A salt is random data in the form of a non-negative exact integer used as an additional input to a hash function in order to defend against dictionary attacks, or (when used in hash tables) against denial-of-service attacks that overcrowd certain hash buckets, increasing the amortized O(1) lookup time to O(n). Salt can also be used to specify which of a family of hash functions should be used for purposes such as cuckoo hashing. This macro provides the current value of the salt, typically for use by user-written hash functions. However, they are not required to make use of the current salt.
The initial value is implementation-dependent, but must be less than the value of (hash-bound), and should be distinct for distinct runs of a program unless otherwise specified by the implementation. Implementations may provide a means to specify the salt value to be used by a particular invocation of a hash function.