HTSlib is an implementation of a unified C library for accessing common file formats, such as SAM, CRAM and VCF, used for high-throughput sequencing data, and is the core library used by samtools and bcftools.

HTSlib implements a generalized BAM index, with file extension .csi (coordinate-sorted index). The HTSlib file reader first looks for the new index and then for the old if the new index is absent.

This project also includes the popular tabix indexer, which indexes both .tbi and .csi formats, and the bgzip compression utility.