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INFO-DIR-SECTION Programming & development tools.
START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Eighth Edition, March 2000, of `Debugging with GDB: the
GNU Source-Level Debugger' for GDB Version 5.0.

   Copyright (C) 1988-2000 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.


File: gdb.info,  Node: Range Checking,  Prev: Type Checking,  Up: Checks

An overview of range checking
-----------------------------

   In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks.  Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.

   For expressions you use in GDB commands, you can tell GDB to treat
range errors in one of three ways: ignore them, always treat them as
errors and abandon the expression, or issue warnings but evaluate the
expression anyway.

   A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member of
any type.  Some languages, however, do not treat overflows as an error.
In many implementations of C, mathematical overflow causes the result
to "wrap around" to lower values--for example, if M is the largest
integer value, and S is the smallest, then

     M + 1 => S

   This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines.  *Note Supported
languages: Support, for further details on specific languages.

   GDB provides some additional commands for controlling the range
checker:

`set check range auto'
     Set range checking on or off based on the current working language.
     *Note Supported languages: Support, for the default settings for
     each language.

`set check range on'
`set check range off'
     Set range checking on or off, overriding the default setting for
     the current working language.  A warning is issued if the setting
     does not match the language default.  If a range error occurs and
     range checking is on, then a message is printed and evaluation of
     the expression is aborted.

`set check range warn'
     Output messages when the GDB range checker detects a range error,
     but attempt to evaluate the expression anyway.  Evaluating the
     expression may still be impossible for other reasons, such as
     accessing memory that the process does not own (a typical example
     from many Unix systems).

`show range'
     Show the current setting of the range checker, and whether or not
     it is being set automatically by GDB.


File: gdb.info,  Node: Support,  Prev: Checks,  Up: Languages

Supported languages
===================

   GDB supports C, C++, Fortran, Java, Chill, assembly, and Modula-2.
Some GDB features may be used in expressions regardless of the language
you use: the GDB `@' and `::' operators, and the `{type}addr' construct
(*note Expressions: Expressions.) can be used with the constructs of
any supported language.

   The following sections detail to what degree each source language is
supported by GDB.  These sections are not meant to be language
tutorials or references, but serve only as a reference guide to what the
GDB expression parser accepts, and what input and output formats should
look like for different languages.  There are many good books written
on each of these languages; please look to these for a language
reference or tutorial.

* Menu:

* C::           C and C++
* Modula-2::    Modula-2
* Chill::        Chill


File: gdb.info,  Node: C,  Next: Modula-2,  Up: Support

C and C++
---------

   Since C and C++ are so closely related, many features of GDB apply
to both languages.  Whenever this is the case, we discuss those
languages together.

   The C++ debugging facilities are jointly implemented by the C++
compiler and GDB.  Therefore, to debug your C++ code effectively, you
must compile your C++ programs with a supported C++ compiler, such as
GNU `g++', or the HP ANSI C++ compiler (`aCC').

   For best results when using GNU C++, use the stabs debugging format.
You can select that format explicitly with the `g++' command-line
options `-gstabs' or `-gstabs+'.  See *Note Options for Debugging Your
Program or GNU CC: (gcc.info)Debugging Options, for more information.

* Menu:

* C Operators::                 C and C++ operators
* C Constants::                 C and C++ constants
* C plus plus expressions::     C++ expressions
* C Defaults::                  Default settings for C and C++
* C Checks::                    C and C++ type and range checks
* Debugging C::                 GDB and C
* Debugging C plus plus::       GDB features for C++


File: gdb.info,  Node: C Operators,  Next: C Constants,  Up: C

C and C++ operators
...................

   Operators must be defined on values of specific types.  For instance,
`+' is defined on numbers, but not on structures.  Operators are often
defined on groups of types.

   For the purposes of C and C++, the following definitions hold:

   * _Integral types_ include `int' with any of its storage-class
     specifiers; `char'; `enum'; and, for C++, `bool'.

   * _Floating-point types_ include `float', `double', and `long
     double' (if supported by the target platform).

   * _Pointer types_ include all types defined as `(TYPE *)'.

   * _Scalar types_ include all of the above.


