Arduino-在草图中使用的C库中包含SD.h

Question

I am working on compiling a certain C library for the Arduino Mega2560. However, I need access to the SD.h library from within my C library. Is this possible if I write a C++ to C wrapper?

Other relevant material:

calling c++ code from c http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html

Answer 1

You should just be able to write a wrapper library with a C interface, that calls the C++ library.

The key here is using extern "C" when building as C++ to ensure the C++ parts call the C functions as C and not C++.

The SD header is more complicated to wrap than, for example, Serial.h as you need to return and wrap the File objects as well. This can be done using a struct declaration in the header, with the real type defined in the .cpp . By working only with pointers to this type in the header, you don't need to define the struct at this point. You could alternatively use void* pointers, but I would avoid this as you loose type safety.

SD_c_iface.h

#ifndef SD_C_IFACE_H
#define SD_C_IFACE_H

#include <Arduino.h>

#ifdef __cplusplus
extern "C" {
#endif

/* Wrapper around File type */

typedef struct _SD_File SD_File;

size_t SD_File_write(SD_File* file, const uint8_t *buf, size_t size);

/* TODO Wrap all required File functions */

/* Wrapper around SD type */

boolean SD_begin(uint8_t csPin);

void SD_open(const char *filename, uint8_t mode, SD_File** file);

/* TODO Wrap all required SD functions */

#ifdef __cplusplus
}
#endif
#endif

SD_c_iface.cpp

#include "SD_c_iface.h"

#include "SD.h"

struct _SD_File
{
  File f;
};

size_t SD_File_write(SD_File* file, const uint8_t *buf, size_t size)
{
  return (file) ? file->f.write(buf, size) : 0;
}

boolean SD_begin(uint8_t csPin)
{
  SD.begin(csPin);
}

void SD_open(const char *filename, uint8_t mode, SD_File** file)
{
  if (!file)
    return;
  *file = new _SD_File();
  (*file)->f = SD.open(filename, mode);
}

// TODO Add more function wrappers

The method above is the generic way to wrap things, in your case you could, for example simplify things and only allow a single file to be opened at once, as the SD.h library only allows one open at once.