MySQL Reference Manual for version 3.23.5-alpha. - 20.4 C API function descriptions

20.4 C API function descriptions

In the descriptions below, a parameter or return value of NULL means NULL in the sense of the C programming language, not a MySQL NULL value.

Functions that return a value generally return a pointer or an integer. Unless specified otherwise, functions returning a pointer return a non-NULL value to indicate success or a NULL value to indicate an error, and functions returning an integer return zero to indicate success or non-zero to indicate an error. Note that ``non-zero'' means just that. Unless the function description says otherwise, do not test against a value other than zero:

if (result)                   /* correct */
    ... error ...

if (result < 0)               /* incorrect */
    ... error ...

if (result == -1)             /* incorrect */
    ... error ...

When a function returns an error, the Errors subsection of the function description lists the possible types of errors. You can find out which of these occurred by calling mysql_errno(). A string representation of the error may be obtained by calling mysql_error().

20.4.1 `mysql_affected_rows()`

my_ulonglong mysql_affected_rows(MYSQL *mysql)

20.4.1.1 Description

Returns the number of rows affected (changed) by the last UPDATE, DELETE or INSERT query. May be called immediately after mysql_query() for UPDATE, DELETE or INSERT statements. For SELECT statements, mysql_affected_rows() works like mysql_num_rows().

mysql_affected_rows() is currently implemented as a macro.

20.4.1.2 Return values

An integer greater than zero indicates the number of rows affected or retrieved. Zero indicates that no records matched the WHERE clause in the query or that no query has yet been executed. -1 indicates that the query returned an error or that, for a SELECT query, mysql_affected_rows() was called prior to calling mysql_store_result().

20.4.1.3 Errors

None.

20.4.1.4 Example

mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
printf("%d products updated",mysql_affected_rows(&mysql));

20.4.2 `mysql_close()`

void mysql_close(MYSQL *mysql)

20.4.2.1 Description

Closes a previously opened connection. mysql_close() also deallocates the connection handle pointed to by mysql if the handle was allocated automatically by mysql_init() or mysql_connect().

20.4.2.2 Return values

None.

20.4.2.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.3 `mysql_connect()`

MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)

20.4.3.1 Description

This function is deprecated. It is preferable to use mysql_real_connect() instead.

mysql_connect() attempts to establish a connection to a MySQL database engine running on host. mysql_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

The meanings of the parameters are the same as for the corresponding parameters for mysql_connect() with the difference that the connection paramater may be NULL. In this case the C API allocates memory for the connection structure automatically and frees it when you call mysql_close(). The disadvantage of this approach is that you can't retrieve an error message if the connection fails. (To get error information from mysql_errno() or mysql_error(), you must provide a valid MYSQL pointer.)

20.4.3.2 Return values

Same as for mysql_real_connect().

20.4.3.3 Errors

Same as for mysql_real_connect().

20.4.4 `mysql_change_user()`

my_bool mysql_change_user(MYSQL *mysql, const char *user, const char *password, const char *db)

20.4.4.1 Description

Changes the user and causes the database specified by db to become the default (current) database on the connection specified by mysql. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

This function was introduced in MySQL 3.23.3.

mysql_change_user() fails unless the connected user can be authenticated or if he doesn't have permission to use the database. In this case the user and database are not changed

The db parameter may be set to NULL if you don't want to have a default database.

20.4.4.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.4.3 Errors

The same that you can get from mysql_real_connect().

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.
ER_UNKNOWN_COM_ERROR: The MySQL server doesn't implement this command (probably an old server)
ER_ACCESS_DENIED_ERROR: The user or password was wrong.
ER_BAD_DB_ERROR: The database didn't exists.
ER_DBACCESS_DENIED_ERROR: The user did not have access rights to the database.
ER_WRONG_DB_NAME: The database name was too long.

20.4.4.4 Example

if (mysql_change_user(&mysql, "user", "password", "new_database"))
{
   fprintf(stderr, "Failed to change user.  Error: %s\n",
           mysql_error(&mysql));
}

20.4.5 `mysql_create_db()`

int mysql_create_db(MYSQL *mysql, const char *db)

20.4.5.1 Description

Creates the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL CREATE DATABASE statement instead.

20.4.5.2 Return values

Zero if the database was created successfully. Non-zero if an error occurred.

20.4.5.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.5.4 Example

if(mysql_create_db(&mysql, "my_database"))
{
   fprintf(stderr, "Failed to create new database.  Error: %s\n",
           mysql_error(&mysql));
}

20.4.6 `mysql_data_seek()`

void mysql_data_seek(MYSQL_RES *result, unsigned int offset)

20.4.6.1 Description

Seeks to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so mysql_data_seek() may be used in conjunction only with mysql_store_result(), not with mysql_use_result().

The offset should be a value in the range from 0 to mysql_num_rows(result)-1.

20.4.6.2 Return values

None.

20.4.6.3 Errors

None.

