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The data returned from the query and placed into the result set required the allocation of resources. Since we are good programmers, we strive to free up the memory no longer needed to avoid memory leaks.2 MySQL AB provides the mysql_free_result() function to help free those resources. This function is defined as void mysql_free_result(MYSQL_RES *result) This function is call-safe, meaning that you can call it using a result set that has already been freed without producing an error. That s just in case you get happy and start flinging free code everywhere. Don t laugh I ve seen programs with more free than new calls. Most of the time this isn t a problem, but if the free calls are not used properly, having too many of them could result in freeing something you don t want freed. As with the new operation, you should use the free operation with deliberate purpose and caution. Here is an example call to this function to free a result set: mysql_free_result(results);
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Disconnecting from and Finalizing the Server
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When you are finished with the embedded server, you need to disconnect and shut it down. This can be accomplished by using the mysql_close() and mysql_server_end() functions. The close function closes the connection and the other finalizes the server and deallocates memory. These functions are defined as void mysql_close(MYSQL *mysql); void mysql_server_end();
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2. It isn t actually leaking so much as it is no longer referenced but still allocated, making that portion of memory unusable.
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CHAPTER 6 EMBEDDED MYSQL
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Example calls for these functions are shown here. Note that these are the last function calls you need to make and are normally called when shutting down your application. mysql_close(mysql); mysql_server_end();
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Now, let s see all of this code together. Listing 6-1 shows a completed embedded server that lists the databases accessible from the given data directory. I ll go through the process of building and running this example in a later section.
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Note The following example is written for Windows. A Linux example is discussed in a later section.
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Listing 6-1. An Example Embedded Server Application #include "my_global.h" #include "mysql.h" MYSQL *mysql; MYSQL_RES *results; MYSQL_ROW record; //the embedded server class //stores results from queries //a single row in a result set
static char *server_options[] = {"mysql_test", "--defaults-file=c:\\mysql_embedded\\my.ini", "--datadir=c:\\mysql_embedded\\data" }; int num_elements=sizeof(server_options) / sizeof(char *); static char *server_groups[] = {"libmyswld_server", "libmysqld_client" }; int main(void) { mysql_server_init(num_elements, server_options, server_groups); mysql = mysql_init(NULL); mysql_options(mysql, MYSQL_READ_DEFAULT_GROUP, "libmysqld_client"); mysql_options(mysql, MYSQL_OPT_USE_EMBEDDED_CONNECTION, NULL); mysql_real_connect(mysql, NULL, NULL, NULL, "information_schema", 0, NULL, 0); mysql_query(mysql, "SHOW DATABASES;"); // issue query results = mysql_store_result(mysql); // get results printf("The following are the databases supported:\n"); while(record=mysql_fetch_row(results)) // fetch row { printf("%s\n", record[0]); // process row }
CHAPTER 6 EMBEDDED MYSQL
mysql_query(mysql, "CREATE DATABASE testdb1;"); mysql_query(mysql, "SHOW DATABASES;"); results = mysql_store_result(mysql); printf("The following are the databases supported:\n"); while(record=mysql_fetch_row(results)) { printf("%s\n", record[0]); } mysql_free_result(results); mysql_query(mysql, "DROP DATABASE testdb1;"); mysql_close(mysql); mysql_server_end(); return 0; }
// issue query // get results // fetch row // process row
// issue query
Error Handling
You may be wondering what happened to all of the error handling that you read about in a previous chapter. Well, the facilities are there in the C API. MySQL AB has provided for error handling using two functions. The first, msyql_errno(), retrieves the error number from the most recent error. The second, mysql_error(), retrieves the associated error message for the most recent error. These functions are defined as unsigned int mysql_errno(MYSQL *mysql) const char *mysql_error(MYSQL *mysql) The parameter passed for both functions is the MYSQL object. Since these methods are error handlers, they are not expected to fail. However, if they are called when no error has occurred, mysql_errno() returns 0 and mysql_error() returns an empty character string. Here are some example calls to these functions: if(somethinggoeshinkyhere) { printf("There was an error! Error number : %d = $s\n", mysql_errno(&mysql), mysql_error(&mysql)); } Whew! That s all there is to it. I hope that my explanations clear the fog from the reference manual. I wrote this section primarily because I feel there aren t any decent examples out there that help you learn how to use the embedded server at least none that capture what is needed in a few short pages.
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