@param idx Index to be written @param fn Input BAM/BCF/etc filename @param fnidx Output filename, or NULL to add .bai/.csi/etc to @a fn @param fmt One of the HTS_FMT_* index formats @return 0 if successful, or negative if an error occurred.
See Implementation
@param idx Index to be written @param fn Input BAM/BCF/etc filename @param fnidx Output filename, or NULL to add .bai/.csi/etc to @a fn @param fmt One of the HTS_FMT_* index formats @return 0 if successful, or negative if an error occurred.