change data segment size
sbrk() functions are historical curiosities left over from earlier days before the advent of virtual memory management.
brk() function sets the break or
lowest address of a process's data segment (uninitialized data) to
addr (immediately above bss). Data addressing is
restricted between addr and the lowest stack pointer
to the stack segment. Memory is allocated by
in page size pieces; if addr is not evenly divisible
by the system page size, it is increased to the next page boundary.
The current value of the program break is reliably
returned by “
sbrk(0)” (see also
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
value returned from a call to
getrlimit(2), e.g., “etext + rlp→rlim_max” (see
end(3) for the definition of
brk() function returns the
value 0 if successful; otherwise the value -1 is returned and
the global variable errno is set to indicate the
sbrk() function returns a pointer to
the base of the new storage if successful; otherwise -1 with
errno set to indicate why the allocation failed.
sbrk() will fail and no additional memory
will be allocated if one of the following are true:
- The limit, as set by setrlimit(2), was exceeded.
- The maximum possible size of a data segment (compiled into the system) was exceeded.
- Insufficient space existed in the swap area to support the expansion.
execve(2), getrlimit(2), mmap(2), end(3), malloc(3)
brk() function call appeared in
Version 7 AT&T UNIX.
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).