Linux _SC_PHYS_PAGES 不适用于 Mac OS X

Question

I am trying to compile a project on Mac OS that doesn't make any problems under Linux. I am using GCC on both operating systems. Besides other problems that I managed to fix, I get the following error when trying to compile on OSX:

error: ‘_SC_PHYS_PAGES’ was not declared in this scope
     long pages = sysconf(_SC_PHYS_PAGES);
                      ^

unistd.h is included in the file where this error occurs.

How can I fix this in a way that the code still compiles under Linux?

Answer 1

Although documented as being available, apparently it is a documentation bug, since it is not.

However, the man page does point to an alternative interface: sysctl() . You can use the sysctlbyname() interface to get the physical memory in bytes.

#include <iostream>
#include <stdint.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/sysctl.h>

int main () {
    size_t pagesz = sysconf(_SC_PAGE_SIZE);
    uint64_t mem;
    size_t len = sizeof(mem);
    sysctlbyname("hw.memsize", &mem, &len, NULL, 0);
    std::cout << mem << '\n';
    std::cout << mem/pagesz << '\n';
    std::cout << mem/1024/1024/1024 << '\n';
}

To mimic the functionality for both compilation environments, I would create a wrapper function and modify what the function does based on conditional compilation.

#define PHYS_PAGES get_phys_pages()

unsigned get_phys_pages () {
    static unsigned phys_pages;
    if (phys_pages == 0) {
        #if USE_SYSCTL_HW_MEMSIZE
            uint64_t mem;
            size_t len = sizeof(mem);
            sysctlbyname("hw.memsize", &mem, &len, NULL, 0);
            phys_pages = mem/sysconf(_SC_PAGE_SIZE);
        #elif USE_SYSCONF_PHYS_PAGES
            phys_pages = sysconf(_SC_PHYS_PAGES);
        #else
        #   error "no way to get phys pages"
        #endif
    }
    return phys_pages;
}