From: utzoo!decvax!harpo!seismo!hao!hplabs!sri-unix!dbj.rice@Rand-Relay
Newsgroups: net.unix-wizards
Title: Re: Questions about nil-pointer dereferences (the "plague")
Article-I.D.: sri-arpa.798
Posted: Wed Mar 16 22:05:15 1983
Received: Mon Apr  4 02:17:48 1983

From:  Dave B Johnson <dbj.rice@Rand-Relay>

For programs made with "cc" (without running "ld" explicitly), Berkeley
4.1BSD DOES (implicitly) guarantee a zero byte at address 0.  The fact
that you had problems with this in porting a locally written C program
from Unix 4.1 to VMS/Eunice is a bug in Eunice, not a problem with Unix.
4.1BSD crt0.o ALWAYS has a zero register save mask in it and is ALWAYS
loaded at address 0 by "cc".  Crt0 is written in assembler and has an
explicit ".word 0x0000" for a register save mask.  In addition, the value
of the register save mask in crt0 has no affect on saving registers,
since the Unix kernel does not "call" the program, but rather jumps
directly into it after the register save mask.  Saving the registers is
not necessary since there is nowhere to return to abover crt0.  In writing
our Unix emulator Phoenix, I have been very careful of details like this
which cause Unix programs to break.  Phoenix will run almost any Unix
program under VMS unmodified, even those that depend on undocumented
details of the Unix environment such as this.


                                        Dave *B* Johnson
                                        Dept. of Mathematical Sciences
                                        Rice University


P.S.  Dave *D* Johnson, huh?  I like your name...