Chapter 28 Connectors and APIs

The examples here use gcc as the compiler. A different compiler might be appropriate on some systems (for example, clang on OS X or FreeBSD, or Sun Studio on Solaris). Adjust the examples as necessary.

You may need to specify an -I option when you compile client programs that use MySQL header files, so that the compiler can find them. For example, if the header files are installed in /usr/local/mysql/include, use this option in the compile command:

-I/usr/local/mysql/include

MySQL clients must be linked using the -lmysqlclient option in the link command. You may also need to specify a -L option to tell the linker where to find the library. For example, if the library is installed in /usr/local/mysql/lib, use these options in the link command:

-L/usr/local/mysql/lib -lmysqlclient

The path names may differ on your system. Adjust the -I and -L options as necessary.

To make it simpler to compile MySQL programs on Unix, use the mysql_config script. See Section 4.7.1, “mysql_config — Display Options for Compiling Clients”.

mysql_config displays the options needed for compiling or linking:

shell> mysql_config --cflags
shell> mysql_config --libs

You can run those commands to get the proper options and add them manually to compilation or link commands. Alternatively, include the output from mysql_config directly within command lines using backticks:

shell> gcc -c `mysql_config --cflags` progname.c
shell> gcc -o progname progname.o `mysql_config --libs`

On Unix, linking uses dynamic libraries by default. To link to the static client library instead, add its path name to the link command. For example, if the library is located in /usr/local/mysql/lib, link like this:

shell> gcc -o progname progname.o /usr/local/mysql/lib/libmysqlclient.a

Or use mysql_config to provide the library name:

shell> gcc -o progname progname.o `mysql_config --variable=pkglibdir`/libmysqlclient.a

mysql_config does not currently provide a way to list all libraries needed for static linking, so it might be necessary to name additional libraries on the link command (for example, -lnsl -lsocket on Solaris). To get an idea which libraries to add, use mysql_config --libs and ldd libmysqlclient.so (or otool -L libmysqlclient.dylib on OS X).

pkg-config can be used as an alternative to mysql_config for obtaining information such as compiler flags or link libraries required to compile MySQL applications. For example, the following pairs of commands are equivalent:

mysql_config --cflags
pkg-config --cflags mysqlclient

mysql_config --libs
pkg-config --libs mysqlclient

To produce flags for static linking, use this command:

pkg-config --static --libs mysqlclient

For more information, see Section 28.7.4.2, “Building C API Client Programs Using pkg-config”.

To specify header and library file locations, use the facilities provided by your development environment.

To build C API clients on Windows, you must link in the C client library, as well as the Windows ws2_32 sockets library and Secur32 security library.

You link your code with either the dynamic or static C client library. On Windows, the static library is named mysqlclient.lib and the dynamic library is named libmysql.dll. In addition, the libmysql.lib static import library is needed for using the dynamic library. If the static C client library is used, the client application must be compiled with the same version of Visual Studio used to compile the C client library (which is Visual Studio 2015 for the static C client library built by Oracle).

When using the Oracle-built MySQL C client library (or MySQL Connector/C), following these rules when it comes to linking the C runtime for your client application:

In general, when linking to a static MySQL C client library, the client library and the client application must use the same compiler option when it comes to linking the C runtime—that is, if your C client library is compiled with the /MT option, your client application should also be compiled with the /MT option, and so on (see the MSDN page describing the C library linking options for more details). Follow this rule when you are building your own static MySQL C client library (or MySQL Connector/C) from source and linking you client application to it.

The MySQL client library includes SSL support built in. It is unnecessary to specify either -lssl or -lcrypto at link time. Doing so may in fact result in problems at runtime.

If the linker cannot find the MySQL client library, you might get undefined-reference errors for symbols that start with mysql_, such as those shown here:

/tmp/ccFKsdPa.o: In function `main':
/tmp/ccFKsdPa.o(.text+0xb): undefined reference to `mysql_init'
/tmp/ccFKsdPa.o(.text+0x31): undefined reference to `mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x69): undefined reference to `mysql_error'
/tmp/ccFKsdPa.o(.text+0x9a): undefined reference to `mysql_close'

You should be able to solve this problem by adding -Ldir_path -lmysqlclient at the end of your link command, where dir_path represents the path name of the directory where the client library is located. To determine the correct directory, try this command:

shell> mysql_config --libs

The output from mysql_config might indicate other libraries that should be specified on the link command as well. You can include mysql_config output directly in your compile or link command using backticks. For example:

shell> gcc -o progname progname.o `mysql_config --libs`

If an error occurs at link time that the floor symbol is undefined, link to the math library by adding -lm to the end of the compile/link line. Similarly, if you get undefined-reference errors for other functions that should exist on your system, such as connect(), check the manual page for the function in question to determine which libraries you should add to the link command.

