minix/lib/libc/sys/brk.2

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.\"     @(#)brk.2	8.4 (Berkeley) 5/1/95
.\"
.Dd July 12, 1999
.Dt BRK 2
.Os
.Sh NAME
.Nm brk ,
.Nm sbrk
.Nd change data segment size
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn brk "void *addr"
.Ft void *
.Fn sbrk "intptr_t incr"
.Sh DESCRIPTION
.Bf -symbolic
The brk and sbrk functions are legacy interfaces from before the
advent of modern virtual memory management.
.Ef
.Pp
The
.Fn brk
and
.Fn sbrk
functions are used to change the amount of memory allocated in a
process's data segment.
They do this by moving the location of the
.Dq break .
The break is the first address after the end of the process's
uninitialized data segment (also known as the
.Dq BSS ) .
.Pp
While the actual process data segment size maintained by the kernel will only
grow or shrink in page sizes, these functions allow setting the break
to unaligned values (i.e. it may point to any address inside the last
page of the data segment).
.Pp
The
.Fn brk
function sets the break to
.Fa addr .
.Pp
The
.Fn sbrk
function raises the break by at least
.Fa incr
bytes, thus allocating at least
.Fa incr
bytes of new memory in the data segment.
If
.Fa incr
is negative,
the break is lowered by
.Fa incr
bytes.
.Pp
.Fn sbrk
returns the prior address of the break.
The current value of the program break may be determined by calling
.Fn sbrk 0 .
(See also
.Xr end 3 ) .
.Pp
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
.Dv RLIMIT_DATA
.Em rlim_max
value returned from a call to
.Xr getrlimit 2 ,
e.g.
.Dq etext + rlim.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.
.Pp
The
.Fn sbrk
function returns the prior break value if successful;
otherwise ((void *)\-1) is returned and
.Va errno
is 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 mmap 2 ,
.Xr end 3 ,
.Xr free 3 ,
.Xr malloc 3 ,
.Xr sysconf 3
.Sh HISTORY
A
.Fn brk
function call appeared in
.At v7 .
.Sh BUGS
Note that
mixing
.Fn brk
and
.Fn sbrk
with
.Xr malloc 3 ,
.Xr free 3 ,
and similar functions may result in non-portable program
behavior.
Caution is advised.
.Pp
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 .