vmem and hdr

The virtual memory system (vmm) is driven off the following two structures:

typedef struct vmem{
  unsigned signature;
  unsigned dirty;
  unsigned ele_size;
  unsigned ele_per_blk;
  long num_ele;
  unsigned cblock;
  char *buf;
} vmem;

typedef struct hdr {
  int active;
  int nrefs;
  unsigned nblocks;
  unsigned offset;
  union { struct hdr *h; //if hdr is in freelist (active=0)
        struct vmem *v;  //virt. mem. element (active = 1)
  } next;
} hdr;

It helps to think of the virtual memory file as a long array of unsigned chars. The hdr structure points to a region in the file. The region may be used or free for use. The used regions of the file are referenced by a vmem structure. The unused regions are kept in a circular linked list of headers. These structures are described in Kernighan and Ritchie. The elements of the hdr structure are:

active: An indicator that the header points to a vmem (1) or is in the freelist (0). It is used for error trapping and debugging.
nrefs: Indicator for the number of matrices using this hdr. See the chapter on the stack for how this variable is used.
nblocks: number of blocks pointed to by the hdr
offset: the offset in the virtual file of the first block pointed to.
next: The next header in the freelist, or the vmem being pointed to

The hdr is analogous to a pointer returned by malloc() of freed by free().

The vmem structure contains the information for an active hdr. The vmm file is organized in blocks of size BLK_SIZE. ^3.2 The elements of the vmem are:

signature: an indicator that the matrix has not been corrupted by heap overwrites. It is 0x5a5a when active, and set to zero when it is freed. This leaves a zero on the heap in case the freed pointer is referenced.
dirty: An indicator that an element in the page buffer has been changed. If this flag is set, then write the buffer out to disk before reading in a new page.
ele_size: size in bytes of an element being stored
ele_per_block: number of elements per block
num_ele: number of elements pointed to
cblock: block currently in buffer
buf: a pointer to the page buffer.

The function vmalloc() creates a hdr and a vmem. The function vfree() frees the vmem and places the hdr in the freelist. Freed space in the file is coalesced with neighboring free blocks.