If you get undefined-reference errors such as the following for functions that do not exist on your system, it usually means that your MySQL client library was compiled on a system that is not 100% compatible with yours:

mf_format.o(.text+0x201): undefined reference to `__lxstat'

In this case, you should download the latest MySQL or Connector/C source distribution and compile the MySQL client library yourself. See Section 2.9, “Installing MySQL from Source”, and MySQL Connector/C Developer Guide.

mysql_affected_rows() may be called immediately after executing a statement with mysql_query() or mysql_real_query(). It returns the number of rows changed, deleted, or inserted by the last statement if it was an UPDATE, DELETE, or INSERT. For SELECT statements, mysql_affected_rows() works like mysql_num_rows().

For UPDATE statements, the affected-rows value by default is the number of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to mysql_real_connect() when connecting to mysqld, the affected-rows value is the number of rows “found”; that is, matched by the WHERE clause.

For REPLACE statements, the affected-rows value is 2 if the new row replaced an old row, because in this case, one row was inserted after the duplicate was deleted.

For INSERT ... ON DUPLICATE KEY UPDATE statements, the affected-rows value per row is 1 if the row is inserted as a new row, 2 if an existing row is updated, and 0 if an existing row is set to its current values. If you specify the CLIENT_FOUND_ROWS flag, the affected-rows value is 1 (not 0) if an existing row is set to its current values.

Following a CALL statement for a stored procedure, mysql_affected_rows() returns the value that it would return for the last statement executed within the procedure, or 0 if that statement would return -1. Within the procedure, you can use ROW_COUNT() at the SQL level to obtain the affected-rows value for individual statements.

mysql_affected_rows() returns a meaningful value for a wide range of statements. For details, see the description for ROW_COUNT() in Section 12.15, “Information Functions”.

An integer greater than zero indicates the number of rows affected or retrieved. Zero indicates that no records were updated for an UPDATE statement, no rows 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().

Because mysql_affected_rows() returns an unsigned value, you can check for -1 by comparing the return value to (uint64_t)-1 (or to (uint64_t)~0, which is equivalent).

None.

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

Sets autocommit mode on if mode is 1, off if mode is 0.

Zero for success. Nonzero if an error occurred.

None.

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 include no explicit database specifier.

mysql_change_user() fails if the connected user cannot be authenticated or does not have permission to use the database. In this case, the user and database are not changed.

Pass a db parameter of NULL if you do not want to have a default database.

This function resets the session state as if one had done a new connect and reauthenticated. (See Section 28.7.28, “C API Automatic Reconnection Control”.) It always performs a ROLLBACK of any active transactions, closes and drops all temporary tables, and unlocks all locked tables. Session system variables are reset to the values of the corresponding global system variables. Prepared statements are released and HANDLER variables are closed. Locks acquired with GET_LOCK() are released. These effects occur even if the user did not change.

To reset the connection state in a more lightweight manner without changing the user, use mysql_reset_connection().

Zero for success. Nonzero if an error occurred.

The same that you can get from mysql_real_connect(), plus:

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

Returns the default character set name for the current connection.

The default character set name

None.

Closes a previously opened connection. mysql_close() also deallocates the connection handler pointed to by mysql if the handler was allocated automatically by mysql_init() or mysql_connect(). Do not use the handler after it has been closed.

None.

None.

Commits the current transaction.

The action of this function is subject to the value of the completion_type system variable. In particular, if the value of completion_type is RELEASE (or 2), the server performs a release after terminating a transaction and closes the client connection. Call mysql_close() from the client program to close the connection from the client side.

Zero for success. Nonzero if an error occurred.

None.

This function is deprecated. 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_real_connect() with the difference that the connection parameter 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 cannot 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.)

Same as for mysql_real_connect().

Same as for mysql_real_connect().

Creates the database named by the db parameter.

This function is deprecated. Use mysql_query() to issue an SQL CREATE DATABASE statement instead.

Zero for success. Nonzero if an error occurred.

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

Seeks to an arbitrary row in a query result set. The offset value is a row number. Specify a value in the range from 0 to mysql_num_rows(result)-1.

This function requires that the result set structure contains the entire result of the query, so mysql_data_seek() may be used only in conjunction with mysql_store_result(), not with mysql_use_result().

None.

None.

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. See Section 29.5.4, “The DBUG Package”.

None.

None.

The call shown here 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");

Drops the database named by the db parameter.

This function is deprecated. Use mysql_query() to issue an SQL DROP DATABASE statement instead.

Zero for success. Nonzero if an error occurred.

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

Instructs the server to write debugging information to the error log. The connected user must have the SUPER privilege.

Zero for success. Nonzero if an error occurred.

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

mysql_eof() determines whether 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(). When used with mysql_store_result(), mysql_eof() always returns true.

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 nonzero 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(). Because those error functions provide the same information, their use is preferred over mysql_eof(), which is deprecated. (In fact, they provide more information, because mysql_eof() returns only a boolean value whereas the error functions indicate a reason for the error when one occurs.)

Zero for success. Nonzero if the end of the result set has been reached.

None.

