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Semicolon's after declaration of structs...

// structure template
struct par
{
double a;
double b;
};

There should be instructions in allegro\docs\build

usully you type something like

cd c:\allegro
fix.bat bcc32
make
make install

and all the files fill be copied to the right folders and...

The compiler will only compile code between #ifndef xxx and #endif, if it hasn't encountered #define xxx before.It's sometimes used to stop the same header file being included twice:
#ifndef...