The following operators are supported.  They are listed here in order
of increasing precedence:

`,'
     The comma or sequencing operator.  Expressions in a
     comma-separated list are evaluated from left to right, with the
     result of the entire expression being the last expression
     evaluated.

`='
     Assignment.  The value of an assignment expression is the value
     assigned.  Defined on scalar types.

`OP='
     Used in an expression of the form `A OP= B', and translated to
     `A = A OP B'.  `OP=' and `=' have the same precedence.  OP is any
     one of the operators `|', `^', `&', `<<', `>>', `+', `-', `*',
     `/', `%'.

`?:'
     The ternary operator.  `A ? B : C' can be thought of as:  if A
     then B else C.  A should be of an integral type.

`||'
     Logical OR.  Defined on integral types.

`&&'
     Logical AND.  Defined on integral types.

`|'
     Bitwise OR.  Defined on integral types.

`^'
     Bitwise exclusive-OR.  Defined on integral types.

`&'
     Bitwise AND.  Defined on integral types.

`==, !='
     Equality and inequality.  Defined on scalar types.  The value of
     these expressions is 0 for false and non-zero for true.

`<, >, <=, >='
     Less than, greater than, less than or equal, greater than or equal.
     Defined on scalar types.  The value of these expressions is 0 for
     false and non-zero for true.

`<<, >>'
     left shift, and right shift.  Defined on integral types.

`@'
     The GDB "artificial array" operator (*note Expressions:
     Expressions.).

`+, -'
     Addition and subtraction.  Defined on integral types,
     floating-point types and pointer types.

`*, /, %'
     Multiplication, division, and modulus.  Multiplication and
     division are defined on integral and floating-point types.
     Modulus is defined on integral types.

`++, --'
     Increment and decrement.  When appearing before a variable, the
     operation is performed before the variable is used in an
     expression; when appearing after it, the variable's value is used
     before the operation takes place.

`*'
     Pointer dereferencing.  Defined on pointer types.  Same precedence
     as `++'.

`&'
     Address operator.  Defined on variables.  Same precedence as `++'.

     For debugging C++, GDB implements a use of `&' beyond what is
     allowed in the C++ language itself: you can use `&(&REF)' (or, if
     you prefer, simply `&&REF') to examine the address where a C++
     reference variable (declared with `&REF') is stored.

`-'
     Negative.  Defined on integral and floating-point types.  Same
     precedence as `++'.

`!'
     Logical negation.  Defined on integral types.  Same precedence as
     `++'.

`~'
     Bitwise complement operator.  Defined on integral types.  Same
     precedence as `++'.

`., ->'
     Structure member, and pointer-to-structure member.  For
     convenience, GDB regards the two as equivalent, choosing whether
     to dereference a pointer based on the stored type information.
     Defined on `struct' and `union' data.

`.*, ->*'
     Dereferences of pointers to members.

`[]'
     Array indexing.  `A[I]' is defined as `*(A+I)'.  Same precedence
     as `->'.

`()'
     Function parameter list.  Same precedence as `->'.

`::'
     C++ scope resolution operator.  Defined on `struct', `union', and
     `class' types.

`::'
     Doubled colons also represent the GDB scope operator (*note
     Expressions: Expressions.).  Same precedence as `::', above.

   If an operator is redefined in the user code, GDB usually attempts
to invoke the redefined version instead of using the operator's
predefined meaning.

* Menu:

* C Constants::


File: gdb.info,  Node: C Constants,  Next: C plus plus expressions,  Prev: C Operators,  Up: C

C and C++ constants
...................

   GDB allows you to express the constants of C and C++ in the
following ways:

   * Integer constants are a sequence of digits.  Octal constants are
     specified by a leading `0' (i.e. zero), and hexadecimal constants
     by a leading `0x' or `0X'.  Constants may also end with a letter
     `l', specifying that the constant should be treated as a `long'
     value.

   * Floating point constants are a sequence of digits, followed by a
     decimal point, followed by a sequence of digits, and optionally
     followed by an exponent.  An exponent is of the form:
     `e[[+]|-]NNN', where NNN is another sequence of digits.  The `+'
     is optional for positive exponents.  A floating-point constant may
     also end with a letter `f' or `F', specifying that the constant
     should be treated as being of the `float' (as opposed to the
     default `double') type; or with a letter `l' or `L', which
     specifies a `long double' constant.