20.4.7 `mysql_debug()`

void mysql_debug(char *debug)

20.4.7.1 Description

Does a DBUG_PUSH with the given string. mysql_debug() uses the Fred Fish debug library. To use this function, you must compile the client library to support debugging. G.1 Debugging a MySQL server. G.2 Debugging a MySQL client.

20.4.7.2 Return values

None.

20.4.7.3 Errors

None.

20.4.7.4 Example

The call shown below causes the client library to generate a trace file in `/tmp/client.trace' on the client machine:

mysql_debug("d:t:O,/tmp/client.trace");

20.4.8 `mysql_drop_db()`

int mysql_drop_db(MYSQL *mysql, const char *db)

20.4.8.1 Description

Drops the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL DROP DATABASE statement instead.

20.4.8.2 Return values

Zero if the database was dropped successfully. Non-zero if an error occurred.

20.4.8.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.8.4 Example

if(mysql_drop_db(&mysql, "my_database"))
  fprintf(stderr, "Failed to drop the database: Error: %s\n",
          mysql_error(&mysql));

20.4.9 `mysql_dump_debug_info()`

int mysql_dump_debug_info(MYSQL *mysql)

20.4.9.1 Description

Instructs the server to write some debug information to the log. The connected user must have the process privilege for this to work.

20.4.9.2 Return values

Zero if the command was successful. Non-zero if an error occurred.

20.4.9.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.10 `mysql_eof()`

my_bool mysql_eof(MYSQL_RES *result)

20.4.10.1 Description

This function is deprecated. mysql_errno() or mysql_error() may be used instead.

mysql_eof() determines whether or not the last row of a result set has been read.

If you acquire a result set from a successful call to mysql_store_result(), the client receives the entire set in one operation. In this case, a NULL return from mysql_fetch_row() always means the end of the result set has been reached and it is unnecessary to call mysql_eof().

On the other hand, if you use mysql_use_result() to initiate a result set retrieval, the rows of the set are obtained from the server one by one as you call mysql_fetch_row() repeatedly. Because an error may occur on the connection during this process, a NULL return value from mysql_fetch_row() does not necessarily mean the end of the result set was reached normally. In this case, you can use mysql_eof() to determine what happened. mysql_eof() returns a non-zero value if the end of the result set was reached and zero if an error occurred.

Historically, mysql_eof() predates the standard MySQL error functions mysql_errno() and mysql_error(). Since those error functions provide the same information, their use is preferred over mysql_eof(), which is now deprecated. (In fact, they provide more information, since mysql_eof() returns only a boolean value whereas the error functions indicate a reason for the error when one occurs.)

20.4.10.2 Return values

Zero if an error occurred. Non-zero if the end of the result set has been reached.

20.4.10.3 Errors

None.

20.4.10.4 Example

The following example shows how you might use mysql_eof():

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(!mysql_eof(result))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

However, you can achieve the same effect with the standard MySQL error functions:

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(mysql_errno(&mysql))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

20.4.11 `mysql_errno()`

unsigned int mysql_errno(MYSQL *mysql)

20.4.11.1 Description

For the connection specified by mysql, mysql_errno() returns the error code for the most recently invoked API function that can succeed or fail. A return value of zero means that no error occurred. Client error message numbers are listed in the MySQL `errmsg.h' header file. Server error message numbers are listed in `mysqld_error.h'

20.4.11.2 Return values:

An error code value. Zero if no error occurred.

20.4.11.3 Errors

None.

20.4.12 `mysql_error()`

char *mysql_error(MYSQL *mysql)

20.4.12.1 Description

For the connection specified by mysql, mysql_error() returns the error message for the most recently invoked API function that can succeed or fail. An empty string ("") is returned if no error occurred. This means the following two tests are equivalent:

if(mysql_errno(&mysql))
{
    // an error occurred
}

if(mysql_error(&mysql)[0] != '\0')
{
    // an error occurred
}

The language of the client error messages may be changed by recompiling the MySQL client library. Currently you can choose error messages in several different languages. 9.1 What languages are supported by MySQL?.

20.4.12.2 Return values

A character string that describes the error. An empty string if no error occurred.

20.4.12.3 Errors

None.

20.4.13 `mysql_escape_string()`

unsigned int mysql_escape_string(char *to, const char *from, unsigned int length)

20.4.13.1 Description

Encodes the string in from to an escaped SQL string that can be sent to the server in a SQL statement, places the result in to, and adds a terminating null byte. Characters encoded are NUL (ASCII 0), `\n', `\r', `\', `'', `"' and Control-Z ( 7.1 Literals: how to write strings and numbers).

The string pointed to by from must be length bytes long. You must allocate the to buffer to be at least length*2+1 bytes long. (In the worse case, each character may need to be encoded as using two bytes, and you need room for the terminating null byte.) When mysql_escape_string() returns, the contents of to will be a null-terminated string. The return value is the length of the encoded string, not including the terminating null character.

