A simple program to allocate and free heap memory:

int main(int argc, char **argv) {
    char *b1, *b2, *b3, *b4, *b_large;
    b1 = malloc(8);
    memset(b1, 0xaa, 8);
    b2= malloc(16);
    memset(b2, 0xbb, 16);
    b3 = malloc(25);
    memset(b3, 0xcc, 25);
    b4= malloc(1000);
    memset(b4, 0xdd, 1000);
    free(b1);
    free(b2);
    free(b3);
    free(b4);

Before first free():

(gdb) x/20gx  0x555555559290 
0x555555559290: 0x0000000000000000  0x0000000000000021
0x5555555592a0: 0xaaaaaaaaaaaaaaaa  0x0000000000000000
0x5555555592b0: 0x0000000000000000  0x0000000000000021
0x5555555592c0: 0xbbbbbbbbbbbbbbbb  0xbbbbbbbbbbbbbbbb
0x5555555592d0: 0x0000000000000000  0x0000000000000031
0x5555555592e0: 0xcccccccccccccccc  0xcccccccccccccccc
0x5555555592f0: 0xcccccccccccccccc  0x00000000000000cc
0x555555559300: 0x0000000000000000  0x00000000000003f1
0x555555559310: 0xdddddddddddddddd  0xdddddddddddddddd
0x555555559320: 0xdddddddddddddddd  0xdddddddddddddddd

And after first free():

(gdb) x/20gx  0x555555559290 
0x555555559290: 0x0000000000000000  0x0000000000000021
0x5555555592a0: 0x0000000555555559  0xd13e7903c502febc
0x5555555592b0: 0x0000000000000000  0x0000000000000021
0x5555555592c0: 0xbbbbbbbbbbbbbbbb  0xbbbbbbbbbbbbbbbb
0x5555555592d0: 0x0000000000000000  0x0000000000000031
0x5555555592e0: 0xcccccccccccccccc  0xcccccccccccccccc
0x5555555592f0: 0xcccccccccccccccc  0x00000000000000cc
0x555555559300: 0x0000000000000000  0x00000000000003f1
0x555555559310: 0xdddddddddddddddd  0xdddddddddddddddd
0x555555559320: 0xdddddddddddddddd  0xdddddddddddddddd

I was expecting to see readable forward and back pointers in the second line of memory, and
in the third line 0x20 in both 8-bytes segments.

Can anyone explain why the free() function would behave in this way?