int
brk(
void *addr
)
void
*
sbrk(
intptr_t incr
)
The
brk()
and
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
``break''.
The break is the first address after the end of the process's
uninitialized data segment (also known as the
``BSS'').
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).
The
brk()
function sets the break to
addr
.
The
sbrk()
function raises the break by at least
incr
bytes, thus allocating at least
incr
bytes of new memory in the data segment.
If
incr
is negative,
the break is lowered by
incr
bytes.
sbrk()
returns the prior address of the break.
The current value of the program break may be determined by calling
sbrk(
0
).
(See also
end(3)).
The
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
RLIMIT_DATA
rlim_max
value returned from a call to
getrlimit(2),
e.g.
``etext + rlim.rlim_max''.
(see
end(3)
for the definition of
etext).
)
returns 0 if successful;
otherwise -1 with
errno
set to indicate why the allocation failed.
The
sbrk()
function returns the prior break value if successful;
otherwise ((void *)-1) is returned and
errno
is set to indicate why the allocation failed.
)
or
sbrk(
)
will fail and no additional memory will be allocated if
one of the following are true:
ENOMEM
]
ENOMEM
]
ENOMEM
]
)
function call appeared in
Version 7 AT&T UNIX
.
)
and
sbrk(
)
with
malloc(3),
free(3),
and similar functions may result in non-portable program
behavior.
Caution is advised.
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 getrlimit(2).