05-06-2011
Quote:
Originally Posted by
alister
Incorrect. The "dynamic portion of the data segment" is delimited by the break. The break is manipulated with the brk() and sbrk() system calls.
Yes you are right the heap is capped by the break pointer...and mmap allocates memory in the hole between break and the upper bound of the stack segment.
Quote:
Originally Posted by
alister
As its name implies, the segment is contiguous. mmap(), however, is allowed to map/allocate memory at random locations in non-contiguous segments beyond the break. Such behavior has nothing to do with the heap.
All segments are contiguous virtually speaking but none are contiguous physically.
Quote:
Originally Posted by
alister
Perhaps your mmap implementation does not do so, but there's nothing to forbid it.
mmap is only used in openbsd for dynamic storage allocation and since my system isnt openbsd it still uses malloc.
Quote:
Originally Posted by
alister
Alternatively, you have a very loose definition of "data segment".
That might be true as anything between the text and stack segment i'm calling collectively the data segment
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LEARN ABOUT OPENDARWIN
brk
BRK(2) BSD System Calls Manual BRK(2)
NAME
brk, sbrk -- change data segment size
SYNOPSIS
#include <unistd.h>
char *
brk(const char *addr);
char *
sbrk(int incr);
DESCRIPTION
The brk and sbrk functions are historical curiosities left over from earlier days before the advent of virtual memory management. The 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 brk 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 end(3)). 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 rlim_max value returned from
a call to getrlimit, e.g. ``qetext + rlp->rlim_max.'' (see end(3) for the definition of etext).
RETURN VALUES
Brk returns a pointer to the new end of memory if successful; otherwise -1 with errno set to indicate why the allocation failed. 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.
ERRORS
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.
SEE ALSO
execve(2), getrlimit(2), malloc(3), mmap(2), end(3)
BUGS
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.
HISTORY
A brk() function call appeared in Version 7 AT&T UNIX.
4th Berkeley Distribution December 11, 1993 4th Berkeley Distribution