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mmap.2: Clarify SIGBUS text and treatment of partial page at end of a mapping 

See https://unix.stackexchange.com/questions/616848/what-is-the-behaviour-of-a-file-backed-memory-map-when-reading-from-or-writing-t

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk 2020-11-02 10:57:14 +01:00
parent 1d5974814d
commit 9807d9afdd
1 changed files with 9 additions and 5 deletions
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@ -635,9 +635,11 @@ Use of a mapped region can result in these signals:
Attempted write into a region mapped as read-only.
.TP
.B SIGBUS
Attempted access to a portion of the buffer that does not correspond
to the file (for example, beyond the end of the file, including the
case where another process has truncated the file).
Attempted access to a page of the buffer that lies beyond the
end of the mapped file.
For an explanation of the treatment of the bytes in the page that
corresponds to the end of a mapped file that is not a multiple
of the page size, see NOTES.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
@ -678,8 +680,10 @@ with the same attributes.
.PP
A file is mapped in multiples of the page size.
For a file that is not
a multiple of the page size, the remaining memory is zeroed when mapped,
and writes to that region are not written out to the file.
a multiple of the page size,
the remaining bytes in the partial page at the end of the mapping
are zeroed when mapped,
and modifications to that region are not written out to the file.
The effect of
changing the size of the underlying file of a mapping on the pages that
correspond to added or removed regions of the file is unspecified.