15. Suba

The suba(3m) module provides a "sub-allocator" that can allocate and free memory from a larger fixed size chunk of memory. This allocator is lock-less but reentrant meaning it will be faster but more consideration is necessary for coordinating multiple threads as opposed to the standard C library allocator.

All objects within the allocator are tracked using offsets relative to the beginning of the sub-allocator and all offsets and state are stored as part of the memory being sub-allocated. Thus the memory backing the allocator can be copied and deleted without being deinitialized to achive a variety of useful effects. The memory of an allocator can be copied temporarily to implement transaction control or checkpoints. Complex data structures can be manipulated by multiple processes in shared memory. When used with the POSIX mmap(2) function (or Windows equivalent), sophisticated (but non-portable) data archives can be created easily.

A very simple and effective use for suba(3m) is as a sub-allocator of stack memory that is implicitly freed when the function returns as the follow code example illustrates:


  int
  myfn(void)
  {
      unsigned char chunk[0x3FFF]; /* 16K */
      struct allocator *al = suba_init(chunk, 0x3FFF, 1, 0);
      struct hashmap map;
  
      hashmap_init(&map, 0, hash_text, cmp_text, NULL, al);
  
      /* use hashmap and allocator ... */
  
      return 0; /* no cleanup necessary; all memory on stack. */
  }

15.1. Suba functions

These functions should be used to initialize a new or existing suba allocator as well as to allocate and free objects from the allocator.

Important: When sharing objects between multiple processes it is important to note that it is not possible to share pointers between processes[1]. However, if the pointer points to an object allocated from a suba allocator it is possible to convert the pointer to an offset relative to the beginning of the sub-allocator using the suba_ref function. The ref_t returned can be passed between processes or stored in a file and later converted back to a valid pointer using the analygous suba_addr function.

[1] Actually, it is possible to share pointers between multiple processes if the pointer points to shared memory and it can be guaranteed that that memory is mapped at the same address in each process. The mmap(2) function accepts a parameter specifying the address at which the memory should be mapped but this is only a hint and use of that option is discouraged. Memory mapped in a parent process, however, will be inherited by children which does meet this guarantee. Pointers to constant data inherited by child processes may also be referenced even though it may not be shared.

The suba_init function
Synopsis


  #include <mba/suba.h>
  struct allocator *suba_init(void *mem, size_t size, int rst, size_t mincell);

Description
The suba_init function initializes a new sub-allocator or attaches to an existing allocator. The memory pointed to by the mem parameter must be at least size bytes. When the rst parameter is non-zero, the beginning of this memory is "reset" meaning it is initialized with the struct allocator structure (discarding any existing allocation state). The remaining memory, which is size bytes minus the header, constitutes the "heap" from which memory will be allocated and freed. If the rst parameter is zero, the existing header is used which presumably came from shared memory or a disk file. If the mincell parameter is non-zero, no memory "cell" will be less than this value (i.e. if mincell is 32 alloc-ing 5 bytes results in a 32 byte cell to back it). The mincell parameter will be increased to accomodate memory alignment requirements if necessary. Larger values for mincell can be faster but results in poorer memory utilization.
Returns
The suba_init function returns a sub-allocator that can be used directly with the other suba(3m) functions or with the more general allocator(3m) functions used by other modules in this package. If an error occurs a null pointer is returned and errno is set accordingly.

The suba_alloc function
Synopsis


  #include <mba/suba.h>
  void *suba_alloc(struct allocator *suba, size_t size, int zero);

Description
The suba_alloc function returns a pointer to memory of size bytes from the sub-allocator identified by the suba parameter. If the zero parameter is non-zero, the memory will be set to zero.

The suba_free function
Synopsis


  #include <mba/suba.h>
  int suba_free(struct allocator *suba, void *ptr);

Description
The suba_free function frees the memory pointed to by ptr back into the allocator identified by suba parameter.
Returns
On success, 0 is returned. On error, -1 is returned, and errno is set appropriately.

The suba_addr function
Synopsis


  #include <mba/suba.h>
  void *suba_addr(const struct allocator *suba, const ref_t ref);

Description
The suba_addr function converts an offset, relative to the beginning of the sub-allocator, of the object idenfied by the ref parameter to a pointer in the current processes address space. This function is equivalent to the expression (char *)suba + ref but with bounds checking.
Returns
The suba_addr function returns a pointer to the object referenced by ref or NULL if the reference was invalid.

The suba_ref function
Synopsis


  #include <mba/suba.h>
  ref_t suba_ref(const struct allocator *suba, const void *ptr);

Description
The suba_ref function converts a pointer ptr that points to an object allocated from the sub-allocator identified by the suba parameter to an offset relative to the beginning of the sub-allocator. This function is equivalent to the expression (char *)ptr - (char *)suba but with bounds checking. See the Suba functions section for a description of when it is necessary to convert pointer to a reference.
Returns
The suba_ref function returns an offset to the object pointed to by ptr or 0 if the pointer was invalid.