Using C++ code in a PostgreSQL function or procedure

Question

I have a compiled library on the same PC with PostgreSQL that I use with #include <seal/seal.h> when writing some C++ code. I must use this library to work on some table values in my PostgresSQL database. Running a separate Client on the same PC with Postgres for the communication with the database is unfortunately not an option.

Is it possible to include and use the functions provided with #include <seal/seal.h> in a PostgreSQL function or a procedure? I read something about using:

external "C" {

#include <seal/seal.h>
// do something with the seal functions here

}

But there are not so many examples out there and I didn't understand it. The section about C++ in the PostgreSQL documentation didn't make it clearer either.

To realize such you'll need to have a compiled module added to your PostgreSQL distribution IIRC. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jul 30, 2018 at 18:37

Laurenz Albe · Accepted Answer · 2018-07-30 19:33:17Z

As the documentation recommends, the functions that are called from PostgreSQL must be declared as extern C so that C code can link with them.

In the places where you have to call he functions from your C++ library, you use an exception handler that uses catch (...) to catch all possible exceptions and converts them into a PostgreSQL error message by calling ereport() (outside the catch clause, so that there is nothing C++ on your call stack).

If you have to call a PostgreSQL C function, make sure that the call stack does not contain any values that are not plain old data structures:

A PDS type in C++, or Plain Old C++ Object, is defined as either a scalar type or a PDS class. A PDS class has no user-defined copy assignment operator, no user-defined destructor, and no non-static data members that are not themselves PDS. Moreover, a PDS class must be an aggregate, meaning it has no user-declared constructors, no private nor protected non-static data, no virtual base classes and no virtual functions.

In short, write your code as C as possible, except where you call the library functions.

So lets see if I got you right. I start declaring a function or a procedure for my table in PostgerSQL and I write my C/C++ in extern "C" { here }. If so far was correct, then can the SQL variables outside extern "C" { } be used by variables inside extern "C" { }?
Poor choice of words. I meant any declared variable outside the extern "C" { } section but inside the PosgresSQL procedure or function.
Yes, certainly. As far as I know, extern C only influences how the linker names the symbol for the decorated function.
Thanks for the help. I'm going to be working on it this Weekend, hopefully it works as planned :).

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