10.3 Caveats when debugging with gdb

There are some peculiarities of Free Pascal which you should be aware of when using gdb. We list the main ones here:

  1. Free Pascal generates information for GDB in uppercase letters. This is a consequence of the fact that Pascal is a case insensitive language. So, when referring to a variable or function, you need to make its name all uppercase.

    As an example, if you want to watch the value of a loop variable count, you should type

    watch COUNT

    Or if you want to stop when a certain function (e.g MyFunction) is called, type

    break MYFUNCTION

  2. gdb does not know sets.
  3. gdb doesn’t know strings. Strings are represented in gdb as records with a length field and an array of char containing the string.

    You can also use the following user function to print strings:

    define pst  
    set $pos=&$arg0  
    set $strlen = {byte}$pos  
    print {char}&$arg0.st@($strlen+1)  
    document pst  
      Print out a Pascal string  

    If you insert it in your gdb.ini file, you can look at a string with this function. There is a sample gdb.ini in appendix E.

  4. Objects are difficult to handle, mainly because gdb is oriented towards C and C++. The workaround implemented in Free Pascal is that object methods are represented as functions, with an extra parameter this (all lowercase!). The name of this function is a concatenation of the object type and the function name, separated by two underscore characters.

    For example, the method TPoint.Draw would be converted to TPOINT__DRAW, and you could stop at it by using:

    break TPOINT__DRAW

  5. Global overloaded functions confuse gdb because they have the same name. Thus you cannot set a breakpoint at an overloaded function, unless you know its line number, in which case you can set a breakpoint at the starting line number of the function.