20.4.13.2 Example

char query[1000],*end;

end = strmov(query,"INSERT INTO test_table values(");
*end++ = '\'';
end += mysql_escape_string(end,"What's this",11);
*end++ = '\'';
*end++ = ',';
*end++ = '\'';
end += mysql_escape_string(end,"binary data: \0\r\n",16);
*end++ = '\'';
*end++ = ')';

if (mysql_real_query(&mysql,query,(unsigned int) (end - query)))
{
   fprintf(stderr, "Failed to insert row, Error: %s\n",
           mysql_error(&mysql));
}

The strmov() function used in the example is included in the mysqlclient library and works like strcpy() but returns a pointer to the terminating null of the first parameter.

20.4.13.3 Return values

The length of the value placed into to, not including the terminating null character.

20.4.13.4 Errors

None.

20.4.14 `mysql_fetch_field()`

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)

20.4.14.1 Description

Returns the definition of one column of a result set as a MYSQL_FIELD structure. Call this function repeatedly to retrieve information about all columns in the result set. mysql_fetch_field() returns NULL when no more fields are left.

mysql_fetch_field() is reset to return information about the first field each time you execute a new SELECT query. The field returned by mysql_fetch_field() is also affected by calls to mysql_field_seek().

If you've called mysql_query() to perform a SELECT on a table but have not called mysql_store_result(), MySQL returns the default blob length (8K bytes) if you call mysql_fetch_field() to ask for the length of a BLOB field. (The 8K size is chosen because MySQL doesn't know the maximum length for the BLOB. This should be made configurable sometime.) Once you've retrieved the result set, field->max_length contains the length of the largest value for this column in the specific query.

20.4.14.2 Return values

The MYSQL_FIELD structure for the current column. NULL if no columns are left.

20.4.14.3 Errors

None.

20.4.14.4 Example

MYSQL_FIELD *field;

while((field = mysql_fetch_field(result)))
{
    printf("field name %s\n", field->name);
}

20.4.15 `mysql_fetch_fields()`

MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES *result)

20.4.15.1 Description

Returns an array of all MYSQL_FIELD structures for a result set. Each structure provides the field definition for one column of the result set.

20.4.15.2 Return values

An array of MYSQL_FIELD structures for all columns of a result set.

20.4.15.3 Errors

None.

20.4.15.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *fields;

num_fields = mysql_num_fields(result);
fields = mysql_fetch_fields(result);
for(i = 0; i < num_fields; i++)
{
   printf("Field %u is %s\n", i, fields[i].name);
}

20.4.16 `mysql_fetch_field_direct()`

MYSQL_FIELD *mysql_fetch_field_direct(MYSQL_RES *result, unsigned int fieldnr)

20.4.16.1 Description

Given a field number fieldnr for a column within a result set, returns that column's field definition as a MYSQL_FIELD structure. You may use this function to retrieve the definition for an arbitrary column. The value of fieldnr should be in the range from 0 to mysql_num_fields(result)-1.

20.4.16.2 Return values

The MYSQL_FIELD structure for the specified column.

20.4.16.3 Errors

None.

20.4.16.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *field;

num_fields = mysql_num_fields(result);
for(i = 0; i < num_fields; i++)
{
    field = mysql_fetch_field_direct(result, i);
    printf("Field %u is %s\n", i, field->name);
}

20.4.17 `mysql_fetch_lengths()`

unsigned long *mysql_fetch_lengths(MYSQL_RES *result)

20.4.17.1 Description

Returns the lengths of the columns of the current row within a result set. If you plan to copy field values, this length information is also useful for optimization, because you can avoid calling strlen(). In addition, if the result set contains binary data, you must use this function to determine the size of the data, because strlen() returns incorrect results for any field containing null characters.

The length for empty columns and for columns containing NULL values is zero. To see how to distinguish these two cases, see the description for mysql_fetch_row().

20.4.17.2 Return values

An array of unsigned long integers representing the size of each column (not including any terminating null characters). NULL if an error occurred.

20.4.17.3 Errors

mysql_fetch_lengths() is valid only for the current row of the result set. It returns NULL if you call it before calling mysql_fetch_row() or after retrieving all rows in the result.

20.4.17.4 Example

MYSQL_ROW row;
unsigned long *lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
{
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
    {
         printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
    }
}

20.4.18 `mysql_fetch_row()`

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)

20.4.18.1 Description

Retrieves the next row of a result set. When used after mysql_store_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve. When used after mysql_use_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve or if an error occurred.

The number of values in the row is given by mysql_num_fields(result). If row holds the return value from a call to mysql_fetch_row(), pointers to the values are accessed as row[0] to row[mysql_num_fields(result)-1]. NULL values in the row are indicated by NULL pointers.

The lengths of the field values in the row may be obtained by calling mysql_fetch_lengths(). Empty fields and fields containing NULL both have length 0; you can distinguish these by checking the pointer for the field value. If the pointer is NULL, the field is NULL; otherwise the field is empty.

20.4.18.2 Return values

A MYSQL_ROW structure for the next row. NULL if there are no more rows to retrieve or if an error occurred.

