On Apr 24 08:03, Ryan Johnson wrote: > Hi all, > > I'm trying to port a linux program that uses mmap to implement a > growable array; the ideas is to mmap(PROT_NONE, MAP_NORESERVE) a > chunk of address space (corresponding to the maximum array size) and > then call mmap(PROT_READ|PROT_WRITE, MAP_FIXED) to allocate actual > memory in the "blank" region. This works well in Linux but fails > with EINVAL in cygwin. > > My code aligns all sizes up to 2MB boundaries, so it's not a 64kB > boundary problem. My code reports the failing call as: > >22 Invalid argument addr=0xffdb0000, sz=2097152 > > A peek in /proc/self/maps confirms that the address is correct: > >FFDB0000-FFFB0000 ===p 00000000 0000:0000 0 > > Oddly, trying to map in blank pages in with mprotect succeeds on > cygwin but fails with ENOMEM on linux... > > Am I missing something here, or is this just a place where different > behavior between the two platforms is a fact of life? Which version > is the posixly "correct" way to reserve a chunk of address space and > later back it with actual memory?
There is no POSIXly correct way to do that since MAP_NORESERVE is a non-POSIX extension in Linux as well as in Cygwin. The general idea of MAP_NORESERVE is to make sure that we get as much memory as requested, but to use only as much memory as is required. On Linux MAP_NORESERVE only performs bookkeeping but doesn't change the state of the memory, so a later mmap works. On Cygwin MAP_NORESERVE uses the Windows way of handling this requirements, so in contrast to what the name of the option suggests, Cygwin actually *reserves* space but does not *commit* it. Cygwin's mmap can't handle this, but you can commit pages by using mprotect or by simply peeking or poking into the address space. This raises a SEGV, and the exception handling code will then commit the page you peeked or poked. Having said that, we *could* also change mmap to handle this scenario gracefully as well. http://cygwin.com/acronyms/#PTC. Corinna -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
