Compiling header using C, instead of C++ - c++

Question

I have a C++ project that I have configured using Cmake to use Eclipse. My problem is that I have added a static C library (namely, svm-struct/svm-light) which doesn't seem to compile- and my guess is that it's compiling as C++ instead of C.
I added the library to my project as follows:
SET(SVM_LIGHT_SRC_DIR "../../Libraries/svm_rank")
INCLUDE_DIRECTORIES(${SVM_LIGHT_SRC_DIR})
ADD_LIBRARY(
svm_rank_lib STATIC
${SVM_LIGHT_SRC_DIR}/svm_light/svm_learn.c
${SVM_LIGHT_SRC_DIR}/svm_light/svm_common.c
${SVM_LIGHT_SRC_DIR}/svm_light/svm_hideo.c
${SVM_LIGHT_SRC_DIR}/svm_struct/svm_struct_learn.c
${SVM_LIGHT_SRC_DIR}/svm_struct/svm_struct_common.c
${SVM_LIGHT_SRC_DIR}/svm_struct/svm_struct_classify.c
${SVM_LIGHT_SRC_DIR}/svm_struct_api.c
${SVM_LIGHT_SRC_DIR}/svm_struct_learn_custom.c
)
add_executable(${PROJECT_NAME} ${SOURCES})
target_link_libraries(${PROJECT_NAME} svm_rank_lib)
Cmake configures fine it seems. Within the output of the configuration it specifies that it finds my C and C++ compilers and that they "work". I add the header to one of my project files using extern as follows:
#ifdef __cplusplus
extern "C" {
# include "svm_struct/svm_struct_common.h"
}
#endif
When I go to build my project the error is here:
../../Libraries/svm_rank/svm_struct/../svm_struct_api_types.h:75:11: error: expected member name or ';' after declaration specifiers
double *class; /* vector of scores that imply ranking */
~~~~~~ ^
1 error generated.
There is a variable within the library header called "class" where the error occurs, and my guess is that it's trying to compile this library header using C++ instead of C. First of all is this the reason for the error? If so, how should I go about fixing this?

Answer 1

As has already been pointed out, the source of the problem is that the C library header declares a variable named class, which is a keyword in C++.
This problem will hit you as soon as that header is pulled in by a C++ source file. Remember that headers are not compiled by themselves, but are merely copy-pasted by the preprocessor into the source file that #includes them. It is the type of the source file that determines whether the code in the header is interpreted as C or C++.
The fact that you wrapped the include in extern "C" does not change this. It simply switches off C++-style name mangling for the declarations in the header, but the code still has to compile as valid C++.
The cleanest solution to this problem is a technique known as insulation or compiler firewall.
You have to make sure that all parts that come into contact with the problematic library are C-source files themselves. The C++ part of your code only interacts with the library through the interface of that C part, but never with the library directly. In particular, you must never #include library headers from any of your header files.
For instance:
my_interface.c
#include "svm_struct/svm_struct_common.h" /* safe to include from a .c file */
struct opaque_ {
/* you can use types from svm_struct_common in here */
};
opaque* initialize()
{
/* you can create an opaque_ on the heap and
manipulate it here, as well as give a
pointer back to the C++ part */
}
void do_stuff(opaque*)
{
/* do whatever you like with the stuff in opaque */
}
my_interface.h
/* no #includes in the header! */
/* the opaque type is only forward declared!
C++ code can obtain a pointer to it,
but cannot look inside */
struct opaque_;
typedef struct opaque_ opaque;
opaque* initialize();
void do_stuff(opaque*);
my_application.cpp
// we only include our own header, which we made sure is valid C++
extern "C" {
#include <my_interface.h>
}
void do_stuff()
{
opaque* context = initialize();
do_stuff(context);
}

Answer 2

double *class; /* vector of scores that imply ranking */
class as highlighted in blue if it helps. Is a reserved word meaning you can't use it as a variable or macro name. Try changing it and it should remove the error.
Edit
I misunderstood you are compiling in C but it seems to compile in C++. But I still stand by my answer its best to change the variable class to keep the code compatible with C++ as class is a reserved word in C++.