20.4.18.3 Errors

CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.18.4 Example

MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;

num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
   unsigned long *lengths;
   lengths = mysql_fetch_lengths(result);
   for(i = 0; i < num_fields; i++)
   {
       printf("[%.*s] ", (int) lengths[i], row[i] ? row[i] : "NULL");
   }
   printf("\n");
}

20.4.19 `mysql_field_count()`

unsigned int mysql_field_count(MYSQL *mysql)

If you are using a version of MySQL earlier than 3.22.24, you should use unsigned int mysql_num_fields(MYSQL *mysql) instead.

20.4.19.1 Description

Returns the number of columns for the most recent query on the connection.

The normal use of this function is when mysql_store_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether or not mysql_store_result() should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a SELECT (or SELECT-like) statement. The example shown below illustrates how this may be done.

NULL mysql_store_result().

20.4.19.2 Return values

An unsigned integer representing the number of fields in a result set.

20.4.19.3 Errors

None.

20.4.19.4 Example

MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if(mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
        else // mysql_store_result() should have returned data
        {
            fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
        }
    }
}

An alternative is to replace the mysql_field_count(&mysql) call with mysql_errno(&mysql). In this case, you are checking directly for an error from mysql_store_result() rather than inferring from the value of mysql_field_count() whether or not the statement was a SELECT.

20.4.20 `mysql_field_seek()`

MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)

20.4.20.1 Description

Sets the field cursor to the given offset. The next call to mysql_fetch_field() will retrieve the field definition of the column associated with that offset.

To seek to the beginning of a row, pass an offset value of zero.

20.4.20.2 Return values

The previous value of the field cursor.

20.4.20.3 Errors

None.

20.4.21 `mysql_field_tell()`

MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)

20.4.21.1 Description

Returns the position of the field cursor used for the last mysql_fetch_field(). This value can be used as an argument to mysql_field_seek().

20.4.21.2 Return values

The current offset of the field cursor.

20.4.21.3 Errors

None.

20.4.22 `mysql_free_result()`

void mysql_free_result(MYSQL_RES *result)

20.4.22.1 Description

Frees the memory allocated for a result set by mysql_store_result(), mysql_use_result(), mysql_list_dbs(), etc. When you are done with a result set, you must free the memory it uses by calling mysql_free_result().

20.4.22.2 Return values

None.

20.4.22.3 Errors

None.

20.4.23 `mysql_get_client_info()`

char *mysql_get_client_info(void)

20.4.23.1 Description

Returns a string that represents the client library version.

20.4.23.2 Return values

A character string that represents the MySQL client library version.

20.4.23.3 Errors

None.

20.4.24 `mysql_get_host_info()`

char *mysql_get_host_info(MYSQL *mysql)

20.4.24.1 Description

Returns a string describing the type of connection in use, including the server host name.

20.4.24.2 Return values

A character string representing the server host name and the connection type.

20.4.24.3 Errors

None.

20.4.25 `mysql_get_proto_info()`

unsigned int mysql_get_proto_info(MYSQL *mysql)

20.4.25.1 Description

Returns the protocol version used by current connection.

20.4.25.2 Return values

An unsigned integer representing the protocol version used by the current connection.

20.4.25.3 Errors

None.

20.4.26 `mysql_get_server_info()`

char *mysql_get_server_info(MYSQL *mysql)

20.4.26.1 Description

Returns a string that represents the server version number.

20.4.26.2 Return values

A character string that represents the server version number.

20.4.26.3 Errors

None.

20.4.27 `mysql_info()`

char *mysql_info(MYSQL *mysql)

20.4.27.1 Description

Retrieves a string providing information about the most recently executed query, but only for the statements listed below. For other statements, mysql_info() returns NULL. The format of the string varies depending on the type of query, as described below. The numbers are illustrative only; the string will contain values appropriate for the query.

INSERT INTO ... SELECT ...: String format: Records: 100 Duplicates: 0 Warnings: 0
INSERT INTO ... VALUES (...),(...),(...)...: String format: Records: 3 Duplicates: 0 Warnings: 0
LOAD DATA INFILE ...: String format: Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
ALTER TABLE: String format: Records: 3 Duplicates: 0 Warnings: 0
UPDATE: String format: Rows matched: 40 Changed: 40 Warnings: 0

Note that mysql_info() returns a non-NULL value for the INSERT ... VALUES statement only if multiple value lists are specified in the statement.

20.4.27.2 Return values

A character string representing additional information about the most recently executed query. NULL if no information is available for the query.

20.4.27.3 Errors

None.

20.4.28 `mysql_init()`

MYSQL *mysql_init(MYSQL *mysql)

20.4.28.1 Description

Allocates or initializes a MYSQL object suitable for mysql_real_connect(). If mysql is a NULL pointer, the function allocates, initializes and returns a new object. Otherwise the object is initialized and the address of the object is returned. If mysql_init() allocates a new object, it will be freed when mysql_close() is called to close the connection.

