Where is the "space itself" in the storage distributor described at the end of the K & R book?

Question

At the very end of the book by Kernighan and Ritchie, written in the C programming language, a memory allocator is described. It says:

Each block contains a size, a pointer to the next block, and space in itself.

But I do not see this in the code:

typedef long Align;  /* for alignment to long boundary */

union header {       /* block header */
   struct {
      union header *ptr; /* next block if on free list */
      unsigned size;     /* size of this block */
   } s;
   Align x;          /* force alignment of blocks */
};

typedef union header Header;

Pointer to the next block *ptr, and the size unsigned size, but which variable is the space itself? Is space itself a variable x?

Answer 1

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void *malloc(unsigned nbytes)
{
    /* ... */
    nunits = (nbytes+sizeof(Header)-1)/sizeof(Header) + 1;

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Answer 2

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{
    …
    nunits = (nbytes+sizeof(Header)-1)/sizeof(header) + 1;

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