BRK(2) System Calls BRK(2)
brk, sbrk - change the amount of space allocated for the calling
process's data segment
#include <unistd.h> int brk
); void *sbrk
functions are used to change dynamically the amount
of space allocated for the calling process's data segment (see exec(2)
The change is made by resetting the process's break value and allocating
the appropriate amount of space. The break value is the address of the
first location beyond the end of the data segment. The amount of
allocated space increases as the break value increases. Newly allocated
space is set to zero. If, however, the same memory space is reallocated
to the same process its contents are undefined.
When a program begins execution using execve()
the break is set at the
highest location defined by the program and data storage areas.
function may be used to determine the maximum
permissible size of the data
segment; it is not possible to set the break
beyond the rlim_max
value returned from a call to getrlimit()
, that is to
say, "end + rlim.rlim_max
." See end(3C)
function sets the break value to endds
and changes the
allocated space accordingly.
function adds incr
function bytes to the break value and
changes the allocated space accordingly. The incr
function can be
negative, in which case the amount of allocated space is decreased.
Upon successful completion, brk()
. Otherwise, it returns -1
to indicate the error.
Upon successful completion, sbrk()
returns the prior break value.
Otherwise, it returns (void *)-1
and sets errno
to indicate the error.
functions will fail and no additional memory will be
allocated if: ENOMEM
The data segment size limit as set by setrlimit()
) would be exceeded; the maximum possible size of a
data segment (compiled into the system) would be exceeded;
insufficient space exists in the swap area to support the
expansion; or the new break value would extend into an area of
the address space defined by some previously established
mapping (see mmap(2)
Total amount of system memory available for private pages is
temporarily insufficient. This may occur even though the space
requested was less than the maximum data segment size (see ulimit(2)
The behavior of brk()
is unspecified if an application also
uses any other memory functions (such as malloc(3C)
functions have been used in specialized cases where
no other memory allocation function provided the same capability. The
use of mmap(2)
is now preferred because it can be used portably with all
other memory allocation functions and with any function that uses other
It is unspecified whether the pointer returned by sbrk()
suitably for any purpose.
for descriptions of the following attributes:
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|MT-Level | MT-Safe |
SEE ALSO exec(2)
The value of incr
may be adjusted by the system before setting the new
break value. Upon successful completion, the implementation guarantees a
minimum of incr
bytes will be added to the data segment if incr
positive value. If incr
is a negative value, a maximum of incr
will be removed from the data segment. This adjustment may not be
necessary for all machine architectures.
The value of the arguments to both brk()
are rounded up for
alignment with eight-byte boundaries.
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()
January 14, 1997 BRK(2)