出现Oracle ORA 24343的一种情况

    对于Oracle ORA-24343这种错误，Oracle的官方解释是：
Error Message: ORA-24343 user defined callback error
Error Cause:
The only valid return value for a user defined callback function is OCI_ CONTINUE. Any other value will cause this error.
Action:
Make sure that OCI_CONTINUE is returned from the user-defined callback function.
最近用Pro*C编写访问Oracle的数据库的程序，遇到了这个错误，最后发现导致这个错误的一个原因是
   “宿主变量”的长度分配不够。
如：
    EXEC SQL BEGIN DECLARE SECTION;
        char completion_time[30];
        char completion_time2[30];
        char SQLSTR[500];
    EXEC SQL END DECLARE SECTION;   
 string SQL = "select to_char(SWITCH.scan_start_time,'YYYY-MM-DD HH24:MI:SS'), (SWITCH.innho_att - SWITCH.innho_fail) / SWITCH.innho_att, (SWITCH.extin_att + SWITCH.extou_att - SWITCH.extin_fail - SWITCH.extou_fail) / (SWITCH.extin_att + SWITCH.extou_att) from u_msc_h_ho@pmdb SWITCH where SWITCH.mscname = :equipe_name and (SWITCH.scan_start_time between to_date(:completion_time, 'YYYY-MM-DD HH24:MI:SS') and to_date(:completion_time, 'YYYY-MM-DD HH24:MI:SS') + 13) and SWITCH.innho_att is not null and SWITCH.innho_fail is not null and SWITCH.extin_att is not null and SWITCH.extin_fail is not null and SWITCH.extou_att is not null and SWITCH.extou_fail is not null and SWITCH.extin_att + SWITCH.extou_att != 0 and SWITCH.innho_att != 0 and to_char(SWITCH.scan_start_time, 'HH24') in ('08','09','10','11','18','19','20','21') ";
   
    strcpy(SQLSTR, SQL.c_str());
   
    EXEC SQL PREPARE SQLCursor from :SQLSTR;
    EXEC SQL DECLARE GetSwitchSceneDataCursor CURSOR for SQLCursor;
   
    EXEC SQL OPEN GetSwitchSceneDataCursor using :equipe_name, :com