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));
}

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. Errors also are listed at Appendix B, Errors, Error Codes, and Common Problems.

MySQL-specific error numbers returned by mysql_errno() differ from SQLSTATE values returned by mysql_sqlstate(). For example, the mysql client program displays errors using the following format, where 1146 is the mysql_errno() value and '42S02' is the corresponding mysql_sqlstate() value:

shell> SELECT * FROM no_such_table;
ERROR 1146 (42S02): Table 'test.no_such_table' doesn't exist

An error code value for the last mysql_xxx() call, if it failed. zero means no error occurred.

None.

For the connection specified by mysql, mysql_error() returns a null-terminated string containing the error message for the most recently invoked API function that failed. If a function did not fail, the return value of mysql_error() may be the previous error or an empty string to indicate no error.

A rule of thumb is that all functions that have to ask the server for information reset mysql_error() if they succeed.

For functions that reset mysql_error(), either of these two tests can be used to check for an error:

if(*mysql_error(&mysql))
{
  // an error occurred
}

if(mysql_error(&mysql)[0])
{
  // an error occurred
}

The language of the client error messages may be changed by recompiling the MySQL client library. You can choose error messages in several different languages. See Section 10.12, “Setting the Error Message Language”.

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

None.

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.

For metadata-optional connections, this function returns NULL when the resultset_metadata system variable is set to NONE. To check whether a result set has metadata, use the mysql_result_metadata() function. For details about managing result set metadata transfer, see Section 28.7.27, “C API Optional Result Set Metadata”.

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 (8KB) if you call mysql_fetch_field() to ask for the length of a BLOB field. (The 8KB size is chosen because MySQL does not 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.

The MYSQL_FIELD structure for the current column. NULL if no columns are left or the result set has no metadata.

None.

MYSQL_FIELD *field;

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

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

For metadata-optional connections, this function returns NULL when the resultset_metadata system variable is set to NONE. To check whether a result set has metadata, use the mysql_result_metadata() function. For details about managing result set metadata transfer, see Section 28.7.27, “C API Optional Result Set Metadata”.

The MYSQL_FIELD structure for the specified column. NULL if the result set has no metadata.

None.

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);
}

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.

For metadata-optional connections, this function returns NULL when the resultset_metadata system variable is set to NONE. To check whether a result set has metadata, use the mysql_result_metadata() function. For details about managing result set metadata transfer, see Section 28.7.27, “C API Optional Result Set Metadata”.

An array of MYSQL_FIELD structures for all columns of a result set. NULL if the result set has no metadata.

None.

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);
}

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().

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

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.

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]);
    }
}

mysql_fetch_row() retrieves the next row of a result set:

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.

A MYSQL_ROW structure for the next row, or NULL. The meaning of a NULL return depends on which function was called preceding mysql_fetch_row():

Errors are not reset between calls to mysql_fetch_row()

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");
}

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 mysql_store_result() should have produced a nonempty result. This enables the client program to take proper action without knowing whether the query was a SELECT (or SELECT-like) statement. The example shown here illustrates how this may be done.

See Section 28.7.29.1, “Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success”.

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

None.

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 the statement was a SELECT.

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

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

The previous value of the field cursor.

None.

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().

The current offset of the field cursor.

None.

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

Do not attempt to access a result set after freeing it.

None.

None.

This function provides information about the default client character set. The default character set may be changed with the mysql_set_character_set() function.

This example shows the fields that are available in the MY_CHARSET_INFO structure:

if (!mysql_set_character_set(&mysql, "utf8"))
{
    MY_CHARSET_INFO cs;
    mysql_get_character_set_info(&mysql, &cs);
    printf("character set information:\n");
    printf("character set+collation number: %d\n", cs.number);
    printf("character set name: %s\n", cs.name);
    printf("collation name: %s\n", cs.csname);
    printf("comment: %s\n", cs.comment);
    printf("directory: %s\n", cs.dir);
    printf("multi byte character min. length: %d\n", cs.mbminlen);
    printf("multi byte character max. length: %d\n", cs.mbmaxlen);
}

Returns a string that represents the MySQL client library version (for example, "8.0.19").

The function value is the version of MySQL or Connector/C that provides the client library. For more information, see Section 28.7.4.5, “C API Server and Client Library Versions”.

A character string that represents the MySQL client library version.

None.

Returns an integer that represents the MySQL client library version. The value has the format XYYZZ where X is the major version, YY is the release level (or minor version), and ZZ is the sub-version within the release level:

major_version*10000 + release_level*100 + sub_version

For example, "8.0.19" is returned as 80019.

The function value is the version of MySQL or Connector/C that provides the client library. For more information, see Section 28.7.4.5, “C API Server and Client Library Versions”.

An integer that represents the MySQL client library version.

None.

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

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

None.

Returns the current value of an option settable using mysql_options(). The value should be treated as read only.