   * Enumerated constants consist of enumerated identifiers, or their
     integral equivalents.

   * Character constants are a single character surrounded by single
     quotes (`''), or a number--the ordinal value of the corresponding
     character (usually its ASCII value).  Within quotes, the single
     character may be represented by a letter or by "escape sequences",
     which are of the form `\NNN', where NNN is the octal representation
     of the character's ordinal value; or of the form `\X', where `X'
     is a predefined special character--for example, `\n' for newline.

   * String constants are a sequence of character constants surrounded
     by double quotes (`"').  Any valid character constant (as described
     above) may appear.  Double quotes within the string must be
     preceded by a backslash, so for instance `"a\"b'c"' is a string of
     five characters.

   * Pointer constants are an integral value.  You can also write
     pointers to constants using the C operator `&'.

   * Array constants are comma-separated lists surrounded by braces `{'
     and `}'; for example, `{1,2,3}' is a three-element array of
     integers, `{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and
     `{&"hi", &"there", &"fred"}' is a three-element array of pointers.

* Menu:

* C plus plus expressions::
* C Defaults::
* C Checks::

* Debugging C::


File: gdb.info,  Node: C plus plus expressions,  Next: C Defaults,  Prev: C Constants,  Up: C

C++ expressions
...............

   GDB expression handling can interpret most C++ expressions.

     _Warning:_ GDB can only debug C++ code if you use the proper
     compiler.  Typically, C++ debugging depends on the use of
     additional debugging information in the symbol table, and thus
     requires special support.  In particular, if your compiler
     generates a.out, MIPS ECOFF, RS/6000 XCOFF, or ELF with stabs
     extensions to the symbol table, these facilities are all
     available.  (With GNU CC, you can use the `-gstabs' option to
     request stabs debugging extensions explicitly.)  Where the object
     code format is standard COFF or DWARF in ELF, on the other hand,
     most of the C++ support in GDB does _not_ work.

  1. Member function calls are allowed; you can use expressions like

          count = aml->GetOriginal(x, y)

  2. While a member function is active (in the selected stack frame),
     your expressions have the same namespace available as the member
     function; that is, GDB allows implicit references to the class
     instance pointer `this' following the same rules as C++.

  3. You can call overloaded functions; GDB resolves the function call
     to the right definition, with some restrictions.  GDB does not
     perform overload resolution involving user-defined type
     conversions, calls to constructors, or instantiations of templates
     that do not exist in the program.  It also cannot handle ellipsis
     argument lists or default arguments.

     It does perform integral conversions and promotions, floating-point
     promotions, arithmetic conversions, pointer conversions,
     conversions of class objects to base classes, and standard
     conversions such as those of functions or arrays to pointers; it
     requires an exact match on the number of function arguments.

     Overload resolution is always performed, unless you have specified
     `set overload-resolution off'.  *Note GDB features for C++:
     Debugging C plus plus.

     You must specify `set overload-resolution off' in order to use an
     explicit function signature to call an overloaded function, as in
          p 'foo(char,int)'('x', 13)

     The GDB command-completion facility can simplify this; see *Note
     Command completion: Completion.

  4. GDB understands variables declared as C++ references; you can use
     them in expressions just as you do in C++ source--they are
     automatically dereferenced.

     In the parameter list shown when GDB displays a frame, the values
     of reference variables are not displayed (unlike other variables);
     this avoids clutter, since references are often used for large
     structures.  The _address_ of a reference variable is always
     shown, unless you have specified `set print address off'.

  5. GDB supports the C++ name resolution operator `::'--your
     expressions can use it just as expressions in your program do.
     Since one scope may be defined in another, you can use `::'
     repeatedly if necessary, for example in an expression like
     `SCOPE1::SCOPE2::NAME'.  GDB also allows resolving name scope by
     reference to source files, in both C and C++ debugging (*note
     Program variables: Variables.).

   In addition, when used with HP's C++ compiler, GDB supports calling
virtual functions correctly, printing out virtual bases of objects,
calling functions in a base subobject, casting objects, and invoking
user-defined operators.


