Questions about mmap()

[Unknown Soldier]

Hello,

I have a couple queries about mmap() that ppl here might be able to help
with.

1. What's the best way to resize an mmap()d area when you've enlarged
the file? Can you call mmap() again with the existing mapped location,
or do you need to remap it from scratch?

2. If I have a file descriptor returned by a call to socket(), can I
used mmap() as an alterantive to read()? Will there be problems
depending on whether the socket is blocking?

Thanks.

 

[Flash Gordon]

Unknown Soldier wrote, On 08/04/08 20:50:

Hello,

I have a couple queries about mmap() that ppl here might be able to help
with.

<snip>

The people over on comp.unix.programmer are more likely to be able to
provide quality help on this since I am guessing you are using the Unix
mmap function. mmap is a system extension rather than being part of the
C language as defined by the C standard.

 

[Antoninus Twink]

Hello,

I have a couple queries about mmap() that ppl here might be able to help
with.

1. What's the best way to resize an mmap()d area when you've enlarged
the file? Can you call mmap() again with the existing mapped location,
or do you need to remap it from scratch?

There's no guarantee that the original mapping is contained in a large
enough space in the process's virtual address space for the new mapping
to fit, so you really need to unmap and the remap the whole thing.

On Linux, there's an mremap function that takes care of this for you
(it's analogous to realloc), but you should be aware that this function
isn't in POSIX, so relying on it will make your program non-portable.

2. If I have a file descriptor returned by a call to socket(), can I
used mmap() as an alterantive to read()? Will there be problems
depending on whether the socket is blocking?

I don't think so - what would that even mean? It doesn't make sense.

 

[Keith Thompson]

I have a couple queries about mmap() that ppl here might be able to
help with.

[snip]

Some people here might try, but you'll get better answers in
comp.unix.programmer. The mmap() function is defined by POSIX, not by
the C language standard.

 

[Antoninus Twink]

I guess that there is a benefit:
Multiple outstanding I/O operations on a single file descriptor using
threads:
http://www.squid-cache.org/mail-archive/squid-dev/199805/0139.html

That's an interesting (albeit bizarre) idea... as far as I can see,
though, all the sample code on that page first mmaps a file, then
read()s from a socket into the mmap'd area, rather than actually
attempting to mmap the socket itself.

 

[ppl]

ppl doesn't participate in this group any more. We killed him and ate
his liver.

But... Why?