20.4.28.2 Return values

An initialized MYSQL* handle. NULL if there was insufficient memory to allocate a new object.

20.4.28.3 Errors

In case of insufficient memory, NULL is returned.

20.4.29 `mysql_insert_id()`

my_ulonglong mysql_insert_id(MYSQL *mysql)

20.4.29.1 Description

Returns the ID generated for an AUTO_INCREMENT column by the previous query. Use this function after you have performed an INSERT query into a table that contains an AUTO_INCREMENT field.

Note that mysql_insert_id() returns 0 if the previous query does not generate an AUTO_INCREMENT value. If you need to save the value for later, be sure to call mysql_insert_id() immediately after the query that generates the value.

Also note that the value of the SQL LAST_INSERT_ID() function always contains the most recently generated AUTO_INCREMENT value, and is not reset between queries since the value of that function is maintained in the server.

20.4.29.2 Return values

The value of the AUTO_INCREMENT field that was updated by the previous query. Returns zero if there was no previous query on the connection or if the query did not update an AUTO_INCREMENT value.

20.4.29.3 Errors

None.

20.4.30 `mysql_kill()`

int mysql_kill(MYSQL *mysql, unsigned long pid)

20.4.30.1 Description

Asks the server to kill the thread specified by pid.

20.4.30.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.30.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.31 `mysql_list_dbs()`

MYSQL_RES *mysql_list_dbs(MYSQL *mysql, const char *wild)

20.4.31.1 Description

Returns a result set consisting of database names on the server that match the simple regular expression specified by the wild parameter. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all databases. Calling mysql_list_dbs() is similar to executing the query SHOW databases [LIKE wild].

You must free the result set with mysql_free_result().

20.4.31.2 Return values

A MYSQL_RES result set for success. NULL if an error occurred.

20.4.31.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.32 `mysql_list_fields()`

MYSQL_RES *mysql_list_fields(MYSQL *mysql, const char *table, const char *wild)

20.4.32.1 Description

Returns a result set consisting of field names in the given table that match the simple regular expression specified by the wild parameter. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all fields. Calling mysql_list_fields() is similar to executing the query SHOW COLUMNS FROM tbl_name [LIKE wild].

Note that it's recommended that you use SHOW COLUMNS FROM tbl_name instead of mysql_list_fields().

You must free the result set with mysql_free_result().

20.4.32.2 Return values

A MYSQL_RES result set for success. NULL if an error occurred.

20.4.32.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.33 `mysql_list_processes()`

MYSQL_RES *mysql_list_processes(MYSQL *mysql)

20.4.33.1 Description

Returns a result set describing the current server threads. This is the same kind of information as that reported by mysqladmin processlist or a SHOW PROCESSLIST query.

You must free the result set with mysql_free_result().

20.4.33.2 Return values

A MYSQL_RES result set for success. NULL if an error occurred.

20.4.33.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.34 `mysql_list_tables()`

MYSQL_RES *mysql_list_tables(MYSQL *mysql, const char *wild)

20.4.34.1 Description

Returns a result set consisting of table names in the current database that match the simple regular expression specified by the wild parameter. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all tables. Calling mysql_list_tables() is similar to executing the query SHOW tables [LIKE wild].

You must free the result set with mysql_free_result().

20.4.34.2 Return values

A MYSQL_RES result set for success. NULL if an error occurred.

20.4.34.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.35 `mysql_num_fields()`

unsigned int mysql_num_fields(MYSQL_RES *result)

unsigned int mysql_num_fields(MYSQL *mysql)

The second form doesn't work on MySQL 3.22.24 or newer. To pass a MYSQL* argument, you must use unsigned int mysql_field_count(MYSQL *mysql) instead.

20.4.35.1 Description

Returns the number of columns in a result set.

Note that you can get the number of columns either from a pointer to a result set or to a connection handle. You would use the connection handle if mysql_store_result() or mysql_user_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether or not mysql_store_result() should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a SELECT (or SELECT-like) statement. The example shown below illustrates how this may be done.

NULL mysql_store_result().

20.4.35.2 Return values

An unsigned integer representing the number of fields in a result set.

20.4.35.3 Errors

None.

20.4.35.4 Example

MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if (mysql_errno(&mysql))
	{
           fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
	}
        else if (mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
    }
}

An alternative (if you KNOW that your query should have returned a result set) is to replace the mysql_errno(&mysql) call with a check if mysql_field_count(&mysql) is = 0. This will only happen if something went wrong.

20.4.36 `mysql_num_rows()`

my_ulonglong mysql_num_rows(MYSQL_RES *result)

20.4.36.1 Description

Returns the number of rows in the result set.

The use of mysql_num_rows() depends on whether you use mysql_store_result() or mysql_use_result() to return the result set. If you use mysql_store_result(), mysql_num_rows() may be called immediately. If you use mysql_use_result(), mysql_num_rows() will not return the correct value until all the rows in the result set have been retrieved.

20.4.36.2 Return values

The number of rows in the result set.

20.4.36.3 Errors

None.