File: gdb.info,  Node: C Defaults,  Next: C Checks,  Prev: C plus plus expressions,  Up: C

C and C++ defaults
..................

   If you allow GDB to set type and range checking automatically, they
both default to `off' whenever the working language changes to C or
C++.  This happens regardless of whether you or GDB selects the working
language.

   If you allow GDB to set the language automatically, it recognizes
source files whose names end with `.c', `.C', or `.cc', etc, and when
GDB enters code compiled from one of these files, it sets the working
language to C or C++.  *Note Having GDB infer the source language:
Automatically, for further details.


File: gdb.info,  Node: C Checks,  Next: Debugging C,  Prev: C Defaults,  Up: C

C and C++ type and range checks
...............................

   By default, when GDB parses C or C++ expressions, type checking is
not used.  However, if you turn type checking on, GDB considers two
variables type equivalent if:

   * The two variables are structured and have the same structure,
     union, or enumerated tag.

   * The two variables have the same type name, or types that have been
     declared equivalent through `typedef'.


   Range checking, if turned on, is done on mathematical operations.
Array indices are not checked, since they are often used to index a
pointer that is not itself an array.


File: gdb.info,  Node: Debugging C,  Next: Debugging C plus plus,  Prev: C Checks,  Up: C

GDB and C
.........

   The `set print union' and `show print union' commands apply to the
`union' type.  When set to `on', any `union' that is inside a `struct'
or `class' is also printed.  Otherwise, it appears as `{...}'.

   The `@' operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function.  *Note Expressions:
Expressions.

* Menu:

* Debugging C plus plus::


File: gdb.info,  Node: Debugging C plus plus,  Prev: Debugging C,  Up: C

GDB features for C++
....................

   Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++.  Here is a summary:

`breakpoint menus'
     When you want a breakpoint in a function whose name is overloaded,
     GDB breakpoint menus help you specify which function definition
     you want.  *Note Breakpoint menus: Breakpoint Menus.

`rbreak REGEX'
     Setting breakpoints using regular expressions is helpful for
     setting breakpoints on overloaded functions that are not members
     of any special classes.  *Note Setting breakpoints: Set Breaks.

`catch throw'
`catch catch'
     Debug C++ exception handling using these commands.  *Note Setting
     catchpoints: Set Catchpoints.

`ptype TYPENAME'
     Print inheritance relationships as well as other information for
     type TYPENAME.  *Note Examining the Symbol Table: Symbols.

`set print demangle'
`show print demangle'
`set print asm-demangle'
`show print asm-demangle'
     Control whether C++ symbols display in their source form, both when
     displaying code as C++ source and when displaying disassemblies.
     *Note Print settings: Print Settings.

`set print object'
`show print object'
     Choose whether to print derived (actual) or declared types of
     objects.  *Note Print settings: Print Settings.

`set print vtbl'
`show print vtbl'
     Control the format for printing virtual function tables.  *Note
     Print settings: Print Settings.  (The `vtbl' commands do not work
     on programs compiled with the HP ANSI C++ compiler (`aCC').)

`set overload-resolution on'
     Enable overload resolution for C++ expression evaluation.  The
     default is on.  For overloaded functions, GDB evaluates the
     arguments and searches for a function whose signature matches the
     argument types, using the standard C++ conversion rules (see *Note
     C++ expressions: C plus plus expressions, for details).  If it
     cannot find a match, it emits a message.

`set overload-resolution off'
     Disable overload resolution for C++ expression evaluation.  For
     overloaded functions that are not class member functions, GDB
     chooses the first function of the specified name that it finds in
     the symbol table, whether or not its arguments are of the correct
     type.  For overloaded functions that are class member functions,
     GDB searches for a function whose signature _exactly_ matches the
     argument types.

`Overloaded symbol names'
     You can specify a particular definition of an overloaded symbol,
     using the same notation that is used to declare such symbols in
     C++: type `SYMBOL(TYPES)' rather than just SYMBOL.  You can also
     use the GDB command-line word completion facilities to list the
     available choices, or to finish the type list for you.  *Note
     Command completion: Completion, for details on how to do this.


