suba(3m)

NAME
suba - Sub-allocate memory from larger chunk of memory.

SYNOPSIS
#include <mba/suba.h>

struct allocator *suba_init(void *mem, size_t size, int rst, size_t mincell);
void *suba_alloc(struct allocator *suba, size_t size, int zero);
int suba_free(struct allocator *suba, void *ptr);
void *suba_addr(const struct allocator *suba, const ref_t ref);
ref_t suba_ref(const struct allocator *suba, const void *ptr);

DESCRIPTION
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
    user_lookup_by_id(const char *filename, int id, tchar *user)
    {
    	char buf[0x3FFF]; /* 16K */
    	struct allocator *al = suba_init(buf, 0x3FFF, 1, 0);
    	struct cfg *cfg = cfg_new(al);
    	if (cfg == NULL || cfg_load(cfg, filename) == -1 ||
    	        cfg_vget_str(cfg, user, 64, NULL, _T("users.%d"), id) == -1) {
    	    return -1;
    	}
    	return 0; /* no cleanup necessary; all memory on stack. */
    }

init
    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.

alloc
    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.

free
    The suba_free function frees the memory pointed to by ptr back into the allocator identified by suba parameter.

addr
    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.

ref 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 theSuba functionssection for a description of when it is necessary to convert pointer to a reference.

RETURNS

init
    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.

free
    On success, 0 is returned. On error, -1 is returned, and errno is set appropriately.

addr
    The suba_addr function returns a pointer to the object referenced by ref or NULL if the reference was invalid.

ref The suba_ref function returns an offset to the object pointed to by ptr or 0 if the pointer was invalid.