How to use custom "C" library with C++ codes

[Igor Vartanov]

In my custom library header, I just have this line of code: "double BankAccountBal ( ) ;".

This is where you're wrong. C and C++ are different languages. So, the same line is interpreted different been compiled in C unit compared to C++ one. Compiled in C the line declares _BankAccountBal symbol while in C++ it is ?BankAccountBal@@YANXZ. So you need a special trick to make this be compiled identically:

#ifdef __cplusplus 
extern "C" {
#endif

/* here C stuff goes */

#ifdef __cplusplus 
}
#endif