The bin-search loop in add_count() assumes nelm*wdth == ulim-llim, but wdth is a rounded quotient so nelm*wdth can be strictly less than ulim-llim. A SGNL value just below ULIM then leaves the loop with i == nelm+1, and pdest = bptr + nelm writes one epicsUInt32 past the nelm-element buffer. This is reachable over Channel Access by putting a value near ULIM to the SGNL field (special SPC_MOD). Clamp the index to nelm. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>