File: gdb.info,  Node: Modula-2,  Next: Chill,  Prev: C,  Up: Support

Modula-2
--------

   The extensions made to GDB to support Modula-2 only support output
from the GNU Modula-2 compiler (which is currently being developed).
Other Modula-2 compilers are not currently supported, and attempting to
debug executables produced by them is most likely to give an error as
GDB reads in the executable's symbol table.

* Menu:

* M2 Operators::                Built-in operators
* Built-In Func/Proc::          Built-in functions and procedures
* M2 Constants::                Modula-2 constants
* M2 Defaults::                 Default settings for Modula-2
* Deviations::                  Deviations from standard Modula-2
* M2 Checks::                   Modula-2 type and range checks
* M2 Scope::                    The scope operators `::' and `.'
* GDB/M2::                      GDB and Modula-2


File: gdb.info,  Node: M2 Operators,  Next: Built-In Func/Proc,  Up: Modula-2

Operators
.........

   Operators must be defined on values of specific types.  For instance,
`+' is defined on numbers, but not on structures.  Operators are often
defined on groups of types.  For the purposes of Modula-2, the
following definitions hold:

   * _Integral types_ consist of `INTEGER', `CARDINAL', and their
     subranges.

   * _Character types_ consist of `CHAR' and its subranges.

   * _Floating-point types_ consist of `REAL'.

   * _Pointer types_ consist of anything declared as `POINTER TO TYPE'.

   * _Scalar types_ consist of all of the above.

   * _Set types_ consist of `SET' and `BITSET' types.

   * _Boolean types_ consist of `BOOLEAN'.

The following operators are supported, and appear in order of
increasing precedence:

`,'
     Function argument or array index separator.

`:='
     Assignment.  The value of VAR `:=' VALUE is VALUE.

`<, >'
     Less than, greater than on integral, floating-point, or enumerated
     types.

`<=, >='
     Less than or equal to, greater than or equal to on integral,
     floating-point and enumerated types, or set inclusion on set
     types.  Same precedence as `<'.

`=, <>, #'
     Equality and two ways of expressing inequality, valid on scalar
     types.  Same precedence as `<'.  In GDB scripts, only `<>' is
     available for inequality, since `#' conflicts with the script
     comment character.

`IN'
     Set membership.  Defined on set types and the types of their
     members.  Same precedence as `<'.

`OR'
     Boolean disjunction.  Defined on boolean types.

`AND, &'
     Boolean conjunction.  Defined on boolean types.

`@'
     The GDB "artificial array" operator (*note Expressions:
     Expressions.).

`+, -'
     Addition and subtraction on integral and floating-point types, or
     union and difference on set types.

`*'
     Multiplication on integral and floating-point types, or set
     intersection on set types.

`/'
     Division on floating-point types, or symmetric set difference on
     set types.  Same precedence as `*'.

`DIV, MOD'
     Integer division and remainder.  Defined on integral types.  Same
     precedence as `*'.

`-'
     Negative. Defined on `INTEGER' and `REAL' data.

`^'
     Pointer dereferencing.  Defined on pointer types.

`NOT'
     Boolean negation.  Defined on boolean types.  Same precedence as
     `^'.

`.'
     `RECORD' field selector.  Defined on `RECORD' data.  Same
     precedence as `^'.

`[]'
     Array indexing.  Defined on `ARRAY' data.  Same precedence as `^'.

`()'
     Procedure argument list.  Defined on `PROCEDURE' objects.  Same
     precedence as `^'.

`::, .'
     GDB and Modula-2 scope operators.

     _Warning:_ Sets and their operations are not yet supported, so GDB
     treats the use of the operator `IN', or the use of operators `+',
     `-', `*', `/', `=', , `<>', `#', `<=', and `>=' on sets as an
     error.


File: gdb.info,  Node: Built-In Func/Proc,  Next: M2 Constants,  Prev: M2 Operators,  Up: Modula-2

Built-in functions and procedures
.................................

   Modula-2 also makes available several built-in procedures and
functions.  In describing these, the following metavariables are used:

A
     represents an `ARRAY' variable.

C
     represents a `CHAR' constant or variable.

I
     represents a variable or constant of integral type.