The option argument is the option for which you want its value. The arg argument is a pointer to a variable in which to store the option value. arg must be a pointer to a variable of the type appropriate for the option argument. The following table shows which variable type to use for each option value.

Zero for success. Nonzero if an error occurred; this occurs for option values that cannot be queried.

The following call tests the MYSQL_OPT_RECONNECT option. After the call returns successfully, the value of reconnect is true or false to indicate whether automatic reconnection is enabled.

bool reconnect;

if (mysql_get_option(mysql, MYSQL_OPT_RECONNECT, &reconnect))
  fprintf(stderr, "mysql_get_options() failed\n");

Returns the protocol version used by current connection.

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

None.

Returns a string that represents the MySQL server version (for example, "8.0.19").

A character string that represents the MySQL server version.

None.

Returns an integer that represents the MySQL server version. The value has the format XYYZZ where X is the major version, YY is the release level (or minor version), and ZZ is the sub-version within the release level:

major_version*10000 + release_level*100 + sub_version

For example, "8.0.19" is returned as 80019.

This function is useful in client programs for determining whether some version-specific server capability exists.

An integer that represents the MySQL server version.

None.

mysql_get_ssl_cipher() returns the encryption cipher used for the given connection to the server. mysql is the connection handler returned from mysql_init().

A string naming the encryption cipher used for the connection, or NULL if the connection is not encrypted.

This function creates a legal SQL string for use in an SQL statement. See Section 9.1.1, “String Literals”.

The string in the from argument is encoded in hexadecimal format, with each character encoded as two hexadecimal digits. The result is placed in the to argument, followed by a terminating null byte.

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. When mysql_hex_string() returns, the contents of to is a null-terminated string. The return value is the length of the encoded string, not including the terminating null byte.

The return value can be placed into an SQL statement using either X'value' or 0xvalue format. However, the return value does not include the X'...' or 0x. The caller must supply whichever of those is desired.

char query[1000],*end;

end = strmov(query,"INSERT INTO test_table values(");
end = strmov(end,"X'");
end += mysql_hex_string(end,"What is this",12);
end = strmov(end,"',X'");
end += mysql_hex_string(end,"binary data: \0\r\n",16);
end = strmov(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 libmysqlclient library and works like strcpy() but returns a pointer to the terminating null of the first parameter.

The length of the encoded string that is placed into to, not including the terminating null character.

None.

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

mysql_info() returns a non-NULL value for INSERT ... VALUES only for the multiple-row form of the statement (that is, only if multiple value lists are specified).

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

None.

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 is freed when mysql_close() is called to close the connection.

In a nonmultithreaded environment, mysql_init() invokes mysql_library_init() automatically as necessary. However, mysql_library_init() is not thread-safe in a multithreaded environment, and thus neither is mysql_init(). Before calling mysql_init(), either call mysql_library_init() prior to spawning any threads, or use a mutex to protect the mysql_library_init() call. This should be done prior to any other client library call.

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

In case of insufficient memory, NULL is returned.

Returns the value generated for an AUTO_INCREMENT column by the previous INSERT or UPDATE statement. Use this function after you have performed an INSERT statement into a table that contains an AUTO_INCREMENT field, or have used INSERT or UPDATE to set a column value with LAST_INSERT_ID(expr).

The return value of mysql_insert_id() is always zero unless explicitly updated under one of the following conditions:

The return value of mysql_insert_id() can be simplified to the following sequence:

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

The value of mysql_insert_id() is affected only by statements issued within the current client connection. It is not affected by statements issued by other clients.

The LAST_INSERT_ID() SQL function will contain the value of the first automatically generated value that was successfully inserted. LAST_INSERT_ID() is not reset between statements because the value of that function is maintained in the server. Another difference from mysql_insert_id() is that LAST_INSERT_ID() is not updated if you set an AUTO_INCREMENT column to a specific nonspecial value. See Section 12.15, “Information Functions”.

mysql_insert_id() returns 0 following a CALL statement for a stored procedure that generates an AUTO_INCREMENT value because in this case mysql_insert_id() applies to CALL and not the statement within the procedure. Within the procedure, you can use LAST_INSERT_ID() at the SQL level to obtain the AUTO_INCREMENT value.

The reason for the differences between LAST_INSERT_ID() and mysql_insert_id() is that LAST_INSERT_ID() is made easy to use in scripts while mysql_insert_id() tries to provide more exact information about what happens to the AUTO_INCREMENT column.

Described in the preceding discussion.

Asks the server to kill the thread specified by pid.

This function is deprecated. Use mysql_query() to issue an SQL KILL statement instead.

mysql_kill() cannot handle values larger than 32 bits, but to guard against killing the wrong thread returns an error in these cases:

Zero for success. Nonzero if an error occurred.

This function finalizes the MySQL client library. Call it when you are done using the library (for example, after disconnecting from the server).

For usage information, see Section 28.7.6, “C API Function Overview”, and Section 28.7.7.41, “mysql_library_init()”.

Call this function to initialize the MySQL client library before you call any other MySQL function.

