4.2BSD: <sys/mtio.h>

[Greg Woods]

  We have just converted to 4.2BSD, and I am having some trouble with
the mag tape ioctl(2) calls. In the include file <sys/mtio.h>, the old
MTIOCTOP is defined as "_IOW(m,1,struct mtop)". When I try to recompile
some libraries I had to do mag tape operations, the variable "m" is
giving a compiler error, since it was not declared in my old software,
and was not needed there. My problem is, I can't find *any* documentation
or source for _IOW, so I have no idea what "m" is or how to fix my program
to work (it used to run under 4.1aBSD). Does anyone out there know where to
find source and/or documentation for _IOW under 4.2, and/or know what the
function of the mysterious "m" is? Thanks for any help. 

		     GREG
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