M
     represents an identifier that belongs to a set.  Generally used in
     the same function with the metavariable S.  The type of S should
     be `SET OF MTYPE' (where MTYPE is the type of M).

N
     represents a variable or constant of integral or floating-point
     type.

R
     represents a variable or constant of floating-point type.

T
     represents a type.

V
     represents a variable.

X
     represents a variable or constant of one of many types.  See the
     explanation of the function for details.

   All Modula-2 built-in procedures also return a result, described
below.

`ABS(N)'
     Returns the absolute value of N.

`CAP(C)'
     If C is a lower case letter, it returns its upper case equivalent,
     otherwise it returns its argument.

`CHR(I)'
     Returns the character whose ordinal value is I.

`DEC(V)'
     Decrements the value in the variable V by one.  Returns the new
     value.

`DEC(V,I)'
     Decrements the value in the variable V by I.  Returns the new
     value.

`EXCL(M,S)'
     Removes the element M from the set S.  Returns the new set.

`FLOAT(I)'
     Returns the floating point equivalent of the integer I.

`HIGH(A)'
     Returns the index of the last member of A.

`INC(V)'
     Increments the value in the variable V by one.  Returns the new
     value.

`INC(V,I)'
     Increments the value in the variable V by I.  Returns the new
     value.

`INCL(M,S)'
     Adds the element M to the set S if it is not already there.
     Returns the new set.

`MAX(T)'
     Returns the maximum value of the type T.

`MIN(T)'
     Returns the minimum value of the type T.

`ODD(I)'
     Returns boolean TRUE if I is an odd number.

`ORD(X)'
     Returns the ordinal value of its argument.  For example, the
     ordinal value of a character is its ASCII value (on machines
     supporting the ASCII character set).  X must be of an ordered
     type, which include integral, character and enumerated types.

`SIZE(X)'
     Returns the size of its argument.  X can be a variable or a type.

`TRUNC(R)'
     Returns the integral part of R.

`VAL(T,I)'
     Returns the member of the type T whose ordinal value is I.

     _Warning:_  Sets and their operations are not yet supported, so
     GDB treats the use of procedures `INCL' and `EXCL' as an error.


File: gdb.info,  Node: M2 Constants,  Next: M2 Defaults,  Prev: Built-In Func/Proc,  Up: Modula-2

Constants
.........

   GDB allows you to express the constants of Modula-2 in the following
ways:

   * Integer constants are simply a sequence of digits.  When used in an
     expression, a constant is interpreted to be type-compatible with
     the rest of the expression.  Hexadecimal integers are specified by
     a trailing `H', and octal integers by a trailing `B'.

   * Floating point constants appear as a sequence of digits, followed
     by a decimal point and another sequence of digits.  An optional
     exponent can then be specified, in the form `E[+|-]NNN', where
     `[+|-]NNN' is the desired exponent.  All of the digits of the
     floating point constant must be valid decimal (base 10) digits.

   * Character constants consist of a single character enclosed by a
     pair of like quotes, either single (`'') or double (`"').  They may
     also be expressed by their ordinal value (their ASCII value,
     usually) followed by a `C'.

   * String constants consist of a sequence of characters enclosed by a
     pair of like quotes, either single (`'') or double (`"').  Escape
     sequences in the style of C are also allowed.  *Note C and C++
     constants: C Constants, for a brief explanation of escape
     sequences.

   * Enumerated constants consist of an enumerated identifier.

   * Boolean constants consist of the identifiers `TRUE' and `FALSE'.

   * Pointer constants consist of integral values only.

   * Set constants are not yet supported.


File: gdb.info,  Node: M2 Defaults,  Next: Deviations,  Prev: M2 Constants,  Up: Modula-2

Modula-2 defaults
.................

   If type and range checking are set automatically by GDB, they both
default to `on' whenever the working language changes to Modula-2.
This happens regardless of whether you or GDB selected the working
language.

   If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with `.mod' sets the working
language to Modula-2.  *Note Having GDB set the language automatically:
Automatically, for further details.


File: gdb.info,  Node: Deviations,  Next: M2 Checks,  Prev: M2 Defaults,  Up: Modula-2

Deviations from standard Modula-2
.................................

