end.3

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th END III 4/28/75
.sh NAME
end, etext, edata \*- last locations in program
.sh SYNOPSIS
.ft B
.nf
extern	end;
extern	etext;
extern	edata;
.ft R
.fi
.sh DESCRIPTION
These names refer neither to routines
nor to locations with interesting contents.
Instead, their addresses coincide with the first
address above the program text region
.it (etext),
above the initialized data region
.it (edata),
or uninitialized data region
.it (end).
The last is the same as the program break.
Values are given to these symbols
by the link editor
.it ld
(I)
when, and only when, they are referred to but not defined
in the set of programs loaded.
.s3
The usage of these symbols is rather specialized, but one plausible
possibility is
.s3
.nf
	extern end;
	...
	... = brk(&end+...);
.fi
.s3
(see
.it break
(II)).
The problem with this is that it ignores any other
subroutines which may want to extend core for their purposes;
these include
.it sbrk
(see
.it break
(II)),
.it alloc
(III),
and also secret subroutines invoked by the profile
(\-p) option of 
.it cc.
Of course it was for the benefit of such systems
that the symbols were invented,
and user programs, unless they are in firm control
of their environment,
are wise not to refer to
the absolute symbols directly.
.s3
One technique sometimes useful
is to call
.it sbrk(0),
which returns the value of the current program break,
instead of referring to
.it &end,
which yields the program break at the instant execution started.
.s3
These symbols are accessible from assembly
language
if it is remembered that they should be prefixed by
`\*_'
.sh "SEE ALSO"
break (II), alloc (III)
.sh BUGS