23 Mar
2016
23 Mar
'16
6:54 a.m.
On Tue, Mar 22, 2016 at 1:21 AM, shawn wilson <ag4ve.us(a)gmail.com> wrote:
Is that the last Unix that had a major number for file
system objects? I
hadn't run into this but, thinking about it, it's kinda smart (probably
slower than specific execve kernel calls but still cool).
I'm trying to parse your question a little and I think you may not quite
understand how “pre-BSD 4.2” UNIX systems worked.
In V7 the file system call works such that mknod(2) is used to create
special files and directories, it does it a little different than you
realize. Generally, there is no need to use it to create a regular file,
although you could. To wit the man page says:
SYNOPSIS
mknod(name, mode, addr)
char *name;
DESCRIPTION
*Mknod* creates a new file whose name is the null-terminated string pointed
to by name. The mode of the new file (including directory and special file
bits) is initialized from mode. (The protection part of the mode is
modified by the process's mode mask; see umask(2)
<http://www.unix.com/man-page/v7/2/umask/>). The first block pointer of the
i-node is initialized from add. For ordinary files and directories addr is
normally zero. In the case of a special file, addr specifies which special
file.
If you look at the code in mkdir.c (V7 mkdir.c source
<http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/mkdir.c>) you
see that what is being done is a mknod with addr=0 is being called and then
the two links.
. . .
if ((mknod(d, 040777, 0)) < 0) { fprintf(stderr,"mkdir: cannot make
directory %s\n", d);
++Errors;
return;
}
chown(d, getuid(), getgid());
strcpy(dname, d);
strcat(dname, "/.");
if((link(d, dname)) < 0) {
fprintf(stderr, "mkdir: cannot link %s\n", dname);
unlink(d);
++Errors;
return;
}
strcat(dname, ".");
if((link(pname, dname)) < 0) { fprintf(stderr, "mkdir: cannot link
%s\n",dname);
dname[strlen(dname)] = '\0'; unlink(dname);
unlink(d);
++Errors;
}
Where as creating special files did mess with the “special file” bits in
the mode.
If you look at the code in mknod.c (V7 mknod.c source
<http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/mknod.c>):
. . .
if(*argv[2] == 'b')
m = 060666; else
if(*argv[2] == 'c')
m = 020666; else
goto usage;
a = number(argv[3]);
if(a < 0)
goto usage;
b = number(argv[4]);
if(b < 0)
goto usage;
if(mknod(argv[1], m, (a<<8)|b) < 0)
perror("mknod");