int sqlite3_blob_read(sqlite3_blob *, void *Z, int N, int iOffset);
This function is used to read data from an open BLOB handle into a caller-supplied buffer. N bytes of data are copied into buffer Z from the open BLOB, starting at offset iOffset.
If offset iOffset is less than N bytes from the end of the BLOB, SQLITE_ERROR is returned and no data is read. If N or iOffset is less than zero, SQLITE_ERROR is returned and no data is read. The size of the blob (and hence the maximum value of N+iOffset) can be determined using the sqlite3_blob_bytes() interface.
This routine only works on a BLOB handle which has been created by a prior successful call to sqlite3_blob_open() and which has not been closed by sqlite3_blob_close(). Passing any other pointer in to this routine results in undefined and probably undesirable behavior.
See also: sqlite3_blob_write().