including external library or package to Qt

[d_stranz]

undefined reference to ''gattlib_connect'

This sounds like a compile-time error, not a link error. You have probably not added the proper #include to your source files for the headers for your library. It isn't enough just to add the header name to the .pro file.

You will probably end up with link errors as well, because your LIBS doesn't contain the name of the library to link to; you have only specified the directory it lives in.

[user03]

In the code #include statement for gattlib.h is present, I simply copied the following code: https://github.com/labapart/gattlib/...can/ble_scan.c
Initially, I was getting compile time error for hci_le_set_scan_parameter and other functions which are a part of lbluetooth library, however, after including the same, i stopped getting error for the same. But I am not sure about how should I include the library in this case.

[d_stranz]

But I am not sure about how should I include the library in this case.

The same way you include the other libraries:

LIBS += -lgattlib (or whatever it is called).