In a nonmultithreaded environment, the call to mysql_library_init() may be omitted, because mysql_init() will invoke it automatically as necessary. However, mysql_library_init() is not thread-safe in a multithreaded environment, and thus neither is mysql_init(), which calls mysql_library_init(). You must either call mysql_library_init() prior to spawning any threads, or else use a mutex to protect the call, whether you invoke mysql_library_init() or indirectly through mysql_init(). Do this prior to any other client library call.

The argc, argv, and groups arguments are unused. In older MySQL versions, they were used for applications linked against the embedded server, which is no longer supported. The call now should be written as mysql_library_init(0, NULL, NULL).

#include <mysql.h>
#include <stdlib.h>

int main(void) {
  if (mysql_library_init(0, NULL, NULL)) {
    fprintf(stderr, "could not initialize MySQL client library\n");
    exit(1);
  }

  /* Use any MySQL API functions here */

  mysql_library_end();

  return EXIT_SUCCESS;
}

Zero for success. Nonzero if an error occurred.

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().

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

Returns an empty result set for which the metadata provides information about the columns 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].

It is preferable to use SHOW COLUMNS FROM tbl_name instead of mysql_list_fields().

You must free the result set with mysql_free_result().

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

int i;
MYSQL_RES *tbl_cols = mysql_list_fields(mysql, "mytbl", "f%");

unsigned int field_cnt = mysql_num_fields(tbl_cols);
printf("Number of columns: %d\n", field_cnt);

for (i=0; i < field_cnt; ++i)
{
  /* col describes i-th column of the table */
  MYSQL_FIELD *col = mysql_fetch_field_direct(tbl_cols, i);
  printf ("Column %d: %s\n", i, col->name);
}
mysql_free_result(tbl_cols);

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().

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

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().

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

This function is used when you execute multiple statements specified as a single statement string, or when you execute CALL statements, which can return multiple result sets.

mysql_more_results() true if more results exist from the currently executed statement, in which case the application must call mysql_next_result() to fetch the results.

TRUE (1) if more results exist. FALSE (0) if no more results exist.

In most cases, you can call mysql_next_result() instead to test whether more results exist and initiate retrieval if so.

See Section 28.7.23, “C API Multiple Statement Execution Support”, and Section 28.7.7.47, “mysql_next_result()”.

None.

mysql_next_result() is used when you execute multiple statements specified as a single statement string, or when you use CALL statements to execute stored procedures, which can return multiple result sets.

mysql_next_result() reads the next statement result and returns a status to indicate whether more results exist. If mysql_next_result() returns an error, there are no more results.

Before each call to mysql_next_result(), you must call mysql_free_result() for the current statement if it is a statement that returned a result set (rather than just a result status).

After calling mysql_next_result() the state of the connection is as if you had called mysql_real_query() or mysql_query() for the next statement. This means that you can call mysql_store_result(), mysql_warning_count(), mysql_affected_rows(), and so forth.

If your program uses CALL statements to execute stored procedures, the CLIENT_MULTI_RESULTS flag must be enabled. This is because each CALL returns a result to indicate the call status, in addition to any result sets that might be returned by statements executed within the procedure. Because CALL can return multiple results, process them using a loop that calls mysql_next_result() to determine whether there are more results.

CLIENT_MULTI_RESULTS can be enabled when you call mysql_real_connect(), either explicitly by passing the CLIENT_MULTI_RESULTS flag itself, or implicitly by passing CLIENT_MULTI_STATEMENTS (which also enables CLIENT_MULTI_RESULTS). CLIENT_MULTI_RESULTS is enabled by default.

It is also possible to test whether there are more results by calling mysql_more_results(). However, this function does not change the connection state, so if it returns true, you must still call mysql_next_result() to advance to the next result.

For an example that shows how to use mysql_next_result(), see Section 28.7.23, “C API Multiple Statement Execution Support”.

Returns the number of columns in a result set.

You can get the number of columns either from a pointer to a result set or to a connection handler. You would use the connection handler if mysql_store_result() or mysql_use_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether mysql_store_result() should have produced a nonempty result. This enables the client program to take proper action without knowing whether the query was a SELECT (or SELECT-like) statement. The example shown here illustrates how this may be done.

See Section 28.7.29.1, “Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success”.

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

None.

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 whether mysql_field_count(&mysql) returns 0. This happens only if something went wrong.

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() does not return the correct value until all the rows in the result set have been retrieved.

mysql_num_rows() is intended for use with statements that return a result set, such as SELECT. For statements such as INSERT, UPDATE, or DELETE, the number of affected rows can be obtained with mysql_affected_rows().

The number of rows in the result set.

None.

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. To retrieve option values, use mysql_get_option().

Call mysql_options() 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, specify a pointer to the value of the integer as the arg argument.

Options for information such as SSL certificate and key files are used to establish an encrypted connection if such connections are available, but do not enforce any requirement that the connection obtained be encrypted. To require an encrypted connection, use the technique described in Section 28.7.22, “C API Encrypted Connection Support”.

