How to output 1s endlessly like /dev/zero?

UNIX for Advanced & Expert Users
1

/dev/zero can output 0's (null characters) endlessly. I am looking for a technique to output 1's (0xFF or 0b11111111) endlessly in a similar manner as /dev/zero.

The following dd statement writes 4 terabytes of 0's to the drive /dev/sdb. This dd statement does not cause any memory shortage.

dd bs=4K count=1G if=/dev/zero of=/dev/sdb

On the other hand, I would like to write 1's almost endlessly. The following perl statement attempts to write 4 terabytes of 1's. However, its huge buffer at the redirection causes the "Out of memory" error, and hence the perl statement fails.

perl -e 'print "\xFF\xFF\xFF\xFF"x1024x1024x1024x1024' > /dev/sdb

Is there any technique to output 1's (0xFF or 0b11111111) endlessly in a similar manner as /dev/zero? I wonder if this goal can be achieved by using /dev/loop or something.

Thanks a lot in advance.

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Mac OS X note: The dd command on Mac OS X has slightly different syntax from that on Linux. The dd command on Mac OS X may expect lowercase letters for k, m and g. If the above dd statement does not work on Mac OS X, try the following.

dd bs=4k count=1g if=/dev/zero of=/dev/disk1

2

The first question I have to ask is "WHY?"

And the resolution is simple. dd reads from stdin by default so just let it:

perl -e 'print chr(0xFF) while(1);' | dd of=/dev/sdb
3

Thanks, pludi. However, is there any faster technique?

The technique "perl -e 'print chr(0xFF) while(1);'" is too slow compared to /dev/zero.

The "perl ... while" method is 400 times slower than /dev/zero, if there is almost no bottleneck by the output destination.

Even in a real world environment where the flow is bottlenecked by the slow hard drive, the "perl ... while" method is 2.5 times slower than /dev/zero.

Can anyone think of a technique to output 1's (0xFF or 0b11111111) endlessly nearly as fast as /dev/zero? Is it possible to custom-make a pseudo-device similar to /dev/zero?

Thanks a lot in advance.

4

This is one of those questions which really begs for:
What do you need to do - exactly? Not how you decided it should be done.

/dev/zero and /dev/null are kernel device drivers implemented by kernel code
Have fun:
linux-v2.6.33-rc6/drivers/char/mem.c | ERTOS Source Browser

One of the reasons perl is "slow" is because it is an interpreted language, not a compiled language. There are lots of disk scrubber apps out there....