Cannot build libpqxx master in QT 5, MinGW810_64, Windows

[Alexk12]

Cannot build libpqxx master in QT 5, MinGW810_64, Windows:

[jtv]

Looks like a bug we've seen in MinGW. I haven't seen it in a long time though... Are you on a recent MinGW version? If not, this may be a simple matter of upgrading.

It may also help to search the Internet for this error.

[Alexk12]

This is standard installation of QT on Windows, QT version 5.15.2 which includes MinGW 8.1 64. I believe QT 5 is still very popular development environment (not all have migrated to QT 6).

[jtv]

FWIW I don't think QT is the problem — this looks like it's entirely a problem within the standard library headers.

That said, I do routine CI builds on Windows/MinGW with both gcc and clang. That works. But it's with the latest MinGW.

[tt4g]

The MinGW that can be installed from the Q Installer does not seem to support the entire std::thread API: QtSql Postgress MinGW8 compilation error. | Qt Forum

This is a known issue with MinGW, and since it stems from the STL's lack of capabilities, the problem cannot be fixed in the library (#543 #424).

There appears to be a library called mingw-std-threads that provides the thread classes missing from MinGW GCC.
See: https://stackoverflow.com/a/37358960

There also seems to be a MinGW implementation of the thread API: https://stackoverflow.com/a/21879380

[Alexk12]

Thank you, jtv and tt4g!

[Alexk12]

I expected that libpqxx is a C++ extension of libpq which may simplify creation and deletion of connection via constructors/destructors and may be simplify data exchange between developer class and query results.
Why libpqxx includes thread header at all?
I have found that the header is included for wait_for() function which is used in transaction class. But why?
Is it possible to rewrite this function so that it not used thread header and modern c++ possibilities for old known simple task - sleep for 1 millisecond?

[jtv]

The reason is that the standard, built-in C++ way of doing it was supposed to be the least hassle.

Before sleep_for() became the official C++ standard way doing it, we had different ways of waiting, for different systems. It was a maintenance burden, and made it easy to get it wrong for some system that I happen not to have. And the build system had to figure out which way of waiting was most appropriate for the current system. Now that sleep_for() is built into C++, the compiler makers have taken over that responsibility.

So you're asking, why does libpqxx use the standard C++ library? It's hard to get anything done without it!

We can try to bring some of that old complexity back to work around the bug in your compiler, but it does raise the question: will that actually solve your problem? To find out, please edit the code to replace the #include and the call on your system, and see whether tests pass.

[Alexk12]

The usage of std c++ library is right. But why the sleep/wait and thread header is used at all? As I know, the native libpq library provides support to transaction. I see, may be it is not correct, but wait_for() function is used for enhanced implementation of transaction. Why not to provide a basic c++ wrapper class over the libpq implementation of transaction, and for those, who want the enhanced implementation (with wait_for() function) to provide a configuration define like PQXX_USE_ENHANCED_TRANSACTION which will require modern compiler and turn off by default?

[tt4g]

Unfortunately, the sleep() API on Windows does not behave as expected for durations shorter than 16ms in most cases.
This is due to the specification of Windows' thread scheduler.
This problem can be avoided by using a high-precision timer, but using a high-precision timer affects all processes.
Either way, it creates a new problem requiring changes to the sleep method depending on the platform, making C++ sleep_for() better.

See also:

• c++ - WinAPI Sleep() function call sleeps for longer than expected - Stack Overflow
• Fun with Windows Timers aka Rounding 1ns up to 16ms : r/programming

[jtv]

The usage of std c++ library is right. But why the sleep/wait and thread header is used at all? As I know, the native libpq library provides support to transaction. I see, may be it is not correct, but wait_for() function is used for enhanced implementation of transaction. Why not to provide a basic c++ wrapper class over the libpq implementation of transaction, and for those, who want the enhanced implementation (with wait_for() function) to provide a configuration define like PQXX_USE_ENHANCED_TRANSACTION which will require modern compiler and turn off by default?

Because there are several places in the tests as well as in transaction management where we do want the process to sleep. Given that I had to implement it anyway, I also made it available to users so that they wouldn't have to worry about how to achieve a sleep on their systems.

Anyway, I don't think there's much point in debating this "why do you have features" question forever. Can you please just try the 8.0.0-rc1 version? I think it had a workaround for your compiler.

[Alexk12]

I have 15 projects that are compiled with qt5 standard toolchain (standard = downloaded from official distributor).
I am investigating, how can I develop the 16th project, a static library, which must support echanging data with PostgreSql.
There are 3 solutions:

1. Using built-in classes (QSqlDarabase, etc.)
2. Using native libpq C-library
3. Using libpqxx C++ library (with libpq C-library).

The solution 3 requres to upgrade compiler for all 15 existing projects, and to be able to reproduce the custom toolchain.
In this situation, the solutions 1 and 2 are more preferable. (Similar reason is why I do not want to upgrade all projects from qt5 to qt6. It requires to make a stategic decision, big work, big upgrade.)

[jtv]

Well like I said, have you tried the libpqxx 8 release candidate? It may solve your problem.

[jtv]

Well like I said, have you tried the libpqxx 8 release candidate? It may solve your problem.

Urgh, sorry about this one — lately I sometimes seem to get notifications about "new" messages that turn out to be old ones. Maybe I'm missing something.

[Alexk12]

libpqxx 8 release

I've just downloaded libpqxx-8.0.0-rc1.zip (selected Tag: 8.0.0-rc1, download as ZIP) and opened CMakeLists.txt in Qt Creator, QT 5.15.2 with MinGW. Build project. The build stops on libpqxx-8.0.0-rc1\src\array.cxx:16, cannot find #include "pqxx/array.hxx".
The file "include/pqxx/array.hxx" really exists, so it seems it is a configuration issue. It seems the include path to "include" directory in the project was not set up correctly when the project was opened in QT.

[jtv]

Thanks, going to look into that!