20.4.37 `mysql_options()`

int mysql_options(MYSQL *mysql, enum mysql_option option, const char *arg)

20.4.37.1 Description

Can be used to set extra connect options and affect behavior for a connection. This function may be called multiple times to set several options.

mysql_options() should be called after mysql_init() and before mysql_connect() or mysql_real_connect().

The option argument is the option that you want to set; the arg argument is the value for the option. If the option is an integer, then arg should point to the value of the integer.

Possible options values:

Option	Argument type	Function
`MYSQL_OPT_CONNECT_TIMEOUT`	`unsigned int *`	Connect timeout in seconds.
`MYSQL_OPT_COMPRESS`	Not used	Use the compressed client/server protocol.
`MYSQL_OPT_NAMED_PIPE`	Not used	Use named pipes to connect to a MySQL server on NT.
`MYSQL_INIT_COMMAND`	`char *`	Command to execute when connecting to the MySQL server. Will automatically be re-executed when reconnecting.
`MYSQL_READ_DEFAULT_FILE`	`char *`	Read options from the named option file instead of from `my.cnf'.
`MYSQL_READ_DEFAULT_GROUP`	`char *`	Read options from the named group from `my.cnf' or the file specified with `MYSQL_READ_DEFAULT_FILE`.

Note that the group client is always read if you use MYSQL_READ_DEFAULT_FILE or MYSQL_READ_DEFAULT_GROUP.

The specified group in the option file may contain the following options:

`compress`	Use the compressed client/server protocol.
`database`	Connect to this database if there no database was specified in the connect command
`debug`	Debug options
`host`	Default host name
`init-command`	Command to execute when connecting to MySQL server. Will automatically be re-executed when reconnecting.
`password`	Default password
`pipe`	Use named pipes to connect to a MySQL server on NT.
`port`	Default port number
`return-found-rows`	Tell `mysql_info()` to return found rows instead of updated rows when using `UPDATE`.
`socket`	Default socket number
`timeout`	Connect timeout in seconds.
`user`	Default user

For more information about option files, see 4.15.4 Option files.

20.4.37.2 Return values

Zero for success. Non-zero if you used an unknown option.

20.4.37.3 Example

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_OPT_COMPRESS,0);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"odbc");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}

The above requests the client to use the compressed client/server protocol and read the additional options from the odbc section in the my.cnf file.

20.4.38 `mysql_ping()`

int mysql_ping(MYSQL *mysql)

20.4.38.1 Description

Checks whether or not the connection to the server is working. If it has gone down, an automatic reconnection is attempted.

This function can be used by clients that remain idle for a long while, to check whether or not the server has closed the connection and reconnect if necessary.

20.4.38.2 Return values

Zero if the server is alive. Non-zero if an error occurred.

20.4.38.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.39 `mysql_query()`

int mysql_query(MYSQL *mysql, const char *query)

20.4.39.1 Description

Executes the SQL query pointed to by the null-terminated string query. The query must consist of a single SQL statement. You should not add a terminating semicolon (`;') or \g to the statement.

mysql_query() cannot be used for queries that contain binary data; you should use mysql_real_query() instead. (Binary data may contain the `\0' character, which mysql_query() interprets as the end of the query string.)

20.4.39.2 Return values

Zero if the query was successful. Non-zero if an error occurred.

20.4.39.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.40 `mysql_real_connect()`

MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned int client_flag)

20.4.40.1 Description

mysql_real_connect() attempts to establish a connection to a MySQL database engine running on host. mysql_real_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

The parameters are specified as follows:

The first parameter should be the address of an existing MYSQL structure. Before calling mysql_real_connect() you must call mysql_init() to initialize the MYSQL structure. See the example below.
The value of host may be either a hostname or an IP address. If host is NULL or the string "localhost", a connection to the local host is assumed. If the OS supports sockets (Unix) or named pipes (Win32), they are used instead of TCP/IP to connect to the server.
The user parameter contains the user's MySQL login ID. If user is NULL, the current user is assumed. Under Unix, this is the current login name. Under Windows ODBC, the current user name must be specified explicitly. 16.4 How to fill in the various fields in the ODBC administrator program.
The passwd parameter contains the password for user. If passwd is NULL, only entries in the user table for the user that have a blank password field will be checked for a match. This allows the database administrator to set up the MySQL privilege system in such a way that users get different privileges depending on whether or not they have specified a password. Note: Do not attempt to encrypt the password before calling mysql_real_connect(); password encryption is handled automatically by the client API.
db is the database name. If db is not NULL, the connection will set the default database to this value.
If port is not 0, the value will be used as the port number for the TCP/IP connection. Note that the host parameter determines the type of the connection.
If unix_socket is not NULL, the string specifies the socket or named pipe that should be used. Note that the host parameter determines the type of the connection.

