Common questions and problems when using the C API

Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success

It is possible for mysql_store_result() to return NULL following a successful call to mysql_query(). When this happens, it means one of the following conditions occurred:

  • There was a malloc() failure (for example, if the result set was too large).

  • The data couldn't be read (an error occurred on the connection).

  • The query returned no data (for example, it was an INSERT, UPDATE, or DELETE).

You can always check whether the statement should have produced a non-empty result by calling mysql_field_count(). If mysql_field_count() returns zero, the result is empty and the last query was a statement that does not return values (for example, an INSERT or a DELETE). If mysql_field_count() returns a non-zero value, the statement should have produced a non-empty result. See the description of the mysql_field_count() function for an example.

You can test for an error by calling mysql_error() or mysql_errno().

What Results You Can Get from a Query

In addition to the result set returned by a query, you can also get the following information:

  • mysql_affected_rows() returns the number of rows affected by the last query when doing an INSERT, UPDATE, or DELETE. An exception is that if DELETE is used without a WHERE clause, the table is re-created empty, which is much faster! In this case, mysql_affected_rows() returns zero for the number of records affected.

  • mysql_num_rows() returns the number of rows in a result set. With mysql_store_result(), mysql_num_rows() may be called as soon as mysql_store_result() returns. With mysql_use_result(), mysql_num_rows() may be called only after you have fetched all the rows with mysql_fetch_row().

  • mysql_insert_id() returns the ID generated by the last query that inserted a row into a table with an AUTO_INCREMENT index. See mysql_insert_id().

  • Some queries (LOAD DATA INFILE ..., INSERT INTO ... SELECT ..., UPDATE) return additional information. The result is returned by mysql_info(). See the description for mysql_info() for the format of the string that it returns. mysql_info() returns a NULL pointer if there is no additional information.

How to Get the Unique ID for the Last Inserted Row

If you insert a record in a table containing a column that has the AUTO_INCREMENT attribute, you can get the most recently generated ID by calling the mysql_insert_id() function.

You can also retrieve the ID by using the LAST_INSERT_ID() function in a query string that you pass to mysql_query().

You can check if an AUTO_INCREMENT index is used by executing the following code. This also checks if the query was an INSERT with an AUTO_INCREMENT index:

if (mysql_error(&mysql)[0] == 0 &&
    mysql_num_fields(result) == 0 &&
    mysql_insert_id(&mysql) != 0)
{
    used_id = mysql_insert_id(&mysql);
}

The most recently generated ID is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0).

If you want to use the ID that was generated for one table and insert it into a second table, you can use SQL statements like this:

INSERT INTO foo (auto,text)
    VALUES(NULL,'text');              # generate ID by inserting NULL
INSERT INTO foo2 (id,text)
    VALUES(LAST_INSERT_ID(),'text');  # use ID in second table

Problems Linking with the C API

When linking with the C API, the following errors may occur on some systems:

gcc -g -o client test.o -L/usr/local/lib/mysql -lmysqlclient -lsocket -lnsl
Undefined        first referenced
 symbol          in file
floor            /usr/local/lib/mysql/libmysqlclient.a(password.o)
ld: fatal: Symbol referencing errors. No output written to client

If this happens on your system, you must include the math library by adding -lm to the end of the compile/link line.