Since the zlib is a C library, to use it in a c++ program one would need to use extern "C" when including the header, as in the following test program:
#include <iostream>
extern "C" {
#include <zlib.h>
}
int main()
{
gzFile file;
file = gzopen("test.gz", "w");
gzwrite(file, "hello\0", 6);
gzclose(file);
char text[6];
file = gzopen("test.gz", "r");
gzread(file, text, 6);
std::cout << text;
return 0;
}
To my surprise, the code compiles and behaves correctly even if I remove the extern "C". Version of zlib is 1.2.13.
Why does the linkage works even without the extern "C"?
If you had looked at zlib.h, you would have found very near the top:
#ifdef __cplusplus
extern "C" {
#endif
(and a corresponding close brace near the bottom).
Related
I just begin new project in eclipse. Try to use C-library from C++ file.
In headers, where are prototypes of functions, here is:
#ifdef __cplusplus
extern "C" {
#endif
// prototypes...
void Init_Configuration(void);
#ifdef __cplusplus
}
#endif
It's initialize.h. In main.cpp i include this file and try use function:
#include "initialize.h"
int main()
{
Init_Configuration();
// Life cycle
while (1)
{
}
}
After compilation i get this error:
initialize.c:54: undefined reference to `SPI_Cmd'
In source C file "initialize.c", where i also include "initialize.h" i use function from C-library for stm32f2xx:
#include "initialize.h"
#include "stm32f2xx_rcc.h"
#include "stm32f2xx_spi.h"
#include "stm32f2xx_gpio.h"
// some code
SPI_Cmd(SPI1, DISABLE);
// some code
In this library also is this code:
#ifdef __cplusplus
extern "C" {
#endif
...
I don't know where a problem. Please, help!
I am very new to C++ and I'm trying to do my best to have a good project structure from the beginning.
I am using the C library libjpeg and was including it in my .cpp with the use of
extern "C" {
#include <jpeglib.h>
}
It worked fine until I removed it to put it in a header file which now gives me the following errors:
inc/jpeg_utils.h: 6: inc/jpeg_utils.h: extern: not found
inc/jpeg_utils.h: 8: inc/jpeg_utils.h: Syntax error: "}" unexpected
My headerjpeg_utils.h :
#ifndef JPEG_UTILS_INCLUDE
#define JPEG_UTILS_INCLUDE
#include <stdio.h>
extern "C" {
#include <jpeglib.h>
}
int read_jpeg_file(char *filename, int decompression);
void write_jpeg_file(char *filename, unsigned char *image_buffer, int image_width, int image_height, int quality);
#endif
And at the top of jpeg_utils.cpp :
#include "../inc/jpeg_utils.h"
Did I misunderstand the use of a header ?
If you include jpeg_utils.h in a C file, that extern "C" directive will not compile (obviously, C is not C++).
Add a pre-processor directive to extern "C" only when in fact you compile as C++.
#ifdef __cplusplus
extern "C" {
#endif
#include <jpeglib.h>
#ifdef __cplusplus
}
#endif
I have a C++ file and its header file. I need to include this header file in a C code and use the functions in it.
When the cpp.h file is compiled through main.c, compilation fails because of the C++ linkage.
On using the macro __cplusplus stream and string are not resolved, is there some way to compile the cpp.h file through and execute?
I have given a outline of my code only.
C++ header file cpp.h:
struct s1
{
string a;
string b;
};
typedef struct s1 s2;
class c1
{
public:
void fun1(s2 &s3);
private:
fun2(std::string &x,const char *y);
};
C++ file cpp.cpp:
c1::fun1(s2 &s3)
{
fstream file;
}
c1::fun2(std::string &x,const char *y)
{
}
C file main.c:
#include "cpp.h"
void main()
{
c1 c2;
s1 structobj;
c2.fun1(&structobj);
printf("\n value of a in struct %s",structobj.a);
}
Basically, you can't.
You need to put only C functions in your header file.
You put them in a extern "C" block this way:
#ifdef __cplusplus
extern "C"
{
#endif
extern void myCppFunction(int n);
#ifdef __cplusplus
}
#endif
The extern "C" block is not recognized by a C compiler, but the C++ compiler need it to understand he have to consider functions inside as C functions.
In your cpp file you can define myCppFunction() so that she uses any C++ code, you will get a function C code can use.
Edit: I add a full example of how to link a program with a C main() using some C++ functions in a module.
stackoverflow.c:
#include "outputFromCpp.h"
int main()
{
myCppFunction(2000);
return 0;
}
outputFromCpp.h:
#ifndef OUTPUT_FROM_CPP_H
#define OUTPUT_FROM_CPP_H
#ifdef __cplusplus
extern "C"
{
#endif
extern void myCppFunction(int n);
#ifdef __cplusplus
}
#endif
#endif
outputFromCpp.cpp:
#include "outputFromCpp.h"
#include <iostream>
using namespace std;
void myCppFunction(int n)
{
cout << n << endl;
}
Compiling and linking:
gcc -Wall -Wextra -Werror -std=gnu99 -c stackoverflow.c
g++ -Wall -Wextra -Werror -std=c++98 -c outputFromCpp.cpp
g++ -o stackoverflow.exe stackoverflow.o outputFromCpp.o -static
You cannot link such a program with gcc.