The value of client_flag is usually 0, but can be set to a combination of the following flags in very special circumstances:

Flag name	Flag meaning
`CLIENT_FOUND_ROWS`	Return the number of found (matched) rows, not the number of affected rows
`CLIENT_NO_SCHEMA`	Don't allow the `db_name.tbl_name.col_name` syntax. This is for ODBC; it causes the parser to generate an error if you use that syntax, which is is useful for trapping bugs in some ODBC programs.
`CLIENT_COMPRESS`	Use compression protocol
`CLIENT_ODBC`	The client is an ODBC client. This changes `mysqld` to be more ODBC-friendly.

20.4.40.2 Return values

A MYSQL* connection handle if the connection was successful. NULL if the connection was unsuccessful. For a successful connection, the return value is the same as the value of the first parameter, unless you pass NULL for that parameter.

20.4.40.3 Errors

CR_CONN_HOST_ERROR: Failed to connect to the MySQL server.
CR_CONNECTION_ERROR: Failed to connect to the local MySQL server.
CR_IPSOCK_ERROR: Failed to create an IP socket.
CR_OUT_OF_MEMORY: Out of memory.
CR_SOCKET_CREATE_ERROR: Failed to create a Unix socket.
CR_UNKNOWN_HOST: Failed to find the IP address for the hostname.
CR_VERSION_ERROR: A protocol mismatch resulted from attempting to connect to a server with a client library that uses a different protocol version. This can happen if you use a very old client library to connect to a new server that wasn't started with the --old-protocol option.
CR_NAMEDPIPEOPEN_ERROR;: Failed to create a named pipe on Win32.
CR_NAMEDPIPEWAIT_ERROR;: Failed to wait for a named pipe on Win32.
CR_NAMEDPIPESETSTATE_ERROR;: Failed to get a pipe handler on Win32.

20.4.40.4 Example

MYSQL mysql;

mysql_init(&mysql);
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}

20.4.41 `mysql_real_query()`

int mysql_real_query(MYSQL *mysql, const char *query, unsigned int length)

20.4.41.1 Description

Executes the SQL query pointed to by query, which should be a string length bytes long. The query must consist of a single SQL statement. You should not add a terminating semicolon (`;') or \g to the statement.

You must use mysql_real_query() rather than mysql_query() for queries that contain binary data, since binary data may contain the `\0' character. In addition, mysql_real_query() is faster than mysql_query() since it does not call strlen() on the query string.

20.4.41.2 Return values

Zero if the query was successful. Non-zero if an error occurred.

20.4.41.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.42 `mysql_reload()`

int mysql_reload(MYSQL *mysql)

20.4.42.1 Description

Asks the MySQL server to reload the grant tables. The connected user must have the reload privilege.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL FLUSH PRIVILEGES statement instead.

20.4.42.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.42.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.43 `mysql_row_seek()`

MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset)

20.4.43.1 Description

Sets the row cursor to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so mysql_row_seek() may be used in conjunction only with mysql_store_result(), not with mysql_use_result().

The offset should be a value returned from a call to mysql_row_tell() or to mysql_row_seek(). This value is not simply a row number; if you want to seek to a row within a result set using a row number, use mysql_data_seek() instead.

20.4.43.2 Return values

The previous value of the row cursor. This value may be passed to a subsequent call to mysql_row_seek().

20.4.43.3 Errors

None.

20.4.44 `mysql_row_tell()`

MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result)

20.4.44.1 Description

Returns the current position of the row cursor for the last mysql_fetch_row(). This value can be used as an argument to mysql_row_seek().

You should use mysql_row_tell() only after mysql_store_result(), not after mysql_use_result().

20.4.44.2 Return values

The current offset of the row cursor.

20.4.44.3 Errors

None.

20.4.45 `mysql_select_db()`

int mysql_select_db(MYSQL *mysql, const char *db)

20.4.45.1 Description

Causes the database specified by db to become the default (current) database on the connection specified by mysql. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

mysql_select_db() fails unless the connected user can be authenticated as having permission to use the database.

20.4.45.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.45.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.46 `mysql_shutdown()`

int mysql_shutdown(MYSQL *mysql)

20.4.46.1 Description

Asks the database server to shutdown. The connected user must have shutdown privileges.

20.4.46.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.46.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.47 `mysql_stat()`

char *mysql_stat(MYSQL *mysql)

20.4.47.1 Description

Returns a character string containing information similar to that provided by the mysqladmin status command. This includes uptime in seconds and the number of running threads, questions, reloads and open tables.

20.4.47.2 Return values

A character string describing the server status. NULL if an error occurred.

20.4.47.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.48 `mysql_store_result()`

MYSQL_RES *mysql_store_result(MYSQL *mysql)

20.4.48.1 Description

You must call mysql_store_result() or mysql_use_result() for every query which successfully retrieves data (SELECT, SHOW, DESCRIBE, EXPLAIN).

mysql_store_result() reads the entire result of a query to the client, allocates a MYSQL_RES structure, and places the result into this structure.

An empty result set is returned if there are no rows returned. (An empty result set differs from a NULL return value.)

Once you have called mysql_store_result(), you may call mysql_num_rows() to find out how many rows are in the result set.

