From Genome Analysis Wiki
→Read only Certain Sections of the File / Using a VCF Index (TABIX) File
bool setReadSection(const char* chromName);
bool set1BasedReadSection(const char* chromName,
int32_t start, int32_t end);
Both methods currently always return true, since in the current implementation this can't fail. It currently allows the VcfIndex file to be read after this call.
<code>set1BasedReadSection</code> will set the code to read the specified chromosome starting at the specified 1-based position up to, but not including, the specified 1-based end position. On the first <code>readRecord</code> call it will jump the file to the beginning of this section. It will continue to read the section for each <code>readRecord</code> call made until it has read the entire section. Once the entire section has been read, <code>readRecord</code> will return false until a new read section is specified.
Implementation NOTE: internally it may read extra records prior to the section, but <code>readRecord</code> will keep reading and will not return until it finds a record in the section or until the section has been passed (no records in the section).