22.9.2. C API Function Overview

The functions available in the C API are summarized here and described in greater detail in a later section. See Section 22.9.3, “C API Function Descriptions”.

Table 22.4. C API Function Names and Descriptions

my_init()Initialize global variables, and thread handler in thread-safe programs
mysql_affected_rows()Returns the number of rows changed/deleted/inserted by the last UPDATE, DELETE, or INSERT query
mysql_autocommit()Toggles autocommit mode on/off
mysql_change_user()Changes user and database on an open connection
mysql_character_set_name()Return default character set name for current connection
mysql_client_find_plugin()Return pointer to plugin
mysql_client_register_plugin()Register a plugin
mysql_close()Closes a server connection
mysql_commit()Commits the transaction
mysql_connect()Connects to a MySQL server (this function is deprecated; use mysql_real_connect() instead)
mysql_create_db()Creates a database (this function is deprecated; use the SQL statement CREATE DATABASE instead)
mysql_data_seek()Seeks to an arbitrary row number in a query result set
mysql_debug()Does a DBUG_PUSH with the given string
mysql_drop_db()Drops a database (this function is deprecated; use the SQL statement DROP DATABASE instead)
mysql_dump_debug_info()Makes the server write debug information to the log
mysql_eof()Determines whether the last row of a result set has been read (this function is deprecated; mysql_errno() or mysql_error() may be used instead)
mysql_errno()Returns the error number for the most recently invoked MySQL function
mysql_error()Returns the error message for the most recently invoked MySQL function
mysql_escape_string()Escapes special characters in a string for use in an SQL statement
mysql_fetch_field()Returns the type of the next table field
mysql_fetch_field_direct()Returns the type of a table field, given a field number
mysql_fetch_fields()Returns an array of all field structures
mysql_fetch_lengths()Returns the lengths of all columns in the current row
mysql_fetch_row()Fetches the next row from the result set
mysql_field_count()Returns the number of result columns for the most recent statement
mysql_field_seek()Puts the column cursor on a specified column
mysql_field_tell()Returns the position of the field cursor used for the last mysql_fetch_field()
mysql_free_result()Frees memory used by a result set
mysql_get_character_set_info()Return information about default character set
mysql_get_client_info()Returns client version information as a string
mysql_get_client_version()Returns client version information as an integer
mysql_get_host_info()Returns a string describing the connection
mysql_get_proto_info()Returns the protocol version used by the connection
mysql_get_server_info()Returns the server version number
mysql_get_server_version()Returns version number of server as an integer
mysql_get_ssl_cipher()Return current SSL cipher
mysql_hex_string()Encode string in hexadecimal format
mysql_info()Returns information about the most recently executed query
mysql_init()Gets or initializes a MYSQL structure
mysql_insert_id()Returns the ID generated for an AUTO_INCREMENT column by the previous query
mysql_kill()Kills a given thread
mysql_library_end()Finalize the MySQL C API library
mysql_library_init()Initialize the MySQL C API library
mysql_list_dbs()Returns database names matching a simple regular expression
mysql_list_fields()Returns field names matching a simple regular expression
mysql_list_processes()Returns a list of the current server threads
mysql_list_tables()Returns table names matching a simple regular expression
mysql_load_plugin()Load a plugin
mysql_load_plugin_v()Load a plugin
mysql_more_results()Checks whether any more results exist
mysql_next_result()Returns/initiates the next result in multiple-result executions
mysql_num_fields()Returns the number of columns in a result set
mysql_num_rows()Returns the number of rows in a result set
mysql_options()Sets connect options for mysql_real_connect()
mysql_ping()Checks whether the connection to the server is working, reconnecting as necessary
mysql_plugin_options()Set a plugin option
mysql_query()Executes an SQL query specified as a null-terminated string
mysql_real_connect()Connects to a MySQL server
mysql_real_escape_string()Escapes special characters in a string for use in an SQL statement, taking into account the current character set of the connection
mysql_real_query()Executes an SQL query specified as a counted string
mysql_refresh()Flush or reset tables and caches
mysql_reload()Tells the server to reload the grant tables
mysql_rollback()Rolls back the transaction
mysql_row_seek()Seeks to a row offset in a result set, using value returned from mysql_row_tell()
mysql_row_tell()Returns the row cursor position
mysql_select_db()Selects a database
mysql_server_end()Finalize the MySQL C API library
mysql_server_init()Initialize the MySQL C API library
mysql_set_character_set()Set default character set for current connection
mysql_set_local_infile_default()Set the LOAD DATA LOCAL INFILE handler callbacks to their default values
mysql_set_local_infile_handler()Install application-specific LOAD DATA LOCAL INFILE handler callbacks
mysql_set_server_option()Sets an option for the connection (like multi-statements)
mysql_sqlstate()Returns the SQLSTATE error code for the last error
mysql_shutdown()Shuts down the database server
mysql_ssl_set()Prepare to establish SSL connection to server
mysql_stat()Returns the server status as a string
mysql_store_result()Retrieves a complete result set to the client
mysql_thread_end()Finalize thread handler
mysql_thread_id()Returns the current thread ID
mysql_thread_init()Initialize thread handler
mysql_thread_safe()Returns 1 if the clients are compiled as thread-safe
mysql_use_result()Initiates a row-by-row result set retrieval
mysql_warning_count()Returns the warning count for the previous SQL statement

Application programs should use this general outline for interacting with MySQL:

  1. Initialize the MySQL library by calling mysql_library_init(). This function exists in both the mysqlclient C client library and the mysqld embedded server library, so it is used whether you build a regular client program by linking with the -libmysqlclient flag, or an embedded server application by linking with the -libmysqld flag.

