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cat real-root-dev.c
#define _GNU_SOURCE
#include stdio.h
#include stdlib.h
#include sys/types.h
#include sys/stat.h
#include unistd.h
#include errno.h
#include fcntl.h

extern int errno;

int main(int argc, char **argv) {
        int r=-1,d=-1;
        FILE *p=NULL;
        struct stat st;
        char *strp=NULL;

        /* arg check */
        if (argc !=2) {
                fprintf(stderr, "Need just one block device\n");
                exit(1);
        }

        /* prevent argument moving during/after checks */
        d= open(argv[1],O_RDONLY);
        if (-1==d) {
                perror("opening parameter");
                exit(1);
        } else if (0>d) {
                fprintf(stderr,"Opened parameter with invalid ret\n");
                close(d);
                exit(1);
        }

        /* get the locked argument's properties */
        r= fstat(d, &st);
        if (0==r) {
        } else if (-1== r) {
                perror("fstating parameter");
                close(d);
                exit(1);
        } else {
                fprintf(stderr, "Stated parametr with invalid ret\n");
                perror("fstating parameter");
                close(d);
                exit(1);
        }

        /* inspect the properties */
        if (! (S_ISBLK(st.st_mode))) {
                fprintf(stderr, "Parameter is not block device\n");
                close(d);
                exit(1);
        }

        close(d);

        /* get the proc file */
        p= fopen("/proc/sys/kernel/real-root-dev", "w");
        if (NULL==p) {
                perror("opening proc");
                exit(1);
        }

        /* write it */
        d= fprintf(p,"%d", st.st_rdev);
        fclose(p);

        /* memorize it */
        r= asprintf( &strp, "%d", st.st_rdev);

        /* compare the result !!! */
        if ((-1 == r) || (-1 == d) || (r != d)) {
                fprintf(stderr, "Trying to sleep for 2 minutes before shit happens\n");
                fprintf(stderr, "It's up to you to push the button and reboot into rescue\n");
                usleep(120000000);
                fprintf(stderr, "Woke up and going to see the shit.\n");
                fprintf(stderr, "...hope you have the interactive rescue on my error\n");
                exit(1);
        }
        exit(0);
}