Import pragma question

["Alex Mentis" <foo@invalid.invalid>]

I've never interfaced Ada with other languages, so I have some
questions about pragma Import. How does an import pragma know where to
find the non-Ada code to be used for the import?

As an illustration of my question, I'll use the GNAT.IO package
(g-io.adb) from GNAT 4.3.6.

In g-io.adb, pragma Import is used to import a C function "get_char"
and associate it with the Ada function Get_Char (lines 53-54).

The C function get_char code is in the file cio.c, located in the same
directory as g-io.adb, but how does the pragma know this? Does it use
the convention parameter to check all .c files in the current directory
for a function of the specified external name? What if multiple .c
files in the current directory have a get_char function? Could the .c
file containing the code to be imported be located in a different
directory? If so, how does the import find it?

Thank you for helping me advance my understanding.

Alex