DragonFly On-Line Manual Pages
BRK(2) DragonFly System Calls Manual BRK(2)
brk, sbrk -- change data segment size
Standard C Library (libc, -lc)
brk(const void *addr);
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 loca-
tion of the ``break''. The break is the first address after the end of
the process's uninitialized data segment (also known as the ``BSS'').
The brk() function sets the break to addr.
The sbrk() function raises the break by 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.
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 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 permis-
sible size of the data segment. It will not be possible to set the break
beyond ``etext + rlim.rlim_max'' where the rlim.rlim_max value is
returned from a call to getrlimit(RLIMIT_DATA, &rlim). (See end(3) for
the definition of etext).
The brk() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The sbrk() function returns the prior break value if successful; other-
wise the value (void *)-1 is returned and the global variable errno is
set to indicate the error.
brk() or sbrk() will fail if:
[EINVAL] The requested break value was beyond the beginning of
the data segment.
[ENOMEM] The data segment size limit, as set by setrlimit(2),
[ENOMEM] Insufficient space existed in the swap area to support
the expansion of the data segment.
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)
A brk() function call appeared in Version 7 AT&T UNIX.
Mixing brk() or sbrk() with malloc(3), free(3), or similar functions will
result in non-portable program behavior.
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).
DragonFly 4.7 July 12, 1999 DragonFly 4.7