Current `mmap` implementation raises a ValueError if a sum of offset and length exceeds file size, as reported by `fstat`. While perfectly valid for most use-cases, it doesn't work for "special" files, for example:

>>> with open("/proc/sys/debug/exception-trace", "r+b") as f:
...     mmap.mmap(f.fileno(), 0)
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError: mmap offset is greater than file size

Same goes for almost any other /proc file, because most of them have S_ISREG() == True and st_size = 0.

How about adding a keyword argument to `mmap.mmap()`, which disables fstat-based size checks?

A couple remarks:
- if st_size == 0, then you have to specify an explicit length (your example passes 0, which would fail with EINVAL without the check)
- most enties in /proc don't support mmap file operation, so it's likely to fail anyway (with ENODEV, EACCESS...). Do you have an example of a /proc entry with st_size == 0 that can be mmapped (mapping /proc/sys/debug/exception-trace fails with EACCESS)?

> How about adding a keyword argument to `mmap.mmap()`, which disables
> fstat-based size checks?

I don't like the idea of adding an argument which doesn't have a counterpart in the POSIX version (especially to address such corner cases).

> I don't like the idea of adding an argument which doesn't have a
> counterpart in the POSIX version (especially to address such corner
> cases).

Indeed, it seems rather messy for a corner case that may well not exist.

If there are definite cases where this usage is needed, a solution may then have to be considered.

> Do you have an example of a /proc entry with st_size == 0 that can be mmapped 
> (mapping /proc/sys/debug/exception-trace fails with EACCESS)?
Yes, I've ran into the issue, while trying to mmap /proc/xen/xsd_kva, which is an 
interface to XenBus [1]. Unfortunately, I'm not aware of any other cases. 

By the way, I've checked mmap(2) manpage -- it looks like the C-version has nothing 
against mmaping 0-sized files, Why does Python's `mmap` still checks file size?

[1] http://wiki.xensource.com/xenwiki/XenBus

> By the way, I've checked mmap(2) manpage -- it looks like the C-version has
> nothing
> against mmaping 0-sized files, Why does Python's `mmap` still checks file
> size?

It doesn't check explicitely that the size is non-0, but rather that
the offset is is less than the file size.
Passing mmap(2) a 0 length doesn't make much sense anyway - for
example, Linux returns EINVAL:
http://lxr.free-electrons.com/source/mm/mmap.c?a=avr32#L979
"""
unsigned long do_mmap_pgoff(...)
[...]
        if (!len)
                 return -EINVAL;
"""

>> Do you have an example of a /proc entry with st_size == 0 that can be
>> mmapped
>> (mapping /proc/sys/debug/exception-trace fails with EACCESS)?
> Yes, I've ran into the issue, while trying to mmap /proc/xen/xsd_kva, which
> is an
> interface to XenBus [1]. Unfortunately, I'm not aware of any other cases.

That's what I thought, it's really uncommon: in that case, I'm
reluctant to making such a change, for the reason explained above.
Ross, Victor?

> That's what I thought, it's really uncommon: in that case, I'm
> reluctant to making such a change, for the reason explained above.
> Ross, Victor?

Why do you want a mmap? Why not using a file?

> That's what I thought, it's really uncommon: in that case, I'm
> reluctant to making such a change, for the reason explained above.
> Ross, Victor?

Even if Linux did accept a a length of 0 (which it doesn't since 2.6.12), what would be the point? Wouldn't you be mapping *nothing*?

Yeah, I'm against this change.

> Passing mmap(2) a 0 length doesn't make much sense anyway - for
example, Linux returns EINVAL.
Yup, but passing mmap(2) a zero-sized file and a non-zero length works just fine.

> Why do you want a mmap? Why not using a file?
Because Xen requires me to mmap it. Anyway, you're right, the use-case is too uncommon.