"Michael Kerrisk (man-pages)" <mtk.manpa...@gmail.com> writes:
> I think the only reasonable solution is to better document existing > behavior and what the programmer should do. With that in mind, I've > drafted the following text for the msync(2) man page: > > NOTES > According to POSIX, exactly one of MS_SYNC and MS_ASYNC must be > specified in flags. However, Linux permits a call to msync() > that specifies neither of these flags, with semantics that are > (currently) equivalent to specifying MS_ASYNC. (Since Linux > 2.6.19, MS_ASYNC is in fact a no-op, since the kernel properly > tracks dirty pages and flushes them to storage as necessary.) > Notwithstanding the Linux behavior, portable, future-proof appli‐ > cations should ensure that they specify exactly one of MS_SYNC > and MS_ASYNC in flags. > > Comments on this draft welcome. I think it's a step backwards to document unspecified behavior. If anything, the man page should make it clear that providing neither flag results in undefined behavior and will lead to failure on systems on than Linux. While I can see the point of not changing the previous behavior to protect buggy code, there's no need to document it in the man page and further enshrine it. There's a larger point, which is that people write code for Linux when they should be writing code for POSIX. Therefore, Linux has an obligation to the larger free software community to avoid encouraging non-portable code. This is somewhat similar (except for the key point that it's unintentional) to bash's allowing "==" in test, which is a gratuitous extension to the standard that has led to large amounts of nonportable code. To mitigate this, it would be reasonable to syslog a warning the first time a process makes a call with flags that POSIX says leads to undefined behavior. That would meet the portability-citizenzhip goals and not break existing systems.
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