   A few changes have been made to make Modula-2 programs easier to
debug.  This is done primarily via loosening its type strictness:

   * Unlike in standard Modula-2, pointer constants can be formed by
     integers.  This allows you to modify pointer variables during
     debugging.  (In standard Modula-2, the actual address contained in
     a pointer variable is hidden from you; it can only be modified
     through direct assignment to another pointer variable or
     expression that returned a pointer.)

   * C escape sequences can be used in strings and characters to
     represent non-printable characters.  GDB prints out strings with
     these escape sequences embedded.  Single non-printable characters
     are printed using the `CHR(NNN)' format.

   * The assignment operator (`:=') returns the value of its right-hand
     argument.

   * All built-in procedures both modify _and_ return their argument.


File: gdb.info,  Node: M2 Checks,  Next: M2 Scope,  Prev: Deviations,  Up: Modula-2

Modula-2 type and range checks
..............................

     _Warning:_ in this release, GDB does not yet perform type or range
     checking.

   GDB considers two Modula-2 variables type equivalent if:

   * They are of types that have been declared equivalent via a `TYPE
     T1 = T2' statement

   * They have been declared on the same line.  (Note:  This is true of
     the GNU Modula-2 compiler, but it may not be true of other
     compilers.)

   As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.

   Range checking is done on all mathematical operations, assignment,
array index bounds, and all built-in functions and procedures.


File: gdb.info,  Node: M2 Scope,  Next: GDB/M2,  Prev: M2 Checks,  Up: Modula-2

The scope operators `::' and `.'
................................

   There are a few subtle differences between the Modula-2 scope
operator (`.') and the GDB scope operator (`::').  The two have similar
syntax:


     MODULE . ID
     SCOPE :: ID

where SCOPE is the name of a module or a procedure, MODULE the name of
a module, and ID is any declared identifier within your program, except
another module.

   Using the `::' operator makes GDB search the scope specified by
SCOPE for the identifier ID.  If it is not found in the specified
scope, then GDB searches all scopes enclosing the one specified by
SCOPE.

   Using the `.' operator makes GDB search the current scope for the
identifier specified by ID that was imported from the definition module
specified by MODULE.  With this operator, it is an error if the
identifier ID was not imported from definition module MODULE, or if ID
is not an identifier in MODULE.


File: gdb.info,  Node: GDB/M2,  Prev: M2 Scope,  Up: Modula-2

GDB and Modula-2
................

   Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of `set print' and `show print' apply specifically to
C and C++: `vtbl', `demangle', `asm-demangle', `object', and `union'.
The first four apply to C++, and the last to the C `union' type, which
has no direct analogue in Modula-2.

   The `@' operator (*note Expressions: Expressions.), while available
with any language, is not useful with Modula-2.  Its intent is to aid
the debugging of "dynamic arrays", which cannot be created in Modula-2
as they can in C or C++.  However, because an address can be specified
by an integral constant, the construct `{TYPE}ADREXP' is still useful.

   In GDB scripts, the Modula-2 inequality operator `#' is interpreted
as the beginning of a comment.  Use `<>' instead.


File: gdb.info,  Node: Chill,  Prev: Modula-2,  Up: Support

Chill
-----

   The extensions made to GDB to support Chill only support output from
the GNU Chill compiler.  Other Chill compilers are not currently
supported, and attempting to debug executables produced by them is most
likely to give an error as GDB reads in the executable's symbol table.

   This section covers the Chill related topics and the features of GDB
which support these topics.

* Menu:

* How modes are displayed::        How modes are displayed
* Locations::                        Locations and their accesses
* Values and their Operations:: Values and their Operations
* Chill type and range checks::
* Chill defaults::


File: gdb.info,  Node: How modes are displayed,  Next: Locations,  Up: Chill

How modes are displayed
.......................

   The Chill Datatype- (Mode) support of GDB is directly related with
the functionality of the GNU Chill compiler, and therefore deviates
slightly from the standard specification of the Chill language. The
provided modes are:

`_Discrete modes:_'
        * _Integer Modes_ which are predefined by `BYTE, UBYTE, INT,
          UINT, LONG, ULONG',

        * _Boolean Mode_ which is predefined by `BOOL',

        * _Character Mode_ which is predefined by `CHAR',

        * _Set Mode_ which is displayed by the keyword `SET'.
               (gdb) ptype x
               type = SET (karli = 10, susi = 20, fritzi = 100)
          If the type is an unnumbered set the set element values are
          omitted.