If you want to link with gcc you need to put all the C++ code in a shared library, I don't put an example as it would be a bit platform dependent.
This can be done by introducing a wrapper to c++ function. The C function calls the wrapper function which inturn calls the desired C++ function (including member functions).
More details are available here
I'm running into a mysterious situation with the GCC compiler. So I've got the following files:
//main.cpp
#include "mleak_cpp.h"
int main(int argc, char* argv[])
{
foo();
__memory_malloc(10,"hello",5);
return 0;
}
//mleak_cpp.h
......
void foo(void);
void* __memory_malloc(size_t size, const char* file, int line);
//mleak_cpp.cpp
//definitions of the functions;
void foo(void){printf("foo\n");
void* __memory_malloc(size_t size, const char* file, int line){
printf("%s,%d\n",file,line);
InitDoubleLinkList();
void* p = malloc(size);
if(p == NULL)
return NULL;
__DuLinkList* pListNode;
pListNode = (__DuLinkList*)malloc(sizeof(__DuLinkList));
pListNode->address = p;
pListNode->memorysize = size;
pListNode->file = file;
pListNode->line = line;
InsertBlockToList(pListNode);
return p;
}
For some reason, the call to void foo(void) is fine, but the call to "__memory_malloc" goes down with a linker error, "undefined reference" blah blah. What's the difference between the two functions that causes the different behaviour?
I tried adding "extern C" to the "#include" directive, so main.cpp reads:
extern "C"{
#include "mleak_cpp.h"
}
and adding the keyword "extern" before the declarations of the functions, and this time the call to "foo()" fails too with the same error.
I appreciate any help from you guys
You're placing extern "C" in the wrong place.
If main.c is truly a C file, and mleak_cpp.cpp is truly a C++ function, then you need to put an extern "C" ahead of the definition of __memory_malloc() like so:
extern "C" void* __memory_malloc(size_t size, const char* file, int line){
// ...
}
If you put extern "C" in the mleak_cpp.h file, it needs to be guarded:
#ifdef __cplusplus
extern "C" {
#endif
/* ... body of header ... */
#ifdef __cplusplus
}
#endif
Also, it's not clear why foo works in your example above, when one file calls __foo() but the other file defines foo(). I assume something more is at play, such as an editing error in your question.
extern "C" is for C++, not C, and tells it that the function's name shouldn't be mangled. In C code, you should never see this. Generally, you put it in header files, and you guard it, like this:
#ifdef __cplusplus
extern "C" {
#endif
/* C and C++ compatible header file body here */
#ifdef __cplusplus
} /* end extern "C" */
#endif
If you do it this way though, you need to include the header file in both your C and C++ files, so that the C++ compiler knows to use C linkage.
You can put the extern "C" in front of the function definition in C++ instead and leave it out of the header, but this only works if you only include the headers in C code, so it's recommended to do it the way I pointed out above.
To teach myself a little C++, I decided to write a little Program to write text to my Saitek X52 Pro joystick display.
I wanted to use Eduards C-library
http://plasma.hasenleithner.at/x52pro/
I know I have to place an "extern C" around the methods if I want to use them in my C++ program. But that means changing the Header file of the library - and then it wouldn't build anymore.
What would be the correct approach in this case?
EDIT: the suggested method worked partially.
Comm.cpp:
...
extern "C"{
#include <x52pro.h>
}
using namespace std;
int main ( int argc, char *argv[] ) {
cout<<"entered main"<<endl;
char *String;
strcpy(String,"testing");
struct x52 *hdl = x52_init();
x52_settext(hdl, 0,String , 7);
x52_close(hdl);
return EXIT_SUCCESS;
}
Error Message:
Comm.o: In function `main':
Comm.cpp|38| undefined reference to `x52_init'
Comm.cpp|39| undefined reference to `x52_settext'
Comm.cpp|40| undefined reference to `x52_close'
which are all methods defined in x52pro.h
To use extern "C" in C header files, wrap it so
#ifdef __cplusplus
extern "C" {
#endif
...
#ifdef __cplusplus
}
#endif
or you can wrap the #includes with extern "C"
extern "C" {
#include <chdr1.h>
#include <chdr2.h>
}
When linking your application, you must tell the linker what library to use and where the library is. From your link, you must add libusb as well. This looks roughly like this
g++ -o app_name Comm.o -L /path/to/library -lx52pro -lusb
When the library is installed in the system lib directory, you can omit the -L /path/... part. If you use a Makefile, you define this in some variables, usually
LDFLAGS = -L /path/to/library
LDLIBS = -lx52pro -lusb
See also Compiling and Linking and Wikipedia - Linker (computing)
In your C++ code, you can surround the included header file with extern "C" like this:
extern "C" {
#include "c_header_file.h"
}
Then, you would not need to modify the header file of the third party library.
#ifdef __cplusplus
extern C {
#endif
...
#ifdef __cplusplus
}
#endif