Debugging with GDB: Additions to Ada

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15.4.10.3 Additions to Ada

As it does for other languages, GDB makes certain generic extensions to Ada (see Expressions):

• If the expression E is a variable residing in memory (typically a local variable or array element) and N is a positive integer, then E@N displays the values of E and the N-1 adjacent variables following it in memory as an array. In Ada, this operator is generally not necessary, since its prime use is in displaying parts of an array, and slicing will usually do this in Ada. However, there are occasional uses when debugging programs in which certain debugging information has been optimized away.
• B::var means “the variable named var that appears in function or file B.” When B is a file name, you must typically surround it in single quotes.
• The expression {type} addr means “the variable of type type that appears at address addr.”
• A name starting with ‘$’ is a convenience variable (see Convenience Vars) or a machine register (see Registers).

In addition, GDB provides a few other shortcuts and outright additions specific to Ada:

• The assignment statement is allowed as an expression, returning its right-hand operand as its value. Thus, you may enter

  (gdb) set x := y + 3
  (gdb) print A(tmp := y + 1)
  

• The semicolon is allowed as an “operator,” returning as its value the value of its right-hand operand. This allows, for example, complex conditional breaks:

  (gdb) break f
  (gdb) condition 1 (report(i); k += 1; A(k) > 100)
  

• Rather than use catenation and symbolic character names to introduce special characters into strings, one may instead use a special bracket notation, which is also used to print strings. A sequence of characters of the form ‘["XX"]’ within a string or character literal denotes the (single) character whose numeric encoding is XX in hexadecimal. The sequence of characters ‘["""]’ also denotes a single quotation mark in strings. For example,

     "One line.["0a"]Next line.["0a"]"
  

  contains an ASCII newline character (Ada.Characters.Latin_1.LF) after each period.

• The subtype used as a prefix for the attributes 'Pos, 'Min, and 'Max is optional (and is ignored in any case). For example, it is valid to write

  (gdb) print 'max(x, y)
  

• When printing arrays, GDB uses positional notation when the array has a lower bound of 1, and uses a modified named notation otherwise. For example, a one-dimensional array of three integers with a lower bound of 3 might print as

  (3 => 10, 17, 1)
  

  That is, in contrast to valid Ada, only the first component has a => clause.

• You may abbreviate attributes in expressions with any unique, multi-character subsequence of their names (an exact match gets preference). For example, you may use a'len, a'gth, or a'lh in place of a'length.
• Since Ada is case-insensitive, the debugger normally maps identifiers you type to lower case. The GNAT compiler uses upper-case characters for some of its internal identifiers, which are normally of no interest to users. For the rare occasions when you actually have to look at them, enclose them in angle brackets to avoid the lower-case mapping. For example,

  (gdb) print <JMPBUF_SAVE>[0]
  

• Printing an object of class-wide type or dereferencing an access-to-class-wide value will display all the components of the object’s specific type (as indicated by its run-time tag). Likewise, component selection on such a value will operate on the specific type of the object.

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