Compile with "-I" Specify the library file
         2. Compile with "-L" Specify Library Directory
5. How to determine the default path of header file in cross compiler ？
   1. Go to the cross compiler Directory , Use find Command to find , Such as
      • perform "find -name "stdio.h""
6. How to determine the default path of Library in cross compiler ？
   1. Go to the cross compiler Directory , perform "find -name lib", Go in and have a look , There's a lot of .so Inside the file is the path to be found .

Some of the concepts

1. Cross compile... In the toolchain include Contents and lib Catalog ：
   1. include Store header file
      1. These header files are usually function declarations , There are also some variable declarations , Namespace , Macro definition ,typedef wait
   2. lib Deposit obj Of documents ( Yes gcc Say for .o)
      1. in other words , Some library files , People don't want you to see the source code , It's just for you to generate in the middle obj file

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GCC compiler

PC The on-board compiler tool chain has gcc、ld、objcopy、objdump wait , They can compile programs in X86 Run on the platform .
To make the compiled program in ARM Up operation , You have to use cross compiler toolchains xxx-gcc、xxx-ld etc. （ Different versions of the compiler have different prefixes , Such as arm-linux-gcc）.

GCC Briefly use

GCC The build process **

One C/C++ The document goes through Preprocessing 、 compile 、 assembly 、 link etc. 4 It takes three steps to become an executable file .（ Use in daily communication compile Collectively, these four steps ）

1. Preprocessing , In the process of pretreatment , For files in the source code file, include (include)、 Precompiled sentences ( Such as macro definition define etc. ) Expand , Generate .i file . Can be understood as the header file code 、 Macro and the like are converted to more pure C Code , However, the generated files are .i For the suffix .
2. compile , Put the pretreated .i Files are compiled into assembly language , Generate .s file , That is, code from C Language into assembly language , This is a GCC What the compiler does .
3. assembly , Assemble the assembly language files , Generate target file .o file , Each source file corresponds to a target file . That is to convert assembly language code into machine code , This is a as What the assembler does .
4. link , Finally, each source file corresponding to .o The files are linked up , Just generate an executable file , This is the linker ld Work done .
   • There are two kinds of links
     • Dynamic links
       • GCC Default options at compile time .
       • When an application is running, it loads the external code base .
     • Static links
       • Use options when linking –static
       • It will package all the libraries used in the compilation phase into its own executable program .

Common compilation options

Options	describe
-E	Preprocessing , During the development process, you want to quickly determine that a macro can be used “-E -dM”
-c	It's been preprocessed 、 compile 、 assembly , But no link
-o	Specify output file
-I	Specify the header directory ( Capitalization i)
-l	Specifies which library file to link to ( A lowercase letter L)
-L	Specify the link time library file directory

Compile multiple files

1. Compile together 、 link ：
   Such as ：

gcc -o hello main.c hello.c

2. Compile separately , Unified link ：
   Such as ：

gcc -c -o main.o main.c
gcc -c -o hello.o hello.c
gcc -o hello main.o hello.o

Make 、 Using dynamic libraries

1. Make 、 compile ：

gcc -c -o main.o main.c
gcc -c -o hello.o hello.c
gcc -shared -o libhello.so hello.o hello1.o // (* Multiple can be used .o Generate a dynamic library *)
gcc -o hello main.o -lhello -L < Directory path > // （* The path to libhello.so File directory *）

2. function
   1. hold libhello.so Put it in PC Or board /lib Under the table of contents , And then run test Program .
   2. If you don't want to put libhello.so Put it in /lib, You can also put it in a new directory , Such as /mylib, And then execute ：

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/a
./hello

( In fact, it's modification The environment variable of the library .)

Make 、 Use static libraries

gcc -c -o main.o main.c
gcc -c -o hello.o hello.c
ar crs libhello.a hello.o hello1.o // (* Multiple can be used .o Generate a static library *)
gcc -o hello main.o libhello.a // (* If .a Not in the current directory , You need to specify its absolute path or relative path *)

Be careful ： There is no need to put hello.a Pull the static library file onto the board

A very useful option

gcc -E main.c // See the preprocessing results , For example, which header file is
gcc -E -dM main.c > h.txt // Expand all the macros , Save to h.txt In the document
gcc -Wp,-MD,yl.dep -c -c -o main.o main.c // Generate dependency file yl.dep, Back Makefile use

Reference resources

• Weidong mountain
• Wildfire
• gcc-gun
• Suggest ： When you want to see the source code of the library file , Such as printf、malloc And so on. , Can enter the glibc Official website Study