The following list describes the possible options, their effect, and how arg is used for each option. For option descriptions that indicate arg is unused, its value is irrelevant; it is conventional to pass 0.

The client group 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.

timeout has been replaced by connect-timeout, but timeout is still supported for backward compatibility.

For more information about option files used by MySQL programs, see Section 4.2.2.2, “Using Option Files”.

Zero for success. Nonzero if you specify an unknown option.

The following mysql_options() calls request the use of compression in the client/server protocol, cause options to be read from the [odbc] group in option files, and disable transaction autocommit mode:

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_OPT_COMPRESS,0);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"odbc");
mysql_options(&mysql,MYSQL_INIT_COMMAND,"SET autocommit=0");
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));
}

mysql_options4() is similar to mysql_options() but has an extra fourth argument so that two values can be passed for the option specified in the second argument.

The following list describes the permitted options, their effect, and how arg1 and arg2 are used.

Zero for success. Nonzero if you specify an unknown option.

This example demonstrates the calls that specify connection attributes:

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_OPT_CONNECT_ATTR_RESET, 0);
mysql_options4(&mysql,MYSQL_OPT_CONNECT_ATTR_ADD, "key1", "value1");
mysql_options4(&mysql,MYSQL_OPT_CONNECT_ATTR_ADD, "key2", "value2");
mysql_options4(&mysql,MYSQL_OPT_CONNECT_ATTR_ADD, "key3", "value3");
mysql_options(&mysql,MYSQL_OPT_CONNECT_ATTR_DELETE, "key1");
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));
}

Checks whether the connection to the server is working. If the connection has gone down and auto-reconnect is enabled an attempt to reconnect is made. If the connection is down and auto-reconnect is disabled, mysql_ping() returns an error.

Auto-reconnect is disabled by default. To enable it, call mysql_options() with the MYSQL_OPT_RECONNECT option. For details, see Section 28.7.7.50, “mysql_options()”.

mysql_ping() can be used by clients that remain idle for a long while, to check whether the server has closed the connection and reconnect if necessary.

If mysql_ping()) does cause a reconnect, there is no explicit indication of it. To determine whether a reconnect occurs, call mysql_thread_id() to get the original connection identifier before calling mysql_ping(), then call mysql_thread_id() again to see whether the identifier has changed.

If reconnect occurs, some characteristics of the connection will have been reset. For details about these characteristics, see Section 28.7.28, “C API Automatic Reconnection Control”.

Zero if the connection to the server is active. Nonzero if an error occurred. A nonzero return does not indicate whether the MySQL server itself is down; the connection might be broken for other reasons such as network problems.

Executes the SQL statement pointed to by the null-terminated string stmt_str. Normally, the string must consist of a single SQL statement without a terminating semicolon (;) or \g. If multiple-statement execution has been enabled, the string can contain several statements separated by semicolons. See Section 28.7.23, “C API Multiple Statement Execution Support”.

mysql_query() cannot be used for statements that contain binary data; you must use mysql_real_query() instead. (Binary data may contain the \0 character, which mysql_query() interprets as the end of the statement string.)

If you want to know whether the statement returns a result set, you can use mysql_field_count() to check for this. See Section 28.7.7.22, “mysql_field_count()”.

Zero for success. Nonzero if an error occurred.

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 other API functions that require a valid MYSQL connection handler structure.

The parameters are specified as follows:

If your program uses CALL statements to execute stored procedures, the CLIENT_MULTI_RESULTS flag must be enabled. This is because each CALL returns a result to indicate the call status, in addition to any result sets that might be returned by statements executed within the procedure. Because CALL can return multiple results, process them using a loop that calls mysql_next_result() to determine whether there are more results.

CLIENT_MULTI_RESULTS can be enabled when you call mysql_real_connect(), either explicitly by passing the CLIENT_MULTI_RESULTS flag itself, or implicitly by passing CLIENT_MULTI_STATEMENTS (which also enables CLIENT_MULTI_RESULTS). CLIENT_MULTI_RESULTS is enabled by default.

If you enable CLIENT_MULTI_STATEMENTS or CLIENT_MULTI_RESULTS, process the result for every call to mysql_query() or mysql_real_query() by using a loop that calls mysql_next_result() to determine whether there are more results. For an example, see Section 28.7.23, “C API Multiple Statement Execution Support”.

For some parameters, it is possible to have the value taken from an option file rather than from an explicit value in the mysql_real_connect() call. To do this, call mysql_options() with the MYSQL_READ_DEFAULT_FILE or MYSQL_READ_DEFAULT_GROUP option before calling mysql_real_connect(). Then, in the mysql_real_connect() call, specify the “no-value” value for each parameter to be read from an option file:

If no value is found in an option file for a parameter, its default value is used as indicated in the descriptions given earlier in this section.

A MYSQL* connection handler 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.

