Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #142145

Re: heap memory indexed as an array

Show all headers | View raw

Scott Lurndal <scott@slp53.sl.home> wrote:
> nobody@example.org (Scott) writes:
>>On Wed, 20 Mar 2019 04:40:25 GMT, Spiros Bousbouras <spibou@gmail.com>
>>wrote:
>>
>>>But you don't get to do error checking either and if there isn't enough
>>>memory to allocate the array , the programme will probably crash with an
>>>uninformative error message.
>>
>>Well, maybe. On some contemporary operating systems, a malloc return
>>value of non-null does not guarantee that the allocated memory
>>actually exists. It can appear to succeed, and only explodes when you
>>try to write to that phantom memory -- and there's no way to trap that
>>fault in C.
> 
> Correction, no "standard C" method.   One may always catch SIGSEGV.

On systems that overcommit memory (e.g. Linux, FreeBSD) a process is usually
killed with SIGKILL and it's not necessarily the process that triggered the
original fault.

Note that overcommit is distinct from unreserved allocations, e.g.
mmap+MAP_NORESERVE. The latter is documented to send SIGSEGV.

On Windows the nomenclature is slightly different: MAP_RESERVE doesn't
commit; one must use MAP_COMMIT in addition to MAP_RESERVE. But Windows
doesn't overcommit memory in the same manner as Linux. For one thing,
overcommit is a system-level option in Linux, beyond the control of a
process. In fact, as I recently had the unfortune to discover, even if you
theoretically mapped all your process memory to a backing store like a disk
file, which *should* guarantee availability, overcommit logic and heuristics
are so pervasive in the kernel that you can still get shot down, both when
triggering your own page fault or someone else triggering a page fault.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

heap memory indexed as an array sinbad <sinbad.sinbad@gmail.com> - 2019-03-19 20:05 -0700
  Re: heap memory indexed as an array Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-03-19 21:37 -0600
    Re: heap memory indexed as an array Spiros Bousbouras <spibou@gmail.com> - 2019-03-20 04:40 +0000
      Re: heap memory indexed as an array nobody@example.org (Scott) - 2019-03-20 15:56 +0000
        Re: heap memory indexed as an array scott@slp53.sl.home (Scott Lurndal) - 2019-03-20 16:43 +0000
          Re: heap memory indexed as an array fir <profesor.fir@gmail.com> - 2019-03-20 10:00 -0700
          Re: heap memory indexed as an array William Ahern <william@25thandClement.com> - 2019-03-20 15:07 -0700
            Re: heap memory indexed as an array fir is a sexy shemale <cdalten@gmail.com> - 2019-03-20 23:52 -0700
          Re: heap memory indexed as an array nobody@example.org (Scott) - 2019-03-21 16:50 +0000
            Re: heap memory indexed as an array scott@slp53.sl.home (Scott Lurndal) - 2019-03-21 17:23 +0000
              Re: heap memory indexed as an array nobody@example.org (Scott) - 2019-03-22 04:29 +0000
                OT: crashing malloc [was Re: heap memory indexed as an array] nobody@example.org (Scott) - 2019-03-23 15:27 +0000
                Re: heap memory indexed as an array Spiros Bousbouras <spibou@gmail.com> - 2019-03-23 19:24 +0000
                Re: heap memory indexed as an array Melzzzzz <Melzzzzz@zzzzz.com> - 2019-03-23 20:20 +0000
      Re: heap memory indexed as an array Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-03-21 22:35 -0700
  Re: heap memory indexed as an array Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-03-20 07:05 +0000
  Re: heap memory indexed as an array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-03-20 12:12 +0000

csiph-web