Thread: ???get max memory before allocate memory?

Hi

checked memory allocate,its possible to allocate memory with nocatch and it leaves NULL pointer,(64bit with many gigabytes)
but there must be a better way than allocate much memory as possible than a loop that trials and errors true allocate with nocatch
like get maxmem and subtract a number of bytes to avoid OS getting unstable,how much?half a gig?1 gig?

Perhaps you should consider using a more sophisticated algorithm such that you can avoid excessive memory usage, or perhaps it may be a good idea to store not currently used data to disk?

Well for one thing, it's sure to depend on your OS of the moment.

For another thing, the amount of memory you could theoretically allocate at any given moment is likely to be a fluid number that depends on whatever else is going on.

Between asking 'can I allocate X bytes' and then requesting 'give me X bytes', it can all change.

The XY Problem
Tell us the problem, not how to work around your solution.

You can allocate more memory than you have physical RAM. The memory that is allocated is "virtual memory" and can in fact exist mostly on disk. This can then be very slow to use, of course, but depending on the algorithm (more specifically, the access pattern) it can work. Still, it's generally something you try to avoid, except perhaps for the case of a memory-mapped file.

Depending on what you are actually trying to do, it may be reasonable to use an OS function to determine how much actual RAM you have and then to request some large fraction of that. As long as you know that your process is the only memory hog on the system then it can work out okay.

To check for total/available memory on Windows: GlobalMemoryStatusEx function (sysinfoapi.h) - Win32 apps | Microsoft Docs

Code:

MEMORYSTATUSEX mem;
if ( GlobalMemoryStatusEx( &mem ) )
{
    cout << "TotalPhys   : " << setw(12) << mem.ullTotalPhys    << '\n';
    cout << "AvailPhys   : " << setw(12) << mem.ullAvailPhys    << '\n';
    cout << "TotalVirtual: " << setw(12) << mem.ullTotalVirtual << '\n';
    cout << "AvailVirtual: " << setw(12) << mem.ullAvailVirtual << '\n';
}
else
{
    cerr << "GlobalMemoryStatusEx failed.\n";
}

thanks John.c, that what I was looking for,and only want use max physical,but try learn take advantage of 64bit strength is to use lots of fast RAM and if not much memory aviable ,fall back on smaller allocate with or without use slower file read/write/append