usr/src/lib/libc/sys/brk.2

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"     @(#)brk.2       4.1 (Berkeley) %G%
.\"
.TH BRK 2
.UC 4
.SH NAME
brk, sbrk, break \- change core allocation
.SH SYNOPSIS
.B char *brk(addr)
.PP
.B char *sbrk(incr)
.SH DESCRIPTION
.I Brk
sets the system's idea of the lowest location not used by the program
(called the break)
to
.I addr
(rounded up to the next multiple of 64 bytes
on the PDP11, 256 bytes on the Interdata 8/32,
and 1024 bytes on a VAX-11).
Locations not less than
.I addr
and below the stack pointer
are not in the address space and will thus
cause a memory violation if accessed.
.PP
In the alternate function
.I sbrk,
.I incr
more bytes are added to the
program's data space and a pointer to the
start of the new area is returned.
.PP
When a program begins execution via
.I exec
the break is set at the
highest location defined by the program
and data storage areas.
Ordinarily, therefore, only programs with growing
data areas need to use
.IR break .
.PP
The
.IR vlimit (2)
system call may be used to determine
the maximum permissible size of the
.I data
region;
it will not be possible to set the break
beyond
\*(lqetext + vlimit(LIM_DATA, \-1).\*(rq
(See
.IR end (3)
for the definition of
.IR etext .)
.SH "SEE ALSO"
exec(2),
vlimit(2),
malloc(3),
end(3)
.SH DIAGNOSTICS
Zero is returned if the
.I brk
could be set;
\-1 if the program requests more
memory than the system limit
or if too many segmentation
registers would be required to implement the break.
.I Sbrk
returns \-1 if the break could not be set.
.SH BUGS
Setting the break in the range
0177701 to 0177777 (on the PDP11) is the same as setting it to zero.
.SH "ASSEMBLER (PDP-11)"
(break = 17.)
.br
.B sys break; addr
.PP
.I Break
performs the function of
.IR brk .
The name of the routine differs from that in C
for historical reasons.