naive-round-to-integer.c

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

int main() {
    for(int i=0; i<1000000; i++) {
        char s[16]; 
        snprintf(s, sizeof(s), "1.%06d", i);
        double d = strtod(s, NULL);
        uint64_t dc = (uint64_t)(d * 1e6);
        if(dc != i + 1000000)
            printf("%s %7" PRId64 " %.14A %.14A\n", s, dc, d, d*1e6);
    }
}

This program shows all values of the form "1.xxxxxx" which, when multiplied by 1e6 and truncated to an integer don't equal the value you expect in decimal arithmetic.

On an x86_64 platform, counting the number of "unexpected" values:

$ for ARCH in -m32 -m64; do for STD in -std=gnu11 -std=c11; do gcc $ARCH $STD -O into.c; printf "%3s %12s " $ARCH $STD; ./a.out | wc -l; done; done 
-m32   -std=gnu11 499718
-m32     -std=c11 499718
-m64   -std=gnu11 11554
-m64     -std=c11 11554

On an armhf platform, the count is:

$ for STD in -std=gnu11 -std=c11 ; do printf "%12s " $STD; gcc $STD into.c && ./a.out | wc -l; done
  -std=gnu11 11554
    -std=c11 11554