Memory leaks and other heap bugs

/* 1. Leak: the only pointer is lost */
char *buf = malloc(100);
buf = malloc(200);            /* the first 100 bytes can never be freed */

/* 2. Use after free */
int *p = malloc(sizeof *p);
free(p);
*p = 5;                       /* writes into memory that may belong to someone else now */

/* 3. Double free */
free(p);
free(p);                      /* corrupts the allocator; often an immediate crash */

/* 4. Heap buffer overflow */
int *a = malloc(4 * sizeof *a);
a[4] = 1;                     /* one past the end */

/* 5. Freeing what was not allocated */
int x;
free(&x);                     /* x is on the stack */

/* 6. Uninitialised read */
int *q = malloc(sizeof *q);
if (*q > 0) { }               /* garbage decides the branch */

gcc -g -O0 prog.c -o prog
valgrind --leak-check=full ./prog

==12345== Invalid write of size 4
==12345==    at 0x10916D: main (prog.c:14)
==12345==  Address 0x4a8e050 is 0 bytes after a block of size 16 alloc'd
==12345==    at 0x483B7F3: malloc (vg_replace_malloc.c:309)
==12345==    by 0x109156: main (prog.c:13)
…
==12345== 100 bytes in 1 blocks are definitely lost in loss record 1 of 1
==12345==    at 0x483B7F3: malloc (vg_replace_malloc.c:309)
==12345==    by 0x109146: main (prog.c:9)

gcc -g -fsanitize=address prog.c -o prog && ./prog
==… ==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000000010
WRITE of size 4 at 0x602000000010 thread T0
    #0 main prog.c:20
freed by thread T0 here:
    #1 main prog.c:19
previously allocated by thread T0 here:
    #1 main prog.c:18