混合C/ C++代码中的结构指针

Question

我在从C++调用C函数时遇到问题。特别是，我有一个类的实例，它有一个C结构成员变量。我将一个指向这个变量的指针传递给一个C函数，该函数是从一个成员函数调用的。我观察到变量指针地址在C函数内部和外部都没有改变。但是，其中一个struct字段的地址实际上发生了变化。

代码大纲：

extern "C"
{
  #define ENAGLO
  #include <rtklib.h>
}

class GNSS
{
  protected: rtcm_t rtcm;   // A C struct type in rtklib.h.
  public: ~GNSS() { free_rtcm(&rtcm); }
  public: GNSS() { init_rtcm(&rtcm); } 
  public: Load() { 

    // Add some stuff to the rtcm.obs element

    // This is the problematic line. If one were to read the address
    // of rtcm.nbyte inside and outside of the function below, its
    // address is different! Thus, if the gen_rtcm3 function sets
    // rtcm->nbyte = 3, it does not persist back into C++.
    gen_rtcm3(&rtcm, 1001, 0);
  }
}

可以从以下位置获得rtklib.h和源文件：RTKLIB

我在Ubuntu 14.04上用的是GCC 4.8.2。

更新:通过以下方式使用CMake构建RTKLIB库和C++代码。我已经检查过了，如果删除了-O3，似乎也没有什么不同。

# Find all RTKLIB source files
set(RTKLIB_SOURCES 
  thirdparty/RTKLIB/src/ephemeris.c
  thirdparty/RTKLIB/src/ppp.c
  thirdparty/RTKLIB/src/preceph.c
  thirdparty/RTKLIB/src/rcvraw.c
  thirdparty/RTKLIB/src/rinex.c
  thirdparty/RTKLIB/src/sbas.c
  thirdparty/RTKLIB/src/rtkpos.c
  thirdparty/RTKLIB/src/datum.c
  thirdparty/RTKLIB/src/rtcm3.c
  thirdparty/RTKLIB/src/options.c
  thirdparty/RTKLIB/src/rtkcmn.c
  thirdparty/RTKLIB/src/rtcm2.c
  thirdparty/RTKLIB/src/stream.c
  thirdparty/RTKLIB/src/ionex.c
  thirdparty/RTKLIB/src/rcv
  thirdparty/RTKLIB/src/rcv/binex.c
  thirdparty/RTKLIB/src/rcv/rt17.c
  thirdparty/RTKLIB/src/rcv/gw10.c
  thirdparty/RTKLIB/src/rcv/nvs.c
  thirdparty/RTKLIB/src/rcv/ublox.c
  thirdparty/RTKLIB/src/rcv/septentrio.c
  thirdparty/RTKLIB/src/rcv/ss2.c
  thirdparty/RTKLIB/src/rcv/rcvlex.c
  thirdparty/RTKLIB/src/rcv/novatel.c
  thirdparty/RTKLIB/src/rcv/javad.c
  thirdparty/RTKLIB/src/rcv/skytraq.c
  thirdparty/RTKLIB/src/rcv/crescent.c
  thirdparty/RTKLIB/src/convkml.c
  thirdparty/RTKLIB/src/pntpos.c
  thirdparty/RTKLIB/src/rtcm.c
  thirdparty/RTKLIB/src/ppp_ar.c
  thirdparty/RTKLIB/src/geoid.c
  thirdparty/RTKLIB/src/rtcm3e.c
  thirdparty/RTKLIB/src/solution.c
  thirdparty/RTKLIB/src/streamsvr.c
  thirdparty/RTKLIB/src/rtklib.h
  thirdparty/RTKLIB/src/rtksvr.c
  thirdparty/RTKLIB/src/qzslex.c
  thirdparty/RTKLIB/src/lambda.c
  thirdparty/RTKLIB/src/tle.c
)

# Create a rtklib library
add_library(crates_gnss_rtklib ${RTKLIB_SOURCES})
target_link_libraries(crates_gnss_rtklib    
  ${LAPACK_LIBRARIES}
  ${BLAS_LIBRARIES}
  ${CMAKE_THREAD_LIBS_INIT}
  ${MATH_LIBRARY}
  ${RT_LIBRARY}
)
SET_TARGET_PROPERTIES(crates_gnss_rtklib PROPERTIES COMPILE_FLAGS "-O3 -DTRACE -DENAGLO -DENAGAL -DENAQZS -DNFREQ=3")

# Crate a gazebo receiver
add_library(crates_gnss_plugin src/GNSS.cc)
target_link_libraries(crates_gnss_plugin    
  ${catkin_LIBRARIES} 
  ${GAZEBO_LIBRARIES} 
  crates_gnss_rtklib
)

# THIS IS THE LINE THAT WAS ULTIMATELY NEEDED!
#SET_TARGET_PROPERTIES(crates_gnss_plugin PROPERTIES COMPILE_FLAGS "-O3 -DTRACE -DENAGLO -DENAGAL -DENAQZS -DNFREQ=3")

Answer 1

我最终发现这是一个编译标志问题。我使用一组不同的编译标志将RTKLIB编译到一个共享库中，而不是链接到它的C++代码。这会导致不同的结构大小和运行时问题。