BRK(2) BSD System Calls Manual BRK(2)
brk, sbrk -- change data segment size
Standard C Library (libc, -lc)
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
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
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 defi-
nition of etext).
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
brk() or sbrk() will fail and no additional memory will be allocated if one of the following are true:
[ENOMEM] The limit, as set by setrlimit(2), was exceeded.
[ENOMEM] The maximum possible size of a data segment (compiled into the system) was exceeded.
[ENOMEM] Insufficient space existed in the swap area to support the expansion.
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3), sysconf(3)
A brk() function call appeared in Version 7 AT&T UNIX.
Note that mixing brk() and sbrk() with malloc(3), free(3), and similar functions may result in non-portable program behavior. Caution is
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).
July 12, 1999 BSD