C++ – How to get available memory C++/g++

ccross platformmemory-management

I want to allocate my buffers according to memory available. Such that, when I do processing and memory usage goes up, but still remains in available memory limits. Is there a way to get available memory (I don't know will virtual or physical memory status will make any difference ?). Method has to be platform Independent as its going to be used on Windows, OS X, Linux and AIX. (And if possible then I would also like to allocate some of available memory for my application, someone it doesn't change during the execution).

Edit: I did it with configurable memory allocation.
I understand it is not good idea, as most OS manage memory for us, but my application was an ETL framework (intended to be used on server, but was also being used on desktop as a plugin for Adobe indesign). So, I was running in to issue of because instead of using swap, windows would return bad alloc and other applications start to fail. And as I was taught to avoid crashes and so, was just trying to degrade gracefully.

Best Answer

On UNIX-like operating systems, there is sysconf.

#include <unistd.h>

unsigned long long getTotalSystemMemory()
{
    long pages = sysconf(_SC_PHYS_PAGES);
    long page_size = sysconf(_SC_PAGE_SIZE);
    return pages * page_size;
}

On Windows, there is GlobalMemoryStatusEx:

#include <windows.h>

unsigned long long getTotalSystemMemory()
{
    MEMORYSTATUSEX status;
    status.dwLength = sizeof(status);
    GlobalMemoryStatusEx(&status);
    return status.ullTotalPhys;
}

So just do some fancy #ifdefs and you'll be good to go.

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');