brk sets the end of the data segment to the value specified by
end_data_segment, when that value is reasonable, the system does have
enough memory and the process does not exceed its max data size (see
sbrk increments the program's data space by increment bytes.
sbrk isn't a system call, it is just a C library wrapper. Calling
sbrk with an increment of 0 can be used to find the current location of
the program break.