Man page 9 : register_blkdev

register_blkdev - register a new block device

int register_blkdev(unsigned int  major , const char *  name );

major
    the requested major device number [1..255]. If major=0, try to allocate any unused major number.

name
    the name of the new block device as a zero terminated string

The name must be unique within the system.

The return value depends on the major input parameter. - if a major device number was requested in range [1..255] then the function returns zero on success, or a negative error code - if any unused major number was requested with major=0 parameter then the return value is the allocated major number in range [1..255] or a negative error code otherwise