8. Configuring MySQL++
Prev |  | Next

## 8. Configuring MySQL++

The default configuration of MySQL++ is suitable for most purposes, but there are a few things you can change to make it meet special needs.

### 8.1. The Location of the MySQL Development Files

MySQL++ is built on top of the MySQL C API. It relies on this low-level library for all communication with the database server. Consequently, the build process for MySQL++ may fail if it can’t find the C API headers and library.

On platforms that use Autoconf[17], the `configure` script can usually figure out the location of the C API development files by itself. It simply tries a bunch of common installation locations until it finds one that works. If your MySQL server was installed in a nonstandard location, you will have to tell the `configure` script where these files are with some combination of the `--with-mysql`, `--with-mysql-include`, and `--with-mysql-lib` flags. See `README-Unix.txt` for details.

No other platform allows this sort of auto-discovery, so the build files for these platforms simply hard-code the default installation location for whatever version of MySQL is in GA state at the time that version of MySQL++ was released. For example, the Visual C++ project files currently assume MySQL is in `c:\Program Files\MySQL\MySQL Server 5.1`. If you’re using some other release of MySQL or you installed it somewhere else, you will have to modify the build files. How you do this, exactly, varies based on platform and what tools you have on hand. See `README-Visual-C++.txt`, `README-MinGW.txt`, or `README-Mac-OS-X.txt`, as appropriate.

### 8.2. The Maximum Number of Fields Allowed

MySQL++ offers two ways to automatically build SQL queries at run time: Template Queries and SSQLS. There’s a limit on the number of fields these mechanisms support, defaulting to 25 fields in the official MySQL++ packages.[18] The files embodying these limits are `lib/querydef.h` and `lib/ssqls.h`, each generated by Perl scripts of the same name but with a `.pl` extension.

The default `querydef.h` is small and its size only increases linearly with respect to maximum field count.

`ssqls.h` is a totally different story. The default 25 field limit makes `ssqls.pl` generate an `ssqls.h` over 1 MB. Worse, the field limit to file size relation is quadratic.[19] This has a number of bad effects:

- Generating header files to support more fields than you actually require is a waste of space and bandwidth.

- Some compilers have arbitrary limits on the size of macros they’re able to parse. Exceeding these limits usually causes the compiler to misbehave badly, rather than fail gracefully.

- Because it increases the size of two key files used in building MySQL++ itself and programs built on it, it increases compile times significantly. One test I did here showed a tripling of compile time from quadrupling the field limit.

- More than 25 fields in a table is a good sign of a bad database design, most likely a denormalization problem.

The default limits try to mitigate against all of these factors while still being high enough to be useful with most DB designs.

If you’re building MySQL++ from source on a platform that uses Autoconf, the easiest way to change these limits is at configuration time:

    ./configure --with-field-limit=50

That causes the configuration script to pass the -f flag to the two Perl scripts named above, overriding the default of 25 fields. Obviously you need a Perl interpreter on the system for this to work, but Perl is usually installed by default on systems MySQL++ supports via Autoconf.

On all other platforms, you’ll have to give the -f flag to these scripts yourself. This may require installing Perl and putting it in the command path first. Having done that, you can do something like this to raise the limits:

    cd lib
    perl ssqls.pl -f 50
    perl querydef.pl -f 50

Note the need to run these commands within the `lib` subdirectory of the MySQL++ source tree. (This is done for you automatically on systems where you are able to use the Autoconf method.)

### 8.3. Buried MySQL C API Headers

It’s common these days on Unixy systems to install the MySQL C API headers in a `mysql` directory under some common `include` directory. If the C API headers are in `/usr/include/mysql`, we say they are “buried” underneath the system’s main include directory, `/usr/include`. Since the MySQL++ headers depend on these C API headers, it can be useful for MySQL++ to know this fact.

When MySQL++ includes one of the C API headers, it normally does so in the obvious way:

    #include <mysql.h>

But, if you define the `MYSQLPP_MYSQL_HEADERS_BURIED` macro, it switches to this style:

    #include <mysql/mysql.h>

In common situations like the `/usr/include/mysql` one, this simplifies the include path options you pass to your compiler.

### 8.4. Building MySQL++ on Systems Without Complete C99 Support

MySQL++ uses the C99 header `stdint.h` for portable fixed-size integer typedefs where possible. The C99 extensions aren’t yet officially part of the C++ Standard, so there are still some C++ compilers that don’t offer this header. MySQL++ works around the lack of this header where it knows it needs to, but your platform might not be recognized, causing the build to break. If this happens, you can define the `MYSQLPP_NO_STDINT_H` macro to make MySQL++ use its best guess for suitable integer types instead of relying on `stdint.h`.

MySQL++ also uses C99’s long long data type where available. MySQL++ has workarounds for platforms where this is known not to be available, but if you get errors in `common.h` about this type, you can define the macro `MYSQLPP_NO_LONG_LONGS` to make MySQL++ fall back to portable constructs.

[17] Linux, Solaris, the BSDs, Mac OS X command line (as opposed to the Xcode IDE), Cygwin... Basically, Unix or anything that works like it.

[18] If you’re using a third-party MySQL++ package, its maintainer may have increased these field counts so the resulting headers more closely approach the size limit of the compiler the package was built with. In that case, you can look at the top of each generated header file to find out how many fields each supports.

[19] The file size equation, for you amateur mathematicians out there, is Nlines = 18.5f2 + 454.5f + 196.4, where `f` is the field count.

Prev |  | Next
7. Using MySQL++ in a Multithreaded Program | Home | 9. Using MySQL++ in Your Own Project