memtest.c

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int global_var = 0;

int main() {
    int fd = open("/proc/self/mem", O_RDWR);
    if(!fd) {
        printf("can't open /proc/self/mem as O_RDWR\n");
        return 1;
    }

    printf("- opened /proc/self/mem as rdwr\n");

    int* addr = &global_var;
    printf("- global var address: %p\n", addr);

    int buf;
    lseek(fd, (off_t) addr, SEEK_SET);
    read(fd, &buf, 4);
    printf("- contents of global_var: 0x%08x (read through /proc/self/mem)\n", buf);

    buf = 0x12345;
    printf("- saving 0x%08lx to global_var through /proc/self/mem...\n", buf); 
    lseek(fd, (off_t) addr, SEEK_SET);
    write(fd, &buf, 4);
    
    printf("- contents of global_var: 0x%08x (read directly)\n", global_var);
    return 0;
}

antek@antekpc ~/dev/tests/proc_mem $ uname -a
Linux antekpc 4.8.13-1-ARCH #1 SMP PREEMPT Fri Dec 9 07:24:34 CET 2016 x86_64 GNU/Linux

antek@antekpc ~/dev/tests/proc_mem $ gcc test.c -o test && ./test
- opened /proc/self/mem as rdwr
- global var address: 0x60105c
- contents of global_var: 0x00000000 (read through /proc/self/mem)
- saving 0x00012345 to global_var through /proc/self/mem...
- contents of global_var: 0x00012345 (read directly)