$ cat a.c
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

int main(void)
{
        char dent[16] = {0};
        const size_t MAX = 1000;
        double nd = 0, nf = 0;
        size_t i1, i2, i3, i4;
        char *p1, *p2, *p3;
        size_t n1, n2, n3;
        mode_t m1 = 0777;
        mode_t m2 = 0666;
        int rt;
        int errnum;
        clock_t start, end;

        start = clock();
        for (i1 = 0; i1 != MAX; i1++){
                snprintf(dent, sizeof dent, "%03d/", i1);
                rt = mkdir(dent, m1);
                errnum = errno;
                nd++;
                if (rt == -1){
                        printf("mkdir: %s, directory: %s, cnt: %.f, file cnt: %.f\n", strerror(errnum), dent, nd, nf);
                        return -1;
                }
                n1 = strlen(dent);
                p1 = dent + n1;
                for (i2 = 0; i2 != MAX; i2++){
                        snprintf(p1, sizeof dent - n1, "%03d/", i2);
                        rt = mkdir(dent, m1);
                        errnum = errno;
                        nd++;
                        if (rt == -1){
                                printf("mkdir: %s, directory: %s, cnt: %.f, file cnt: %.f\n", strerror(errnum), dent, nd, nf);
                                return -1;
                        }
                        n2 = strlen(dent);
                        p2 = dent + n2;
                        for (i3 = 0; i3 != MAX; i3++){
                                snprintf(p2, sizeof dent - n2, "%03d/", i3);
                                rt = mkdir(dent, m1);
                                errnum = errno;
                                nd++;
                                if (rt == -1){
                                        printf("mkdir: %s, directory: %s, cnt: %.f, file cnt: %.f\n", strerror(errnum), dent, nd, nf);
                                        return -1;
                                }
                                n3 = strlen(dent);
                                p3 = dent + n3;
                                for (i4 = 0; i4 != MAX; i4++){
                                        snprintf(p3, sizeof dent - n3, "%03d", i4);
                                        rt = creat(dent, m2);
                                        errnum = errno;
                                        close(rt);
                                        nf++;
                                        if (rt == -1){
                                                printf("creat: %s, file: %s, cnt: %.f, directory cnt: %.f\n", strerror(errnum), dent, nf, nd);
                                                return -1;
                                        }
                                }
                        }
                }
        }
        end = clock();
        printf("Time Elapsed: %lu, file cnt: %.f, directory cnt: %.f\n", (unsigned long)(end - start) / CLOCKS_PER_SEC, nf, nd);
        return 0;
}

$
$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              19G  2.6G   16G  15% /
tmpfs                 125M     0  125M   0% /lib/init/rw
udev                  121M  160K  120M   1% /dev
tmpfs                 125M     0  125M   0% /dev/shm
$ df -iT
Filesystem    Type    Inodes   IUsed   IFree IUse% Mounted on
/dev/sda1     ext3   1256640   41395 1215245    4% /
tmpfs        tmpfs     31843       5   31838    1% /lib/init/rw
udev         tmpfs     30732     587   30145    2% /dev
tmpfs        tmpfs     31843       1   31842    1% /dev/shm
$ make
gcc -std=c99 -Wall -W -I ../include  -c -o a.o a.c
gcc  a.o -o main.out
$ echo > a.txt
$ ./main.out
creat: No space left on device, file: 000/001/214/026, cnt: 1214027, directory cnt: 1218
$ echo > b.txt
-bash: b.txt: No space left on device
$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              19G  2.7G   16G  15% /
tmpfs                 125M     0  125M   0% /lib/init/rw
udev                  121M  160K  120M   1% /dev
tmpfs                 125M     0  125M   0% /dev/shm
$ df -iT
Filesystem    Type    Inodes   IUsed   IFree IUse% Mounted on
/dev/sda1     ext3   1256640 1256640       0  100% /
tmpfs        tmpfs     31843       5   31838    1% /lib/init/rw
udev         tmpfs     30732     587   30145    2% /dev
tmpfs        tmpfs     31843       1   31842    1% /dev/shm
$ ls
000  a.c  a.d  a.o  a.txt  main.out  Makefile
$ rm -fr 000
$ echo > b.txt
$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              19G  2.6G   16G  15% /
tmpfs                 125M     0  125M   0% /lib/init/rw
udev                  121M  160K  120M   1% /dev
tmpfs                 125M     0  125M   0% /dev/shm
$ df -iT
Filesystem    Type    Inodes   IUsed   IFree IUse% Mounted on
/dev/sda1     ext3   1256640   41397 1215243    4% /
tmpfs        tmpfs     31843       5   31838    1% /lib/init/rw
udev         tmpfs     30732     587   30145    2% /dev
tmpfs        tmpfs     31843       1   31842    1% /dev/shm
$