All compile time constants are defined in the header file
bc.h. The following constants may be altered
for different hardware environments. The values used in the
example given are from the original UNIX implementation.
- ZBPW
The number of bytes in a word.
- ZBYTEW
The number of bits in a byte.
- ZMXBLK
Maximum number of in-memory disk blocks that can be stored. The minimum value for this parameter is 3; there is no maximum. The more in-memory blocks defined, the lower the disk I/O requirements will be.
- ZBLKSZ
The number of bytes allocated to a disk block. This value should be set to a multiple of the physical disk block size. This must be defined as a power of two.
- ZKEYSZ
The maximum size (in bytes) of a key.
- ZTHRES
Threshold for block joining. This value determines the number of free key slots that must exist before two blocks are considered candidates for joining.
- ZMXACT
The maximum number of B Tree files that may be open concurrently.
- ZSLEEP
The number of seconds to wait for a B Tree file to become unlocked, when in shared mode.
- ZRNAMESZ
The maximum number of bytes returned for the name of the function reporting a BT error (via btcerr)/
- ZMSGSZ
The maximum number of bytes returned for the error message of the corresponding to a BT library error (via btcerr).
These compile time constants are assigned the following values in the distributed version:
ZBPW = 4
ZBYTEW = 8
ZMXBLK = 3 (16 when LFS=1)
ZBLKSZ = 1024 (8192 when LFS=1)
ZKEYSZ = 32
ZTHRES = 3
ZMXACT = 5
ZSLEEP = 5
ZRNAMESZ = 16
ZMSGSZ = 123
These values result in the minimum memory usage. If memory is not a constraint, increasing the values for ZMXBLK and ZBLKSZ will make the B Tree implementation much faster, e.g try:
ZMXBLK = 100
ZBLKSZ = 8192
The number of keys that can be stored in a block is determined at compile time, using the following definition:
#define ZMXKEY ((ZBLKSZ-ZBPW-ZINFSZ*ZBPW)/(ZKYLEN+2*ZBPW))
N.B. ZINFSZ is the number of information words that a block must carry as overhead. This value is five in this implementation.