Re: [Libguestfs] [libnbd PATCH v3 07/29] lib/utils: add async-signal-safe assert()

On 2/20/23 19:21, Laszlo Ersek wrote: > On 2/15/23 21:57, Eric Blake wrote: >> On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek wrote: >>> + >>> + xwrite (STDERR_FILENO, file, strlen (file)); >>> + xwrite (STDERR_FILENO, ":", 1); >> >> Presumably, if our first best-effort xwrite() fails to produce full >> output, all later xwrite() will hopefully hit the same error condition >> so that we aren't producing even more-mangled output where later >> syscalls succeed despite missing context earlier in the overall >> output. If it were something we truly wanted to worry about, the >> solution would be pre-loading the entire message into a single buffer, >> then calling xwrite() just once - but that's far more effort for >> something we don't anticipate hitting in normal usage anyways. I'm >> happy if you ignore this whole paragraph of mine. > > Any single buffer presents the problem of sizing the buffer > appropriately, which we can't do in this context :) Actually, we *can* do better: https://pubs.opengroup.org/onlinepubs/9699919799/functions/writev.html What we're doing here is the textbook use case for scatter-gather (well, in this case, gather). It's strange that it has taken me one night to realize this (it occurred to me before falling asleep last night), given that I heavily used scatter-gather in other, not-so-old, code. (Namely the edk2 virtiofs driver.) I'll attempt to replace xwrite() with xwritev().