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)
     end
     
     document pst
       Print out a Pascal string
     end

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.