How do I programmatically check memory use in a fairly portable way? (C/C++)

unsigned long m0 = GetMemoryInUse();
char *p = new char[ random_number ];
unsigned long m1 = GetMemoryInUse();
printf( "%d bytes usedn", (m1-m0) );

#ifdef __linux__
# include <sys/sysinfo.h>
#endif

#ifdef __APPLE__
# include <mach/task.h>
# include <mach/mach_init.h>
#endif

#ifdef _WINDOWS
# include <windows.h>
#else
# include <sys/resource.h>
#endif

/// The amount of memory currently being used by this process, in bytes.
/// By default, returns the full virtual arena, but if resident=true,
/// it will report just the resident set in RAM (if supported on that OS).
size_t memory_used (bool resident=false)
{
#if defined(__linux__)
    // Ugh, getrusage doesn't work well on Linux.  Try grabbing info
    // directly from the /proc pseudo-filesystem.  Reading from
    // /proc/self/statm gives info on your own process, as one line of
    // numbers that are: virtual mem program size, resident set size,
    // shared pages, text/code, data/stack, library, dirty pages.  The
    // mem sizes should all be multiplied by the page size.
    size_t size = 0;
    FILE *file = fopen("/proc/self/statm", "r");
    if (file) {
        unsigned long vm = 0;
        fscanf (file, "%ul", &vm);  // Just need the first num: vm size
        fclose (file);
       size = (size_t)vm * getpagesize();
    }
    return size;
#endif

#elif defined(__APPLE__)
    // Inspired by:
    // http://miknight.blogspot.com/2005/11/resident-set-size-in-mac-os-x.html
    struct task_basic_info t_info;
    mach_msg_type_number_t t_info_count = TASK_BASIC_INFO_COUNT;
    task_info(current_task(), TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count);
    size_t size = (resident ? t_info.resident_size : t_info.virtual_size);
    return size;

#elif defined(_WINDOWS)
    // According to MSDN...
    PROCESS_MEMORY_COUNTERS counters;
    if (GetProcessMemoryInfo (GetCurrentProcess(), &count, sizeof (count)))
        return count.PagefileUsage;
    else return 0;

#else
    // No idea what platform this is
    return 0;   // Punt
#endif
}