196 lines
3.6 KiB
Markdown
196 lines
3.6 KiB
Markdown
# extern "C"
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## About Author：
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## 1. Compiler difference between C and C ++
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In C + +, we often see extern "C" modifier function in the header file. What's the effect. Is a function defined in C language module for C + + link.
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Although C + + is compatible with C, the symbols generated by function compilation in C + + files are different from those generated by C language.Because C + + supports function overloading, the symbols generated by C + + function compilation have the information of function parameter type, while C does not.
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Take `int add(int a, int b)` for example. The C + + compiler generates the. O file, `add` becomes `add_int_int` and so on, while C would be like this `_add`, that is：For the same function, in C and C + +, the symbols generated after compilation are different.
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This leads to a problem: if the function implemented in C + + is implemented in C language, an error will occur when compiling the link, indicating that the corresponding symbol cannot be found. At this time`extern "C"` works：Tell the linker to find its `_ C `language symbols such as `add` are not modified by C ++.
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## 2.C ++ calls C functions
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When referring to the header file of C, you need to add `extern "C"`
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```c++
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//add.h
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#ifndef ADD_H
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#define ADD_H
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int add(int x,int y);
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#endif
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//add.c
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#include "add.h"
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int add(int x,int y) {
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return x+y;
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}
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//add.cpp
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#include <iostream>
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#include "add.h"
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using namespace std;
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int main() {
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add(2,3);
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return 0;
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}
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```
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Compile：
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```
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//Generate add.o file
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gcc -c add.c
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```
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Link：
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```
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g++ add.cpp add.o -o main
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```
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Without extern "C"：
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```c++
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> g++ add.cpp add.o -o main
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add.o：在函数‘main’中：
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add.cpp:(.text+0x0): `main'被多次定义
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/tmp/ccH65yQF.o:add.cpp:(.text+0x0)：第一次在此定义
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/tmp/ccH65yQF.o：在函数‘main’中：
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add.cpp:(.text+0xf)：对‘add(int, int)’未定义的引用
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add.o：在函数‘main’中：
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add.cpp:(.text+0xf)：对‘add(int, int)’未定义的引用
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collect2: error: ld returned 1 exit status
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```
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With extern "C"：
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`add.cpp`
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```c++
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#include <iostream>
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using namespace std;
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extern "C" {
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#include "add.h"
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}
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int main() {
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add(2,3);
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return 0;
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}
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```
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When compiling, you must pay attention to generating intermediate files add.o through GCC
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```
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gcc -c add.c
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```
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Compile：
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```
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g++ add.cpp add.o -o main
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```
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Code：
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- [add.h](extern_c++/add.h)
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- [add.c](extern_c++/add.c)
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- [add.cpp](extern_c++/add.cpp)
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## 2.Calling C++ function in C
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`extern "C"` It is a syntax error in C, which needs to be put in the C + + header file.
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```c
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// add.h
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#ifndef ADD_H
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#define ADD_H
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extern "C" {
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int add(int x,int y);
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}
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#endif
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// add.cpp
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#include "add.h"
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int add(int x,int y) {
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return x+y;
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}
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// add.c
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extern int add(int x,int y);
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int main() {
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add(2,3);
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return 0;
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}
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```
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Compile：
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```c
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g++ -c add.cpp
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```
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Link：
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```
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gcc add.c add.o -o main
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```
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Code：
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- [add.h](extern_c/add.h)
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- [add.c](extern_c/add.c)
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- [add.cpp](extern_c/add.cpp)
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In the header file of C language, the external function can only be specified as extern type. The declaration of extern "C" is not supported in C language. There will be compiler syntax error when the. C file contains extern "C". Therefore, the use of external "C" is all placed in CPP program related files or its header files.
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The following forms are summarized:
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（1）C + + calls C functions:
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```c++
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//xx.h
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extern int add(...)
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//xx.c
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int add(){
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}
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//xx.cpp
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extern "C" {
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#include "xx.h"
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}
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```
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（2）C calls C + + functions
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```c
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//xx.h
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extern "C"{
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int add();
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}
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//xx.cpp
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int add(){
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}
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//xx.c
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extern int add();
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```
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