V3/usr/man/man2/read.2

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NAME		read  --  read from file
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SYNOPSIS	(file descriptor in r0)
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sys read; buffer; nbytes  / read = 3.
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(nread in r0)
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DESCRIPTION	A file descriptor is a word
returned from a successful open____ or creat_____ call.
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Buffer______ is the location of nbytes______ contiguous
bytes into which the input will be placed.  It is not guaranteed
that all nbytes______ bytes will be read; for example
if the file refers to a typewriter at most one line
will be returned.
In any event the number of characters read is returned
in r0.
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If r0 returns with value 0, then
end-of-file has been reached.
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SEE ALSO	open(II), creat(II)
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DIAGNOSTICS	As mentioned,
r0 is 0 on return when the end of the file has been reached.
If the read was otherwise unsuccessful
the error bit (c-bit) is set.  Many conditions,
can generate an error:
physical I/O errors, bad buffer address,
preposterous nbytes______, file descriptor not that of
an input file.
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BUGS		--