  2. Initialize a connection handler by calling mysql_init() and connect to the server by calling mysql_real_connect().

  3. Issue SQL statements and process their results. (The following discussion provides more information about how to do this.)

  4. Close the connection to the MySQL server by calling mysql_close().

  5. End use of the MySQL library by calling mysql_library_end().

The purpose of calling mysql_library_init() and mysql_library_end() is to provide proper initialization and finalization of the MySQL library. For applications that are linked with the client library, they provide improved memory management. If you don't call mysql_library_end(), a block of memory remains allocated. (This does not increase the amount of memory used by the application, but some memory leak detectors will complain about it.) For applications that are linked with the embedded server, these calls start and stop the server.

In a nonmulti-threaded 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 multi-threaded 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(). This should be done prior to any other client library call.

To connect to the server, call mysql_init() to initialize a connection handler, then call mysql_real_connect() with that handler (along with other information such as the host name, user name, and password). 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. When you are done with the connection, call mysql_close() to terminate it.

While a connection is active, the client may send SQL statements to the server using mysql_query() or mysql_real_query(). The difference between the two is that mysql_query() expects the query to be specified as a null-terminated string whereas mysql_real_query() expects a counted string. If the string contains binary data (which may include null bytes), you must use mysql_real_query().

For each non-SELECT query (for example, INSERT, UPDATE, DELETE), you can find out how many rows were changed (affected) by calling mysql_affected_rows().

For SELECT queries, you retrieve the selected rows as a result set. (Note that some statements are SELECT-like in that they return rows. These include SHOW, DESCRIBE, and EXPLAIN. They should be treated the same way as SELECT statements.)

There are two ways for a client to process result sets. One way is to retrieve the entire result set all at once by calling mysql_store_result(). This function acquires from the server all the rows returned by the query and stores them in the client. The second way is for the client to initiate a row-by-row result set retrieval by calling mysql_use_result(). This function initializes the retrieval, but does not actually get any rows from the server.

In both cases, you access rows by calling mysql_fetch_row(). With mysql_store_result(), mysql_fetch_row() accesses rows that have previously been fetched from the server. With mysql_use_result(), mysql_fetch_row() actually retrieves the row from the server. Information about the size of the data in each row is available by calling mysql_fetch_lengths().

After you are done with a result set, call mysql_free_result() to free the memory used for it.

The two retrieval mechanisms are complementary. Client programs should choose the approach that is most appropriate for their requirements. In practice, clients tend to use mysql_store_result() more commonly.

An advantage of mysql_store_result() is that because the rows have all been fetched to the client, you not only can access rows sequentially, you can move back and forth in the result set using mysql_data_seek() or mysql_row_seek() to change the current row position within the result set. You can also find out how many rows there are by calling mysql_num_rows(). On the other hand, the memory requirements for mysql_store_result() may be very high for large result sets and you are more likely to encounter out-of-memory conditions.

An advantage of mysql_use_result() is that the client requires less memory for the result set because it maintains only one row at a time (and because there is less allocation overhead, mysql_use_result() can be faster). Disadvantages are that you must process each row quickly to avoid tying up the server, you don't have random access to rows within the result set (you can only access rows sequentially), and you don't know how many rows are in the result set until you have retrieved them all. Furthermore, you must retrieve all the rows even if you determine in mid-retrieval that you've found the information you were looking for.

The API makes it possible for clients to respond appropriately to statements (retrieving rows only as necessary) without knowing whether the statement is a SELECT. You can do this by calling mysql_store_result() after each mysql_query() (or mysql_real_query()). If the result set call succeeds, the statement was a SELECT and you can read the rows. If the result set call fails, call mysql_field_count() to determine whether a result was actually to be expected. If mysql_field_count() returns zero, the statement returned no data (indicating that it was an INSERT, UPDATE, DELETE, and so forth), and was not expected to return rows. If mysql_field_count() is nonzero, the statement should have returned rows, but didn't. This indicates that the statement was a SELECT that failed. See the description for mysql_field_count() for an example of how this can be done.

Both mysql_store_result() and mysql_use_result() enable you to obtain information about the fields that make up the result set (the number of fields, their names and types, and so forth). You can access field information sequentially within the row by calling mysql_fetch_field() repeatedly, or by field number within the row by calling mysql_fetch_field_direct(). The current field cursor position may be changed by calling mysql_field_seek(). Setting the field cursor affects subsequent calls to mysql_fetch_field(). You can also get information for fields all at once by calling mysql_fetch_fields().

For detecting and reporting errors, MySQL provides access to error information by means of the mysql_errno() and mysql_error() functions. These return the error code or error message for the most recently invoked function that can succeed or fail, enabling you to determine when an error occurred and what it was.

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