Using C++ for Extensibility

35.15. Using C++ for Extensibility

It is possible to use a compiler in C++ mode to build PostgreSQL extensions by following these guidelines:

  • All functions accessed by the backend must present a C interface to the backend; these C functions can then call C++ functions. For example, extern C linkage is required for backend-accessed functions. This is also necessary for any functions that are passed as pointers between the backend and C++ code.

  • Free memory using the appropriate deallocation method. For example, most backend memory is allocated using palloc(), so use pfree() to free it, i.e. using C++ delete() in such cases will fail.

  • Prevent exceptions from propagating into the C code (use a catch-all block at the top level of all extern C functions). This is necessary even if the C++ code does not throw any exceptions because events like out-of-memory still throw exceptions. Any exceptions must be caught and appropriate errors passed back to the C interface. If possible, compile C++ with -fno-exceptions to eliminate exceptions entirely; in such cases, you must check for failures in your C++ code, e.g. check for NULL returned by new().

  • If calling backend functions from C++ code, be sure that the C++ call stack contains only plain old data structures (POD). This is necessary because backend errors generate a distant longjmp() that does not properly unroll a C++ call stack with non-POD objects.

In summary, it is best to place C++ code behind a wall of extern C functions that interface to the backend, and avoid exception, memory, and call stack leakage.