You can call mysql_fetch_row() to fetch rows from the result set, or mysql_row_seek() and mysql_row_tell() to obtain or set the current row position within the result set.

You must call mysql_free_result() once you are done with the result set.

NULL mysql_store_result().

20.4.48.2 Return values

A MYSQL_RES result structure with the results. NULL if an error occurred.

20.4.48.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.49 `mysql_thread_id()`

unsigned long mysql_thread_id(MYSQL *mysql)

20.4.49.1 Description

Returns the thread ID of the current connection. This value can be used as an argument to mysql_kill() to kill the thread.

If the connection is lost and you reconnect with mysql_ping(), the thread ID will change. This means you should not get the thread ID and store it for later, you should get it when you need it.

20.4.49.2 Return values

The thread ID of the current connection.

20.4.49.3 Errors

None.

20.4.50 `mysql_use_result()`

MYSQL_RES *mysql_use_result(MYSQL *mysql)

20.4.50.1 Description

You must call mysql_store_result() or mysql_use_result() for every query which successfully retrieves data (SELECT, SHOW, DESCRIBE, EXPLAIN).

mysql_use_result() initiates a result set retrieval but does not actually read the result set into the client like mysql_store_result() does. Instead, each row must be retrieved individually by making calls to mysql_fetch_row(). This reads the result of a query directly from the server without storing it in a temporary table or local buffer, which is somewhat faster and uses much less memory than mysql_store_result(). The client will only allocate memory for the current row and a communication buffer that may grow up to max_allowed_packet bytes.

On the other hand, you shouldn't use mysql_use_result() if you are doing a lot of processing for each row on the client side, or if the output is sent to a screen on which the user may type a ^S (stop scroll). This will tie up the server and prevent other threads from updating any tables from which the data are fetched.

When using mysql_use_result(), you must execute mysql_fetch_row() until a NULL value is returned, otherwise the unfetched rows will be returned as part of the result set for your next query. The C API will give the error Commands out of sync; You can't run this command now if you forget to do this!

You may not use mysql_data_seek(), mysql_row_seek(), mysql_row_tell(), mysql_num_rows() or mysql_affected_rows() with a result returned from mysql_use_result(), nor may you issue other queries until the mysql_use_result() has finished. (However, after you have fetched all the rows, mysql_num_rows() will accurately return the number of rows fetched.)

You must call mysql_free_result() once you are done with the result set.

20.4.50.2 Return values

A MYSQL_RES result structure. NULL if an error occurred.

20.4.50.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

20.4.51 Why is it that after `mysql_query()` returns success, `mysql_store_result()` sometimes returns `NULL?`

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 (e.g., it was an INSERT, UPDATE or DELETE).

You can always check whether or not 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().

20.4.52 What results can I 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 truncated, 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. mysql_insert_id().
Some queries (LOAD DATA INFILE ..., INSERT INTO ... SELECT ..., UPDATE) return additional info. 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.

20.4.53 How can I 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

20.4.54 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.

20.4.55 How to make a thread-safe client

The client is ``almost'' thread-safe. The biggest problem is that the subroutines in `net.c' that read from sockets are not interrupt-safe. This was done with the thought that you might want to have your own alarm that can break a long read to a server.

The standard client libraries are not compiled with the thread options.

To get a thread-safe client, use the -lmysys, -lstring and -ldbug libraries and net_serv.o that the server uses.

When using a threaded client, you can make great use of the routines in the `thr_alarm.c' file. If you are using routines from the mysys library, the only thing you must remember is to call my_init() first!

All functions except mysql_real_connect() are currently thread-safe. The following notes describe how to compile a thread-safe client library and use it in a thread-safe manner. (The notes below for mysql_real_connect() actually apply to mysql_connect() as well, but since mysql_connect() is deprecated, you should be using mysql_real_connect() anyway.)

To make mysql_real_connect() thread-safe, you must recompile the client library with this command:

shell> CPPFLAGS=-DTHREAD_SAFE_CLIENT ./configure ...

You may get some errors because of undefined symbols when linking the standard client, because the pthread libraries are not included by default.

The resulting `libmysqlclient.a' library is now thread-safe. What this means is that client code is thread-safe as long as two threads don't query the same connection handle returned by mysql_real_connect() at the same time; the client/server protocol allows only one request at a time on a given connection. If you want to use multiple threads on the same connection, you must have a mutex lock around your mysql_query() and mysql_store_result() call combination. Once mysql_store_result() is ready, the lock can be released and other threads may query the same connection. (In other words, different threads can use different MYSQL_RES pointers that were created with mysql_store_result(), as long as they use the proper locking protocol.) If you program with POSIX threads, you can use pthread_mutex_lock() and pthread_mutex_unlock() to establish and release a mutex lock.

If you used mysql_use_result() rather than mysql_store_result(), the lock would need to surround mysql_use_result() and the calls to mysql_fetch_row(). However, it really is best for threaded clients not to use mysql_use_result().