Article Directory
In the project development process, our underlying code is often
Cused for implementation, and most of the upper-level applications areC++implemented, so we are involved in the situation of callingCandC++calling each other. So,C/C++how to achieve mutual calls?
1. Why is there a difference?
- Compilation methods are different :
Cfiles are oftengcccompiledCppusingg++ C++Support function overloading : Due to this feature, the same function inC++andCin will generate different function names after compilation.
This has led to the Cinability C++to directly call between and, to solve this problem, you have to rely on extern "C"help.
2、extern “C”
- extern
externWe are not unfamiliar with keywords. It is an attribute in programming languages, which is used to indicate the scope of variables, functions and other types.
We often
.cdefine variables or implement functions in source files,.hand useexternkeywords to declare them in header files, which is convenient for other files to call.
- “C”
There are many kinds of programming languages, and different languages have different compilation rules. If you want to call each other, you must tell the compiler what rules to compile the file, so that it can be called normally.
Its main function is : use it “C”as a flag to tell the compiler that the following code Cis compiled in the way!
After understanding the principle, let's practice it!
3. C++ calls C
We create 3 files, respectively main.cpp, cal.c, cal.h.
We use gccand g++compile files separately, and compile cal.otwo main.ointermediate files. It is very simple and defines a embedded_artfunction.
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [15:57:32]
$ ls
cal.c cal.h main.cpp
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [15:57:43]
$ gcc -c cal.c
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [15:57:49]
$ g++ -c main.cpp
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [15:57:55]
$ ls
cal.c cal.h cal.o main.cpp main.o
Let's take a look at the compiled intermediate file cal.oand the symbol table to see the difference between different compilation methods for the main.osame function .embedded_art
It can be seen that g++after compilation, the function name is processed, and a new function name is finally generated according to its own compilation rules, so if we directly call cal.cit, it will embedded_artdefinitely not work.
correct way
Use extern "C"to make the g++compiler compile Cin the same way.
In the main.cppfile, where we import cal.h, addextern "C"
extern "C" {
#include "cal.h"
}
Compile again and you're done!
You can see that in the symbol table, the function name is normal, and then we link the intermediate file, execute it, and output the correct result!
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [16:18:36]
$ g++ main.o cal.o
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [16:19:54]
$ ls
a.out cal.c cal.h cal.o main.cpp main.o
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test on git:main x [16:19:57]
$ ./a.out
main entry
嵌入式艺术
4. C calls C++
We create 3 files, respectively main.c, cal.cpp, cal.h.
We use gccand g++compile files separately, and compile cal.otwo main.ointermediate files, which is very simple and also defines a embedded_artfunction.
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:24:45]
$ g++ -c cal.cpp
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:24:52]
$ gcc -c main.c
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:24:56]
$ ls
cal.cpp cal.h cal.o main.c main.o
Let's take a look at the compiled intermediate file cal.oand the symbol table to see the difference between different compilation methods for the main.osame function .embedded_art
Also, different compilers handle it differently, and the function names are still different! Also, need to be added extern "C"to tell the compiler to Ccompile by .
We cal.hadd in the declaration section, and then recompile!
extern "C" {
extern void embedded_art(void);
}
You can see that in the symbol table, the function name is normal, and then we link the intermediate files.
At this time, an error will be reported
extern "C". What is the situation?
In the main.cfile, c++the header file is introduced cal.h, because it can only be recognized "C"at compile time , and there is no such keyword in the language.C++C
Therefore, we need g++to add extern "C"it when compiling, and gccskip it during compiling. At this time, we need to mention c++the specific macros __cplusplusduring compiling, which is equivalent to a valve.
We modify the cal.hfile :
#ifdef __cplusplus
extern "C" {
#endif
extern void embedded_art(void);
#ifdef __cplusplus
}
#endif
In this way, it is ensured that when the function is compiled, the c++syntax is used to compile, and when it is compiled, it is not processed.embedded_artCgcc
Link again, execute!
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:45:06] C:1
$ gcc -no-pie cal.o main.o -o main
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:46:46]
$ ls
cal.cpp cal.h cal.o main main.c main.o
# dong @ ubuntu in ~/WorkSpace/Donge_Programs/Unix_Programming_Learning/c_c++_call_test/c_call_c++ on git:main x [16:49:01]
$ ./main
main entry
嵌入式艺术
5. Summary
C/C++In the final analysis, the mutual calls between them are: different languages have different compilation rules. If you want to achieve universal use, you must tell the compiler to compile according to the rules of the target language!
Like + follow, never get lost
