brk.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 102 行

.\" SCCSID: @(#)brk.2	8.1	9/11/90
.TH brk 2 VAX
.SH Name
brk, sbrk \- change core allocation
.SH Syntax
#include <sys/types.h>
.PP
caddr_t brk(\fIaddr\fP)
.br
caddr_t \fIaddr\fP;
.PP
caddr_t sbrk(\fIincr\fP)
.br
int \fIincr\fP;
.SH Description
.NXR "brk system call"
.NXR "sbrk system call"
.NXR "data memory" "changing size"
The
.PN brk
system call sets the system's idea of the lowest data segment 
location not used by the program (called the break)
to
.I addr
(rounded up to the next multiple of the system's page size).
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
.PN 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 using
.PN 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
.PN sbrk .
.PP
The
.PN getrlimit
system call may be used to determine
the maximum permissible size of the
.I data
segment.
It will not be possible to set the break
beyond the
.I rlim_max
value returned from a call to
.PN getrlimit ,
for example, 
.B etext + rlp \-\fIrlim_max.\fR
See
.MS end 3
for the definition of
.IR etext .
.SH Return Values
If the call is successful, 
.PN brk
returns a 0 value.
If the program requests more
memory than the system limit, 
.PN brk
returns \-1. 
If the break could not be set, 
.PN brk
returns \-1.
.SH Restrictions
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 
.PN getrlimit .
.SH Diagnostics
The
.PN sbrk
call fails and no additional memory is allocated under the following
conditions:
.TP 15
[ENOMEM]
The limit, as set by
.PN setrlimit , 
is exceeded.
.TP 15
[ENOMEM]
The maximum possible size of a data segment (compiled into the
system) is exceeded.
.TP 15
[ENOMEM]
Insufficient space exists in the swap area
to support the expansion.
.SH See Also
execve(2), getrlimit(2), setrlimit(2), end(3), malloc(3)