Below is a sample program which shows how the large object interface in LIBPQ can be used. Parts of the program are commented out but are left in the source for the readers benefit. This program can be found in ../src/test/examples Frontend applications which use the large object interface in LIBPQ should include the header file libpq/libpq-fs.h and link with the libpq library.
Read the upstream documentation. For corrections, first check the current manual.