The brk and sbrk functions are legacy interfaces from before the advent of modern virtual memory management.
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).