undefined reference to Errors in Linking Static C Library with C Code

When attempting to link a static C library with C code, you may encounter "undefined reference to" errors, despite modifying the link order. This issue arises from the differing symbol names created by C and C compilation known as 'name mangling'.

In C , the linker displays demangled symbol names in error messages, which can be confusing. Inspecting the object file (*.o) with "nm -u" reveals that the referenced symbol names do not match those in the library.

To resolve this issue, functions linked in as externals that were compiled using the C compiler must have their function declarations enclosed in an "extern "C" {}" block. This suppresses C name mangling for everything within the block.

For example:

extern "C" {
    #include 
    #include 
}

Alternatively, you can wrap function declarations in header files as follows:

#if defined (__cplusplus)
extern "C" {
#endif

/*
 * Put plain C function declarations here ...
 */

#if defined (__cplusplus)
}
#endif