BIP 42

BIP-42 is the consensus fix that closed a subtle bug in Bitcoin's original supply code. The bug, discovered by Pieter Wuille in 2014, was that integer arithmetic in the original GetBlockSubsidy function would, after enough halvings, overflow and start producing positive subsidies again - meaning Bitcoin would inflate indefinitely starting around year 2214.

The original code looked roughly like:

int64_t nSubsidy = 50 * COIN;
nSubsidy >>= (nHeight / 210000);  // halve for each 210,000-block era

The >> operator is C++ right-shift on a 64-bit signed integer. After ~64 halvings, the shift would technically be undefined behavior; in practice on most compilers it would either zero out (correct) or wrap around (catastrophic). Different compilers behaving differently here could cause a chain split.

The fix in BIP-42 was simple: explicitly return zero after enough halvings have occurred, regardless of what the underlying arithmetic would do.

if (halvings >= 64) return 0;

This made the 21-million cap genuinely permanent rather than dependent on undefined C++ behavior. As a consensus change, BIP-42 was deployed as a soft fork (since it tightens behavior to "no, you really can't get a positive subsidy after halving 33").

The episode is a small but instructive moment in Bitcoin's history. The 21M cap isn't an aspiration; it's a property of the code, enforced by every node, every block, forever. BIP-42 made sure the code actually said what everyone thought it said.