brk.2

Unix操作系统minix 2.0源码

.\" 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	6.3 (Berkeley) 5/22/86
.\"
.TH BRK 2 "May 22, 1986"
.UC 4
.SH NAME
brk, sbrk \- change data segment size
.SH SYNOPSIS
.nf
#include <unistd.h>
.PP
.ft B
char *brk(char *\fIaddr\fP)
.PP
.ft B
char *sbrk(int \fIincr\fP)
.fi
.SH DESCRIPTION
.B Brk
sets the system's idea of the lowest data segment 
location not used by the program (called the break)
to
.IR addr .
Locations greater 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
.BR 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
.B execve
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
.BR sbrk .
.SH "RETURN VALUE
The address of the new break is returned if
.B brk
succeeds;
.B \-1
if the program requests more
memory than the system limit.
.B Sbrk
returns
.B \-1
if the break could not be set.
.SH ERRORS
.B Sbrk
will fail and no additional memory will be allocated if
one of the following are true:
.TP 15
[ENOMEM]
The maximum possible size of a data segment (as set by
.BR chmem (1))
was exceeded.
.TP 15
[ENOMEM]
Insufficient virtual memory space existed
to support the expansion.  (Minix-vmd)
.SH "SEE ALSO"
.BR chmem (1),
.BR execve (2),
.BR malloc (3),
.BR end (3).
.SH NOTES
Minix-vmd rounds a small data segment limit up to 3 megabytes.
.SH BUGS
Setting the break may fail due to a temporary lack of
virtual memory under Minix-vmd.  It is not possible to distinguish this
from a failure caused by exceeding the maximum size of
the data segment.