On Wed, Apr 14, 2010 at 12:14:13PM +0000, Paul Stoeber wrote:
> The command
> cat /dev/zero | dd bs=1000 count=20000
> doesn't always produce the desired number of bytes.
> Is this supposed to happen?
Yes, I think so. Reading from a pipe may turn up a block of data
smaller than requested. To avoid short reads, use a buffer equal to
PIPE_BUF (512), or read directly from /dev/zero, using if=/dev/zero.
-Otto
>
> # cat bug
> t=`mktemp`
> for i in `jot 100`; do
> cat /dev/zero | dd bs=1000 count=20000 > /dev/null 2> $t
> grep -q '^20000+0 records in' $t || cat $t
> done
> rm $t
> # sh bug
> 18840+1160 records in
> 18840+1160 records out
> 19285968 bytes transferred in 0.052 secs (364753338 bytes/sec)
> 19779+221 records in
> 19779+221 records out
> 19864200 bytes transferred in 0.059 secs (333040490 bytes/sec)
> 19763+237 records in
> 19763+237 records out
> 19854024 bytes transferred in 0.059 secs (333238624 bytes/sec)
> 19556+444 records in
> 19556+444 records out
> 19726120 bytes transferred in 0.058 secs (338558654 bytes/sec)
> 18824+1176 records in
> 18824+1176 records out
> 19275584 bytes transferred in 0.053 secs (360001942 bytes/sec)
> 19902+98 records in
> 19902+98 records out
> 19939328 bytes transferred in 0.060 secs (330077606 bytes/sec)
> 19780+220 records in
> 19780+220 records out
> 19864920 bytes transferred in 0.059 secs (332617585 bytes/sec)
> #
>
> dmesg:
>
> OpenBSD 4.7-current (GENERIC) #603: Mon Apr 12 16:28:26 MDT 2010
> dera...@i386.openbsd.org:/usr/src/sys/arch/i386/compile/GENERIC
> cpu0: Intel(R) Pentium(R) M processor 1600MHz ("GenuineIntel" 686-class) 1.61
> GHz
> cpu0:
> FPU,V86,DE,PSE,TSC,MSR,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,TM,SBF,EST,TM2
> real mem = 527265792 (502MB)
> avail mem = 501395456 (478MB)
> mainbus0 at root
> bios0 at mainbus0: AT/286+ BIOS, date 06/10/03, SMBIOS rev. 2.3 @ 0xf5620 (37
> entries)
> bios0: vendor American Megatrends Inc. version "0206" date 06/10/2003
> bios0: ASUSTeK Computer Inc. M2N
> apm0 at bios0: Power Management spec V1.2
> apm0: AC on, no battery
> acpi at bios0 function 0x0 not configured
> pcibios at bios0 function 0x1a not configured
> bios0: ROM list: 0xc0000/0xd000
> cpu0 at mainbus0: (uniprocessor)
> cpu0: Enhanced SpeedStep 1601 MHz: speeds: 1600, 1400, 1200, 1000, 800, 600
> MHz
> pci0 at mainbus0 bus 0: configuration mode 1 (bios)
> pchb0 at pci0 dev 0 function 0 "Intel 82855GM Host" rev 0x02
> "Intel 82855GM Memory" rev 0x02 at pci0 dev 0 function 1 not configured
> "Intel 82855GM Config" rev 0x02 at pci0 dev 0 function 3 not configured
> vga1 at pci0 dev 2 function 0 "Intel 82855GM Video" rev 0x02
> wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)
> wsdisplay0: screen 1-5 added (80x25, vt100 emulation)
> intagp0 at vga1
> agp0 at intagp0: aperture at 0xf0000000, size 0x8000000
> inteldrm0 at vga1: irq 5
> drm0 at inteldrm0
> "Intel 82855GM Video" rev 0x02 at pci0 dev 2 function 1 not configured
> uhci0 at pci0 dev 29 function 0 "Intel 82801DB USB" rev 0x03: irq 5
> uhci1 at pci0 dev 29 function 1 "Intel 82801DB USB" rev 0x03: irq 5
