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:

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


gdb does not know sets.
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.

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:


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.