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DESCRIPTION

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.