From: Heesob Park <phasis@...>
Date: 2009-01-29T14:47:11+09:00
Subject: Re: Errno::ENOMEM reading a device in Ruby, not in Java though

2009/1/29 Roger Pack <rogerpack2005@gmail.com>:
> Question on how to avoid an Errno::ENOMEM
>
> Currently if I open a tape device from my machine [/dev/st2]
> in ruby reading the second file from it results in:
>
> read.rb:2:in `read': Cannot allocate memory - /dev/st2 (Errno::ENOMEM)
>        from read.rb:2
>
> Java to do the same works, however.
>
> the "cat" command [i.e. cat </dev/st2] also fails with "Not enough
> memory"
>
> Here is an strace of the two:
>
> ruby:
>
> open("/dev/st2", O_RDONLY)              = 3
> fstat(3, {st_mode=S_IFCHR|0660, st_rdev=makedev(9, 2), ...}) = 0
> fstat(3, {st_mode=S_IFCHR|0660, st_rdev=makedev(9, 2), ...}) = 0
> ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff5721d900) = -1 EINVAL
> (Invalid argument)
> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
> = 0x2b5b53902000
> read(3, 0x2b5b53902000, 4096)           = -1 ENOMEM (Cannot allocate
> memory)
>
>
> java:
>
> [pid 25476] open("/dev/nst2", O_RDONLY) = 4
> [pid 25476] fstat(4, {st_mode=S_IFCHR|0660, st_rdev=makedev(9, 130),
> ...}) = 0
> [pid 25476] mprotect(0x2aab3409b000, 65536, PROT_READ|PROT_WRITE) = 0
> [pid 25476] read(4, "<familysearch-format>\n<type name"..., 65536) = 116
>
> Anybody know if the difference between mmap and mprotect might be
> causing this?
The another difference between ruby and java is read buffer size.
Ruby's buffer size is 4096 and java's buffer size is 65536.

The man page of st says:

RETURN VALUE

       ENOMEM        The byte count in read() is smaller than the next  physi-
                     cal block on the tape. (Before 2.2.18 and 2.4.0-test6 the
                     extra bytes have been silently ignored.)

Refer to http://fts.ifac.cnr.it/cgi-bin/dwww?type=runman&location=st/4

Regards,

Park Heesob