jrmacmillan@watdragon.waterloo.edu (John R. MacMillan) (08/15/88)
With the discussion of placement of /lib/crt0[s].o on the ld
command line to get a -F file, I decided to look into why, and
what I came up with is that it doesn't really work.
According to the ld(1) man page, -F files have virtual addresses
equal to the file offsets modulo 4096. Files compiled as above
don't.
So why does file(1) say it worked? Because /etc/magic decides that
and object is -F if the _entry point_ is > 0x080000. Putting
/lib/crt0.o _after_ other things makes this true, and thus tricks
the file(1) command.
The grisly details are included below.
So this brings us back to: how do we get a _real_ -F file? Has anyone
got a /lib/ifile.0413-F (this is what ld(1) looks for if you specify
-F)? Is anyone enough of an ld(1) wizard to reconstruct it?
Here's the whole of my little test, with some comments:
$ cat foo.c
main()
{}
$ cc -c foo.c
$ ld -m foo.o /lib/crt0.o -lc
LINK EDITOR MEMORY MAP
output input virtual
section section address size
.text 80000 6c
^^^^^
>> The text is starting at this address
.text 80000 14 foo.o
.text 80014 2c crt0.o
^^^^^
>> The entry point is around here, which will fool file(1)
.text 80040 18 libc.a
.text 80058 08 libc.a
.text 80060 0c libc.a
*avail* 8006c ff94
.data 90000 00 uninitialized
.bss 90000 08 uninitialized
.bss 90000 08 crt0.o
*avail* 90008 26eff8
$ file a.out
a.out: mc68k executable (shared demand paged) not stripped -F (0413 demand paged)
$ dump -h a.out
***SECTION HEADER***
Name Paddr Vaddr Scnptr Relptr Lnnoptr
Flags Size Nreloc Nlnno
a.out:
.text 0x00080000 0x00080000 0x00000400 0x00000000 0x00000000
^^^^^^^^^^
>> This is the offset of the .text section in the file. This means that
>> the address for .text (above) should be 0x080400 for a -F file
0x00000020 0x0000006c 0 0
.data 0x00090000 0x00090000 0x00000000 0x00000000 0x00000000
0x00000040 0x00000000 0 0
.bss 0x00090000 0x00090000 0x00000000 0x00000000 0x00000000
0x00000080 0x00000008 0 0
Note that if you do "ld -m /lib/crt0.o foo.o -lc" you'll get exactly
the same map as above; ie. our -F file is the same as a normal one.
Oh well, back to the drawing board. :-(
--
John R. MacMillan
jrmacmillan@lily.waterloo.edu If the universe fits, wear it.
...!watmath!lily!jrmacmillanjrmacmillan@watdragon.waterloo.edu (John R. MacMillan) (08/15/88)
A closer look at the a.out files shows that what you get if you don't give any flags to ld is the same as if you specify -z, so I guess the manual is wrong; -z, not -F, is on by default. -- John R. MacMillan jrmacmillan@dragon.waterloo.edu If the universe fits, wear it. ...!watmath!dragon!jrmacmillan