Linking error of _ntl_gbigint_body in using NTL

I use NTL on ArchLinux, and there is a struct _ntl_gbigint_body which is actually not defined (refer this post):

/*
 * This way of defining the bigint handle type is a bit non-standard,
 * but better for debugging.
 */

struct _ntl_gbigint_body;
typedef _ntl_gbigint_body *_ntl_gbigint;  

You should pay attention to functions who depend on this struct, such as:

void _ntl_gcopy(_ntl_gbigint a, _ntl_gbigint *bb)   

Because for old NTL library, the function prototype generated by compiler is _ntl_gcopy(void*, void**):

$ readelf -sW libntl.so | c++filt | grep ntl_gcopy
2511: 000000000012b750   184 FUNC    GLOBAL DEFAULT   12 _ntl_gcopy(void*, void**)

While for new one it is _ntl_gcopy(_ntl_gbigint_body*, _ntl_gbigint_body**):

$ readelf -sW libntl.so | c++filt | grep ntl_gcopy
615: 0000000000148500   202 FUNC    GLOBAL DEFAULT   11 _ntl_gcopy(_ntl_gbigint_body*, _ntl_gbigint_body**)

So if you meet linking error as following:

undefined reference to `_ntl_gcopy(void*, void**)'

It should be NTL header files and library mismatch. The header files are old, while library is new.