| BRK(2) | System Calls Manual | BRK(2) | 
brk, sbrk —
#include <unistd.h>
int
  
  brk(void
    *addr);
void *
  
  sbrk(intptr_t
    incr);
brk() and
  sbrk() functions are legacy interfaces from before the
  advent of modern virtual memory management. brk() is
  subject to removal and sbrk() is destined for full
  compat, where the system call will exist in the kernel, but no longer be
  exposed.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
    address at which the process's heap ends. This address is known as the
    “break”.
The brk() function sets the break to
    addr.
The sbrk() function changes the break by
    incr bytes. If incr is positive,
    this allocates incr bytes of new memory in the data
    segment. If incr is negative, this releases the
    corresponding number of bytes.
While the break may be set to any address, actual allocation takes place in page-sized quantities. For allocation and access control purposes the address of the break is always rounded up to the next page boundary. Thus, changes to the break that do not cross a page boundary have no material effect. Any new pages that are allocated, however, always appear freshly zeroed.
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 so
    that the sum of the heap size and the data segment is greater than the
    RLIMIT_DATA rlim_max value
    returned from a call to
    getrlimit(2). One can use
    the “_etext” symbol to find the end of the program text and
    thus the beginning of the data segment. See
    end(3) regarding
    “_etext”.
Historically and in NetBSD the heap immediately follows the data segment, and in fact is considered part of it. Thus the initial break is the first address after the end of the process's uninitialized data (also known as the “BSS”). This address is provided by the linker as “_end”; see end(3).
There exist implementations in the wild where this is not the
    case, however, or where the initial break is rounded up to a page boundary,
    or other minor variations, so the recommended more-portable way to retrieve
    the initial break is by calling
    sbrk(0) at program startup.
    (This returns the current break without changing it.)
In any event, the break may not be set to an address below its initial position.
Note that ordinary application code should use
    malloc(3) and related
    functions to allocate memory, or
    mmap(2) for lower-level
    page-granularity control. While the brk() and/or
    sbrk() functions exist in most Unix-like
    environments, their semantics sometimes vary subtly and their use is not
    particularly portable. Also, one must take care not to mix calls to
    malloc(3) or related functions
    with calls to brk() or
    sbrk() as this will ordinarily confuse
    malloc(3); this can be
    difficult to accomplish given that many things in the C library call
    malloc(3) themselves.
brk() 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.
brk() or sbrk() will fail and no
  additional memory will be allocated if one of the following are true:
ENOMEM]sbrk() function call appeared in
  Version 4 AT&T UNIX. A
  brk() function call appeared in
  Version 6 AT&T UNIX.
| September 7, 2019 | NetBSD 10.0 |