freebsd-dev/lib/libc/sys/brk.2

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.\"     @(#)brk.2	8.4 (Berkeley) 5/1/95
.\" $FreeBSD$
.\"
.Dd May 1, 1995
.Dt BRK 2
.Os BSD 4
.Sh NAME
.Nm brk ,
.Nm sbrk
.Nd change data segment size
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char *
.Fn brk "const char *addr"
.Ft char *
.Fn sbrk "int incr"
.Sh DESCRIPTION
.Bf -symbolic
The brk and sbrk functions are historical curiosities
left over from earlier days before the advent of virtual memory management.
.Ef
The
.Fn brk
function
sets the break or lowest address
of a process's data segment (uninitialized data) to
.Fa addr
(immediately above bss).
Data addressing is restricted between
.Fa addr
and the lowest stack pointer to the stack segment.
Memory is allocated by
.Fa brk
in page size pieces;
if
.Fa addr
is not evenly divisible by the system page size, it is
increased to the next page boundary.
.Pp
.\" The
.\" .Nm sbrk
.\" function
.\" allocates chunks of
.\" .Fa incr
.\" bytes
.\" to the process's data space
.\" and returns an address pointer.
.\" The
.\" .Xr malloc 3
.\" function utilizes
.\" .Nm sbrk .
.\" .Pp
The current value of the program break is reliably returned by
.Dq Li sbrk(0)
(see also 
.Xr end 3 ) .
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
.Em rlim_max
value returned from a call to
.Xr getrlimit 2 ,
e.g.
.Dq etext + rlp\(->rlim_max.
(see
.Xr end 3
for the definition of
.Em etext ) .
.Sh RETURN VALUES
.Fn Brk
returns 0 if successful;
otherwise -1 with
.Va errno
set to indicate why the allocation failed.
The
.Fn sbrk
function returns a pointer to the base of the new storage if successful;
otherwise -1 with
.Va errno
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 end 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 .