I wrote: > Consider the following approach: > 1. Teach src/port/snprintf.c about %m. While I've not written a patch > for this, it looks pretty trivial. > 2. Teach configure to test for %m and if it's not there, use the > replacement snprintf. (Note: we're already forcing snprintf replacement > in cross-compiles, so the added run-time test isn't losing anything.) > 3. Get rid of elog.c's hand-made substitution of %m strings, and instead > just let it pass the correct errno value down. (We'd likely need to do > some fooling in appendStringInfoVA and related functions to preserve > call-time errno, but that's not complicated, nor expensive.) > 4. (Optional) Get rid of strerror(errno) calls in favor of %m, even in > frontend code.
So I started to hack on this, and soon noticed that actually, what elog.c replaces %m with is *not* the result of strerror(), it's the result of useful_strerror(). Which, primarily, does this: /* * Some strerror()s return an empty string for out-of-range errno. This * is ANSI C spec compliant, but not exactly useful. Also, we may get * back strings of question marks if libc cannot transcode the message to * the codeset specified by LC_CTYPE. If we get nothing useful, first try * get_errno_symbol(), and if that fails, print the numeric errno. */ I don't know offhand whether glibc's implementation delivers anything useful for out-of-range errno, but I do know that we've seen the transcoding problem with it, cf commit 8e68816cc which arose from this discussion: https://www.postgresql.org/message-id/flat/2782A2665E8342DF8695F396DBA80C88%40maumau We could easily move useful_strerror() into snprintf.c, I think (might need to move pgwin32_socket_strerror there too). But then we'd lose its functionality when using glibc. So now I'm about ready to propose that we just *always* use snprintf.c, and forget all of the related configure probing. This'd have some advantages, notably that we'd get the useful_strerror() behavior in frontend as well as backend, assuming we converted all our frontend code to use %m. And we'd not exactly be the first project to decide that. But it's kind of a big move from where we are today. Thoughts? regards, tom lane