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"your_prog_name");
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));
}

By using mysql_options() the MySQL client library reads the [client] and [your_prog_name] sections in the my.cnf file which ensures that your program works, even if someone has set up MySQL in some nonstandard way.

Upon connection, mysql_real_connect() sets the reconnect flag (part of the MYSQL structure) to a value of 1 in versions of the API older than 5.0.3, or 0 in newer versions. A value of 1 for this flag indicates that if a statement cannot be performed because of a lost connection, to try reconnecting to the server before giving up. You can use the MYSQL_OPT_RECONNECT option to mysql_options() to control reconnection behavior.

This function creates a legal SQL string for use in an SQL statement. See Section 9.1.1, “String Literals”.

The mysql argument must be a valid, open connection because character escaping depends on the character set in use by the server.

The string in the from argument is encoded to produce an escaped SQL string, taking into account the current character set of the connection. The result is placed in the to argument, followed by a terminating null byte.

Characters encoded are \, ', ", NUL (ASCII 0), \n, \r, and Control+Z. Strictly speaking, MySQL requires only that backslash and the quote character used to quote the string in the query be escaped. mysql_real_escape_string() quotes the other characters to make them easier to read in log files. For comparison, see the quoting rules for literal strings and the QUOTE() SQL function in Section 9.1.1, “String Literals”, and Section 12.5, “String Functions and Operators”.

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 worst case, each character may need to be encoded as using two bytes, and there must be room for the terminating null byte.) When mysql_real_escape_string() returns, the contents of to is a null-terminated string. The return value is the length of the encoded string, not including the terminating null byte.

If you must change the character set of the connection, use the mysql_set_character_set() function rather than executing a SET NAMES (or SET CHARACTER SET) statement. mysql_set_character_set() works like SET NAMES but also affects the character set used by mysql_real_escape_string(), which SET NAMES does not.

The following example inserts two escaped strings into an INSERT statement, each within single quote characters:

char query[1000],*end;

end = my_stpcpy(query,"INSERT INTO test_table VALUES('");
end += mysql_real_escape_string(&mysql,end,"What is this",12);
end = my_stpcpy(end,"','");
end += mysql_real_escape_string(&mysql,end,"binary data: \0\r\n",16);
end = my_stpcpy(end,"')");

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

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

The length of the encoded string that is placed into the to argument, not including the terminating null byte, or -1 if an error occurs.

Because mysql_real_escape_string() returns an unsigned value, you can check for -1 by comparing the return value to (unsigned long)-1 (or to (unsigned long)~0, which is equivalent).

This function creates a legal SQL string for use in an SQL statement. See Section 9.1.1, “String Literals”.

The mysql argument must be a valid, open connection because character escaping depends on the character set in use by the server.

The string in the from argument is encoded to produce an escaped SQL string, taking into account the current character set of the connection. The result is placed in the to argument, followed by a terminating null byte.

Characters encoded are \, ', ", NUL (ASCII 0), \n, \r, Control+Z, and `. Strictly speaking, MySQL requires only that backslash and the quote character used to quote the string in the query be escaped. mysql_real_escape_string_quote() quotes the other characters to make them easier to read in log files. For comparison, see the quoting rules for literal strings and the QUOTE() SQL function in Section 9.1.1, “String Literals”, and Section 12.5, “String Functions and Operators”.

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 worst case, each character may need to be encoded as using two bytes, and there must be room for the terminating null byte.) When mysql_real_escape_string_quote() returns, the contents of to is a null-terminated string. The return value is the length of the encoded string, not including the terminating null byte.

The quote argument indicates the context in which the escaped string is to be placed. Suppose that you intend to escape the from argument and insert the escaped string (designated here by str) into one of the following statements:

1) SELECT * FROM table WHERE name = 'str'
2) SELECT * FROM table WHERE name = "str"
3) SELECT * FROM `str` WHERE id = 103

To perform escaping properly for each statement, call mysql_real_escape_string_quote() as follows, where the final argument indicates the quoting context:

1) len = mysql_real_escape_string_quote(&mysql,to,from,from_len,'\'');
2) len = mysql_real_escape_string_quote(&mysql,to,from,from_len,'"');
3) len = mysql_real_escape_string_quote(&mysql,to,from,from_len,'`');

If you must change the character set of the connection, use the mysql_set_character_set() function rather than executing a SET NAMES (or SET CHARACTER SET) statement. mysql_set_character_set() works like SET NAMES but also affects the character set used by mysql_real_escape_string_quote(), which SET NAMES does not.

The following example inserts two escaped strings into an INSERT statement, each within single quote characters:

char query[1000],*end;

end = my_stpcpy(query,"INSERT INTO test_table VALUES('");
end += mysql_real_escape_string_quote(&mysql,end,"What is this",12,'\'');
end = my_stpcpy(end,"','");
end += mysql_real_escape_string_quote(&mysql,end,"binary data: \0\r\n",16,'\'');
end = my_stpcpy(end,"')");

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

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

The length of the encoded string that is placed into the to argument, not including the terminating null byte.

