The following listing shows how to establish a connection to a MySQL database server and retrieve the results of a SQL query. Only a small number of functions offered by libdbi are shown here. For a more extensive example check out the test program tests/test_dbi.c in the libdbi-drivers source tarball. #include int main() { dbi_conn conn; dbi_result result; double threshold = 4.333333; unsigned int idnumber; const char *fullname; dbi_initialize(NULL); conn = dbi_conn_new("mysql"); dbi_conn_set_option(conn, "host", "localhost"); dbi_conn_set_option(conn, "username", "your_name"); dbi_conn_set_option(conn, "password", "your_password"); dbi_conn_set_option(conn, "dbname", "your_dbname"); dbi_conn_set_option(conn, "encoding", "UTF-8"); if (dbi_conn_connect(conn) < 0) { printf("Could not connect. Please check the option settings\n"); } else { result = dbi_conn_queryf(conn, "SELECT id, name FROM coders " "WHERE hours_of_sleep > %0.2f", threshold); if (result) { while (dbi_result_next_row(result)) { idnumber = dbi_result_get_uint(result, "id"); fullname = dbi_result_get_string(result, "name"); printf("%i. %s\n", idnumber, fullname); } dbi_result_free(result); } dbi_conn_close(conn); } dbi_shutdown(); return 0; } ]]> Compile with: gcc -lm -ldl -ldbi -o foo foo.c The -ldl option is not required on systems that implement the dynamic linking in their libc (like FreeBSD). You may also have to throw in something like -I/usr/local/include and -L/usr/local/lib to help gcc and ld find the libdbi headers and libraries. Of course, a complete program should be checking for errors more thoroughly. This example keeps error-checking at a minimum for the sake of clarity. There are also other ways to retrieve data after a successful query. Keep reading on to see the rest.

The generic example shown in the previous section assumed that the program is linked against libdbi. This is in fact the recommended way to add libdbi functionality to your programs. However, there are situations where this approach will not work. Some programs are designed to load modules at runtime to extend their capabilities. A well-known example is the web server Apache, which uses loadable modules to custom-tailor its capabilities. If such a module were to use libdbi, we'd look at the following pattern: Parent => dl_open(module) => dl_open(libdbi) => dl_open(driver) Both libdbi and the drivers have to be loaded using the dl_open function as it is not possible to link the dynamically loaded module against libdbi. For this pattern to work, the drivers need a little tweak, like this: ~/libdbi-drivers #./configure --enable-libdbi --with-mysql Drivers built with this option are supposed to work both with the procedure shown above and with the generic procedure shown in the previous section. This has been tested on Linux, FreeBSD, and Cygwin. You may want to test the drivers on other platforms in both modes to avoid surprises.

If your project uses autoconf to manage the build process on the target machine, you should add some tests to your ./configure script to check for the presence and usability of libdbi. The following example shows how this can be done: dnl check for dynamic linking functions AC_CHECK_LIB(dl,dlopen) dnl check for the libdbi library AC_CHECK_LIB(dbi,dbi_initialize) dnl to check for the availability and function of a particular dnl driver we need a runtime check (since the driver is loaded dnl dynamically). This example checks for the mysql driver AC_MSG_CHECKING("for libdbi mysql driver (dynamic load)") AC_RUN_IFELSE( [AC_LANG_PROGRAM(, [[dbi_initialize(0); return(dbi_conn_new("mysql") ? 0 : 1);]])], [AC_MSG_RESULT("yes")], [AC_MSG_FAILURE("mysql driver not installed?")]) The first two tests add the appropriate flags to the LIBS variable to link against the required libraries. In addition, you have to make sure that both the directory which contains the libdbi header file directory (usually /usr/include or /usr/local/include) as well as the directory which contains the libdbi library (usually /usr/lib or /usr/local/lib) are accessible to the compiler and to the linker by using the -I and -L compiler flags, respectively.