> uhci2 at pci0 dev 29 function 2 "Intel 82801DB USB" rev 0x03: irq 5
> ehci0 at pci0 dev 29 function 7 "Intel 82801DB USB" rev 0x03: irq 4
> ehci0: timed out waiting for BIOS
> usb0 at ehci0: USB revision 2.0
> uhub0 at usb0 "Intel EHCI root hub" rev 2.00/1.00 addr 1
> ppb0 at pci0 dev 30 function 0 "Intel 82801BAM Hub-to-PCI" rev 0x83
> pci1 at ppb0 bus 1
> cbb0 at pci1 dev 3 function 0 "Ricoh 5C476 CardBus" rev 0xa9: couldn't map
> interrupt
> cbb1 at pci1 dev 3 function 1 "Ricoh 5C476 CardBus" rev 0xa9: couldn't map
> interrupt
> "Ricoh 5C552 Firewire" rev 0x01 at pci1 dev 3 function 2 not configured
> rl0 at pci1 dev 4 function 0 "Realtek 8139" rev 0x10: irq 5, address
> 00:e0:18:06:ac:25
> rlphy0 at rl0 phy 0: RTL internal PHY
> ipw0 at pci1 dev 5 function 0 "Intel PRO/Wireless 2100" rev 0x04: irq 5,
> address 00:04:23:77:1d:5e
> ichpcib0 at pci0 dev 31 function 0 "Intel 82801DBM LPC" rev 0x03: 24-bit
> timer at 3579545Hz
> pciide0 at pci0 dev 31 function 1 "Intel 82801DBM IDE" rev 0x03: DMA, channel
> 0 configured to compatibility, channel 1 configured to compatibility
> wd0 at pciide0 channel 0 drive 0: <IC25N060ATMR04-0>
> wd0: 16-sector PIO, LBA48, 57231MB, 117210240 sectors
> wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 5
> atapiscsi0 at pciide0 channel 1 drive 0
> scsibus0 at atapiscsi0: 2 targets
> cd0 at scsibus0 targ 0 lun 0: <TOSHIBA, DVD-ROM SD-R2312, 1708> ATAPI 5/cdrom
> removable
> cd0(pciide0:1:0): using PIO mode 4, Ultra-DMA mode 2
> auich0 at pci0 dev 31 function 5 "Intel 82801DB AC97" rev 0x03usb1 at uhci0:
> USB revision 1.0
> uhub1 at usb1 "Intel UHCI root hub" rev 1.00/1.00 addr 1
> usb2 at uhci1: USB revision 1.0
> uhub2 at usb2 "Intel UHCI root hub" rev 1.00/1.00 addr 1
> usb3 at uhci2: USB revision 1.0
> uhub3 at usb3 "Intel UHCI root hub" rev 1.00/1.00 addr 1
> isa0 at ichpcib0
> isadma0 at isa0
> com1 at isa0 port 0x2f8/8 irq 3: ns16550a, 16 byte fifo
> pckbc0 at isa0 port 0x60/5
> pckbd0 at pckbc0 (kbd slot)
> pckbc0: using irq 1 for kbd slot
> wskbd0 at pckbd0: console keyboard, using wsdisplay0
> pms0 at pckbc0 (aux slot)
> pckbc0: using irq 12 for aux slot
> wsmouse0 at pms0 mux 0
> pcppi0 at isa0 port 0x61
> midi0 at pcppi0: <PC speaker>
> spkr0 at pcppi0
> lpt0 at isa0 port 0x378/4 irq 7
> npx0 at isa0 port 0xf0/16: reported by CPUID; using exception 16
> fdc0 at isa0 port 0x3f0/6 irq 6 drq 2
> pcic0 at isa0 port 0x3e0/2 iomem 0xd0000/65536
> pcic0 controller 0: <Intel 82365SL rev 1> has sockets A and B
> pcmcia0 at pcic0 controller 0 socket 0
> pcmcia1 at pcic0 controller 0 socket 1
> pcic0: irq 9, polling enabled
> biomask ed75 netmask ed75 ttymask ffff
> mtrr: Pentium Pro MTRR support
> umass0 at uhub0 port 2 configuration 1 interface 0 "SMI Corporation USB DISK"
> rev 2.00/11.00 addr 2
> umass0: using SCSI over Bulk-Only
> scsibus1 at umass0: 2 targets, initiator 0
> sd0 at scsibus1 targ 1 lun 0: <USB, Stick 2.0 ME, 1100> SCSI0 0/direct
> removable
> sd0: 967MB, 512 bytes/sec, 1981440 sec total
> vscsi0 at root
> scsibus2 at vscsi0: 256 targets
> softraid0 at root
> root on sd0a swap on sd0b dump on sd0b
