How to find the physical address of a variable from user-space in Linux?

I want to find the physical address of a variable defined in a user-space process? Is there any way to do it using root privileges?

As partially answered before, normal programs should not need to worry about physical addresses as they run in a virtual address space with all its conveniences. Furthermore, not every virtual address has a physical address, the may belong to mapped files or swapped pages. However, sometimes it may be interesting to see this mapping, even in userland.

For this purpose, the Linux kernel exposes its mapping to userland through a set of files in the /proc. The documentation can be found here. Short summary:

1. /proc/$pid/maps provides a list of mappings of virtual addresses together with additional information, such as the corresponding file for mapped files.
2. /proc/$pid/pagemap provides more information about each mapped page, including the physical address if it exists.

This website provides a C program that dumps the mappings of all running processes using this interface and an explanation of what it does.

#include "stdio.h"
#include "unistd.h"
#include "inttypes.h"

uintptr_t vtop(uintptr_t vaddr) {
    FILE *pagemap;
    intptr_t paddr = 0;
    int offset = (vaddr / sysconf(_SC_PAGESIZE)) * sizeof(uint64_t);
    uint64_t e;

    // https://www.kernel.org/doc/Documentation/vm/pagemap.txt
    if ((pagemap = fopen("/proc/self/pagemap", "r"))) {
        if (lseek(fileno(pagemap), offset, SEEK_SET) == offset) {
            if (fread(&e, sizeof(uint64_t), 1, pagemap)) {
                if (e & (1ULL << 63)) { // page present ?
                    paddr = e & ((1ULL << 54) - 1); // pfn mask
                    paddr = paddr * sysconf(_SC_PAGESIZE);
                    // add offset within page
                    paddr = paddr | (vaddr & (sysconf(_SC_PAGESIZE) - 1));
                }   
            }   
        }   
        fclose(pagemap);
    }   

    return paddr;
}