3d94d77159
returns EINVAL for an existing shared memory segment. Although it's not terribly sensible, that behavior does meet the POSIX spec because EINVAL is the appropriate error code when the existing segment is smaller than the requested size, and the spec explicitly disclaims any particular ordering of error checks. Moreover, it does in fact happen on OS X and probably other BSD-derived kernels. (We were able to talk NetBSD into changing their code, but purging that behavior from the wild completely seems unlikely to happen.) We need to distinguish collision with a pre-existing segment from invalid size request in order to behave sensibly, so it's worth some extra code here to get it right. Per report from Gavin Kistner and subsequent investigation. Back-patch to all supported versions, since any of them could get used with a kernel having the debatable behavior.
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