brk.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.TH brk 2 RISC
.SH Name
brk, sbrk \- change data segment space allocation
.SH Syntax
.B #include <sys/types.h>
.PP
.B char \(**brk(addr)
.br
.B char \(**addr;
.PP
.B  char \(**sbrk(incr)
.br
.B int incr;
.SH Description
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 thus will
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 by 
.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 brk .
.PP
The
.MS getrlimit 2
system call may be used to determine the maximum permissible size of
the data segment; it will not be possible to set the break beyond
the 
.I rlim_max
value returned from a call to
.MS getrlimit 2 .
For example:
.EX
0x10000000 + rlp -> rlim_max
.EE
.SH Return Values
Upon successful completion, the
.PN brk
system call
returns a value of 0 or \-1 if the program requests more memory than
the system limit.  The
.PN sbrk
system call returns \-1 if the break could not be set.
.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
system call fails and no additionally memory is allocated if one of 
the following is true:
.RS
.TP 13
.SM [ENOMEM]
The limit, as set by
.MS setrlimit 2
was exceeded.
.TP
.SM [ENOMEM]
The maximum possible size a data segment (compiled into the system)
was exceeded.
.TP
.SM [ENOMEM]
Insufficient space existed in the swap area to support the expansion.
.RE
.SH See Also
execve(2), getrlimit(2), setrlimit(2), ulimit(2)