Hi Uwe,
On 9/16/26 11:40 AM, you wrote:
Hello Carl,
On Mon, Aug 31, 2026 at 12:23:35PM -0700, Carl Wuebker wrote:
Package: linux-libc-dev
Version: 6.12.107-1
Severity: normal
Dear Maintainer,
The problem is that sbrk(2) checks each new request against the machine's
total RAM + swap space. It should check each new request against total RAM
+ swap space - memory that has already been allocated to the current
process.
Steps to reproduce:
1. Compile & run the program below on a machine with 32 GiB memory & 24 GiB
swap
--- eatmem_bad.c
#include <stdio.h>
#include <unistd.h>
intptr_t bigMem = (intptr_t) 128LL*1024*1024*1024;
int main(int argc,char *argv[]) {
intptr_t i;
unsigned long long mem;
if (argc != 1)
{ (void) fprintf(stderr,"Usage: %s\n", argv[0]); return 2; }
mem=0;
for (i = bigMem; i >= 16; i /= 2)
if (sbrk(i) != (void *) -1)
mem |= i;
(void) printf("Memory=%llu MiB\n",mem / (1024*1024));
return 0;
}
---
Expected (and correct) result:
Memory=56718 MiB
Actual result:
Memory=65535 MiB
I wonder what your actual problem is.
It's quite usual that a process can get more memory assigned than there
is physically available. That's called "overcommitting" if you want to
research about that.
Unless there is a real problem to solve I suggest to either ignore it or
discuss with upstream what to do.
Thanks for your reply. The "real problem" comes from the current
situation in CAD, AI & other single programs running on a Linux
machine. Some programs need a way to find out how much RAM is available
and might use a technique similar to this one, which might report that
almost 2x the available RAM is available because, as the man page says:
"On success, sbrk() returns the previous program break. (If the break
was increased, then this value is a pointer to the start of the newly
allocated memory). On error, (void *) -1 is returned, and errno is set
to ENOMEM."
When I read this, I think "if the memory allocation was unsuccessful,
I'll get an error." But that's not the case -- I guess that if I ask it
to allocate 128 GBy and get no error, I still need to check the brk(2)
value to see if the memory was allocated.
So, in my mind (but maybe not obvious to others), there is a problem
with 2 solutions -- one is to return an error if the last memory
allocation fails, the other is to update the sbrk(2) documentation to
tell the user that -- even if the sbrk(2) doesn't return an error --
they need to compare the sbrk(2) return value with the last sbrk(2)
value to see if it moved -- if it didn't move, the allocation failed.
Where can I find a reasonably up-to-date copy of Linux's sbrk(2) code?
I might be better able to understand your comments and/or recommend a
solution if I read that code.
Thanks,
Carl
