CODEWITHSUNDEEP
c++linuxmmap
zer0stimulus
zer0stimulus

Reputation: 23606

mmap(): what happens if underlying file changes (shrinks)?

If you memory map a file using mmap(), but then the underlying file changes to a much smaller size. What happens if you access a memory offset that was shaved off from the file?

Upvotes: 6

Views: 6245

Answers (1)

osgx
osgx

Reputation: 94235

IBM says it is undefined http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=%2Fapis%2Fmmap.htm

If the size of the mapped file is decreased after mmap(), attempts to reference beyond the end of the file are undefined and may result in an MCH0601 exception.

If the size of the file increases after the mmap() function completes, then the whole pages beyond the original end of file will not be accessible via the mapping.

The same is said in SingleUnixSpecification: http://pubs.opengroup.org/onlinepubs/7908799/xsh/mmap.html

If the size of the mapped file changes after the call to mmap() as a result of some other operation on the mapped file, the effect of references to portions of the mapped region that correspond to added or removed portions of the file is unspecified.

'undefined' or 'unspecified' means - the OS is allowed to start formatting of disk or anything. Most probable is SIGSEGV-killing your application.

Upvotes: 4

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