I'm cleaning up the brk/sbrk man page, and I thought I'd send this out 
for review.

Note that the standard says

   The newly-allocated space is set to 0. However, if the application
   first decrements and then increments the break value, the contents of
   the reallocated space are unspecified.

What is our behavior, and how should I document it?

Perry


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.\"     @(#)brk.2	8.4 (Berkeley) 5/1/95
.\"
.Dd July 6, 1999
.Dt BRK 2
.Os
.Sh NAME
.Nm brk ,
.Nm sbrk
.Nd change data segment size
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn brk "void *addr"
.\" Note: XPG says sbrk should take an intptr_t and should return a
.\" void. Our usage conflicts with that.
.Ft char *
.Fn sbrk "int incr"
.Sh DESCRIPTION
.Bf -symbolic
The brk and sbrk functions are legacy interfaces from before the
advent of virtual memory management.
.Ef
.Pp
The
.Fn brk
and
.Fn sbrk
functions are used to change the amount of memory allocated in a
process's data segment. They do this by moving the location of the
.Dq break .
The break is the first address after the end of the process's
uninitialized data segment (also known as the
.Dq BSS ) .
.Pp
The break must always lie on an even page boundary. If
.Fn brk
or
.Fn sbrk
are asked for a non-integral number of pages, the address of the break
will be rounded up to the next highest page boundary.
.Pp
The
.Fn brk
function sets the break to
.Fa addr .
.Pp
The
.Fn sbrk
function raises the break by at least
.Fa incr
bytes, thus allocating at least
.Fa incr
bytes of new memory in the data segment.
If 
.Fa incr
is negative,
the break is lowered by
.Fa incr
bytes.
.Pp
.Fn sbrk
returns the prior address of the break.
The current value of the program break may be determined by calling
.Fn sbrk 0 .
(See also
.Xr end 3 ) .
.Pp
The
.Xr getrlimit 2
system call may be used to determine
the maximum permissible size of the
.Em data
segment;
it will not be possible to set the break
beyond the
.Dv RLIMIT_DATA
.Em rlim_max
value returned from a call to
.Xr getrlimit 2 ,
e.g.
.Dq etext + rlim.rlim_max .
(see
.Xr end 3
for the definition of
.Em etext ) .
.Pp
Note that
mixing
.Fn brk
and
.Fn sbrk
with
.Xr malloc 3 ,
.Xr free 3 ,
and similar functions may result in non-portable program
behavior. Caution is advised.
.Sh RETURN VALUES
.Fn brk
returns 0 if successful;
otherwise -1 with
.Va errno
set to indicate why the allocation failed.
.Pp
The
.Fn sbrk
function returns the prior break value if successful;
otherwise ((void *)-1) is returned and
.Va errno
is set to indicate why the allocation failed.
.Sh ERRORS
.Fn brk
or
.Fn sbrk
will fail and no additional memory will be allocated if
one of the following are true:
.Bl -tag -width Er
.It Bq Er ENOMEM
The limit, as set by
.Xr setrlimit 2 ,
was exceeded.
.It Bq Er ENOMEM
The maximum possible size of a data segment (compiled into the
system) was exceeded.
.It Bq Er ENOMEM
Insufficient space existed in the swap area
to support the expansion.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr getrlimit 2 ,
.Xr mmap 2 ,
.Xr end 3 ,
.Xr free 3 ,
.Xr malloc 3
.Sh BUGS
Setting the break may fail due to a temporary lack of swap space.
It is not possible to distinguish this from a failure caused by
exceeding the maximum size of the data segment without consulting
.Xr getrlimit 2 .
.Sh HISTORY
A
.Fn brk
function call appeared in
.At v7 .