Date: Sat, 21 Aug 1999 19:04:25 -0600 From: Wes Peters <wes@softweyr.com> To: Don Lewis <Don.Lewis@tsc.tdk.com> Cc: hackers@FreeBSD.ORG Subject: Re: mmap mapped segment length Message-ID: <37BF4C99.67814B7@softweyr.com> References: <199908210906.CAA17706@salsa.gv.tsc.tdk.com>
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Don Lewis wrote:
>
> On Aug 21, 2:10am, Wes Peters wrote:
> } Subject: mmap mapped segment length
> } I discovered to my dismay today that the length field in the mmap call is
> } a size_t, not an off_t. I was attempting to process a large (~50 MByte) file
> } and found I was only processing the first 4 MBytes of it.
>
> 50 MB should comfortably fit in a size_t.
Oh, duh. Notice the time on my message? Yeah, that's what I get for jumping
into a mailing list when I've been up for way too long.
> } Is this intentional, or just an artifact of the implementation? Is there any
> } reason NOT to change this to an off_t?
>
> The type of size_t is supposed to be large enough to express the length of
> any object that will fit in the virtual address space of a process. Since
> a size_t is 32 bits on an i386 and pointers are also 32 bits, there wouldn't
> be any advantage to changing mmap() to use a 64 bit wide length parameter,
> since you wouldn't be able to access all of such a large object.
Obviously. I'm not sure which memory space my HEAD was in last night, the
moment I read your message I realized size_t spans 4 GB. Duh. Sorry to
embarass myself in such a public manner.
Now I've got to go figure out what *I've* screwed up. I fstat the file before
mapping it and pass S.st_size as the map length. Is there any reason why
mmap would return non-NULL but map less than the requested size?
Scratching my head,
--
"Where am I, and what am I doing in this handbasket?"
Wes Peters Softweyr LLC
http://softweyr.com/ wes@softweyr.com
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