None.

mysql_real_query() executes the SQL statement pointed to by stmt_str, a string length bytes long. Normally, the string must consist of a single SQL statement without a terminating semicolon (;) or \g. If multiple-statement execution has been enabled, the string can contain several statements separated by semicolons. See Section 28.7.23, “C API Multiple Statement Execution Support”.

mysql_query() cannot be used for statements that contain binary data; you must use mysql_real_query() instead. (Binary data may contain the \0 character, which mysql_query() interprets as the end of the statement string.) In addition, mysql_real_query() is faster than mysql_query() because it does not call strlen() on the statement string.

If you want to know whether the statement returns a result set, you can use mysql_field_count() to check for this. See Section 28.7.7.22, “mysql_field_count()”.

Zero for success. Nonzero if an error occurred.

This function flushes tables or caches, or resets replication server information. The connected user must have the RELOAD privilege.

The options argument is a bitmask composed from any combination of the following values. Multiple values can be OR'ed together to perform multiple operations with a single call.

Zero for success. Nonzero if an error occurred.

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

This function is deprecated. Use mysql_query() to issue an SQL FLUSH PRIVILEGES statement instead.

Zero for success. Nonzero if an error occurred.

Resets the connection to clear the session state.

mysql_reset_connection() has effects similar to mysql_change_user() or an auto-reconnect except that the connection is not closed and reopened, and reauthentication is not done. The write set session history is reset. See Section 28.7.7.3, “mysql_change_user()”, and Section 28.7.28, “C API Automatic Reconnection Control”.

The connection-related state is affected as follows:

Zero for success. Nonzero if an error occurred.

Clears from the client library any cached copy of the public key required by the server for RSA key pair-based password exchange. This might be necessary when the server has been restarted with a different RSA key pair after the client program had called mysql_options() with the MYSQL_SERVER_PUBLIC_KEY option to specify the RSA public key. In such cases, connection failure can occur due to key mismatch. To fix this problem, the client can use either of the following approaches:

None.

None.

mysql_result_metadata() returns a value that indicates whether a result set has metadata. It can be useful for metadata-optional connections when the client does not know in advance whether particular result sets have metadata. For example, if a client executes a stored procedure that returns multiple result sets and might change the resultset_metadata system variable, the client can invoke mysql_result_metadata() for each result set to determine whether it has metadata.

For details about managing result set metadata transfer, see Section 28.7.27, “C API Optional Result Set Metadata”.

mysql_result_metadata() returns one of these values:

enum enum_resultset_metadata {
 RESULTSET_METADATA_NONE= 0,
 RESULTSET_METADATA_FULL= 1
};

Rolls back the current transaction.

The action of this function is subject to the value of the completion_type system variable. In particular, if the value of completion_type is RELEASE (or 2), the server performs a release after terminating a transaction and closes the client connection. Call mysql_close() from the client program to close the connection from the client side.

Zero for success. Nonzero if an error occurred.

None.

Sets the row cursor to an arbitrary row in a query result set. The offset value is a row offset, typically a value returned from mysql_row_tell() or from mysql_row_seek(). This value is not a row number; to seek to a row within a result set by number, use mysql_data_seek() instead.

This function requires that the result set structure contains the entire result of the query, so mysql_row_seek() may be used only in conjunction with mysql_store_result(), not with mysql_use_result().

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

None.

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().

Use mysql_row_tell() only after mysql_store_result(), not after mysql_use_result().

The current offset of the row cursor.

None.

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 include no explicit database specifier.

mysql_select_db() fails unless the connected user can be authenticated as having permission to use the database or some object within it.

Zero for success. Nonzero if an error occurred.

This function finalizes the MySQL client library, which should be done when you are done using the library. However, mysql_server_end() is deprecated and mysql_library_end() should be used instead. See Section 28.7.7.40, “mysql_library_end()”.

None.

This function initializes the MySQL client library, which must be done before you call any other MySQL function. However, mysql_server_init() is deprecated and you should call mysql_library_init() instead. See Section 28.7.7.41, “mysql_library_init()”.

Zero for success. Nonzero if an error occurred.

MySQL implements a session tracker mechanism whereby the server returns information about session state changes to clients. To control which notifications the server provides about state changes, client applications set system variables having names of the form session_track_xxx, such as session_track_state_change, session_track_schema, and session_track_system_variables. See Section 5.1.15, “Server Tracking of Client Session State Changes”.

Change notification occurs in the MySQL client/server protocol, which includes tracker information in OK packets so that session state changes can be detected. To enable client applications to extract state-change information from OK packets, the MySQL C API provides a pair of functions:

The mysql_session_track_get_first() parameters are used as follows. These descriptions also apply to mysql_session_track_get_next(), which takes the same parameters.

The following discussion describes how to interpret the data and length values according to the type value. It also indicates which system variable enables notifications for each tracker type.