V4/man/man2/mount.2

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.th MOUNT II 8/5/73
.sh NAME
mount \*- mount file system
.sh SYNOPSIS
(mount = 21.)
.br
.ft B
sys  mount; special; name
.ft R
.sh DESCRIPTION
.it Mount
announces to the system that a removable file system has
been mounted on
the block-structured special file
.it special;
from now on, references to file
.it name
will refer to
the root file on the newly mounted file system.
.it Special
and
.it name
are pointers to null-terminated strings
containing the appropriate path names.
.s3
.it Name
must exist already. 
Its old contents
are inaccessible while the file system
is mounted.
.sh "SEE ALSO"
mount(I), umount(II)
.sh DIAGNOSTICS
Error bit
(c-bit) set if:
.it special
is inaccessible or not an appropriate file;
.it name
does not exist;
.it special
is already mounted;
there are already too many
file systems mounted.