        * _Range Mode_ which is displayed by
               `type = <basemode>(<lower bound> : <upper bound>)'
          where `<lower bound>, <upper bound>' can be of any discrete
          literal expression (e.g. set element names).

`_Powerset Mode:_'
     A Powerset Mode is displayed by the keyword `POWERSET' followed by
     the member mode of the powerset.  The member mode can be any
     discrete mode.
          (gdb) ptype x
          type = POWERSET SET (egon, hugo, otto)

`_Reference Modes:_'
        * _Bound Reference Mode_ which is displayed by the keyword `REF'
          followed by the mode name to which the reference is bound.

        * _Free Reference Mode_ which is displayed by the keyword `PTR'.

`_Procedure mode_'
     The procedure mode is displayed by `type = PROC(<parameter list>)
     <return mode> EXCEPTIONS (<exception list>)'. The `<parameter
     list>' is a list of the parameter modes.  `<return mode>' indicates
     the mode of the result of the procedure if any.  The exceptionlist
     lists all possible exceptions which can be raised by the procedure.

`_Synchronization Modes:_'
        * _Event Mode_ which is displayed by
               `EVENT (<event length>)'
          where `(<event length>)' is optional.

        * _Buffer Mode_ which is displayed by
               `BUFFER (<buffer length>)<buffer element mode>'
          where `(<buffer length>)' is optional.

`_Timing Modes:_'
        * _Duration Mode_ which is predefined by `DURATION'

        * _Absolute Time Mode_ which is predefined by `TIME'

`_Real Modes:_'
     Real Modes are predefined with `REAL' and `LONG_REAL'.

`_String Modes:_'
        * _Character String Mode_ which is displayed by
               `CHARS(<string length>)'
          followed by the keyword `VARYING' if the String Mode is a
          varying mode

        * _Bit String Mode_ which is displayed by
               `BOOLS(<string
               length>)'

`_Array Mode:_'
     The Array Mode is displayed by the keyword `ARRAY(<range>)'
     followed by the element mode (which may in turn be an array mode).
          (gdb) ptype x
          type = ARRAY (1:42)
                    ARRAY (1:20)
                       SET (karli = 10, susi = 20, fritzi = 100)

`_Structure Mode_'
     The Structure mode is displayed by the keyword `STRUCT(<field
     list>)'.  The `<field list>' consists of names and modes of fields
     of the structure.  Variant structures have the keyword `CASE
     <field> OF <variant fields> ESAC' in their field list.  Since the
     current version of the GNU Chill compiler doesn't implement tag
     processing (no runtime checks of variant fields, and therefore no
     debugging info), the output always displays all variant fields.
          (gdb) ptype str
          type = STRUCT (
              as x,
              bs x,
              CASE bs OF
              (karli):
                  cs a
              (ott):
                  ds x
              ESAC
          )


File: gdb.info,  Node: Locations,  Next: Values and their Operations,  Prev: How modes are displayed,  Up: Chill

Locations and their accesses
............................

   A location in Chill is an object which can contain values.

   A value of a location is generally accessed by the (declared) name of
the location.  The output conforms to the specification of values in
Chill programs.  How values are specified is the topic of the next
section, *Note Values and their Operations::.

   The pseudo-location `RESULT' (or `result') can be used to display or
change the result of a currently-active procedure:

     set result := EXPR

This does the same as the Chill action `RESULT EXPR' (which is not
available in GDB).

   Values of reference mode locations are printed by `PTR(<hex value>)'
in case of a free reference mode, and by `(REF <reference mode>)
(<hex-value>)' in case of a bound reference.  `<hex value>' represents
the address where the reference points to.  To access the value of the
location referenced by the pointer, use the dereference operator `->'.

   Values of procedure mode locations are displayed by
     `{ PROC
     (<argument modes> ) <return mode> } <address> <name of procedure
     location>'
   `<argument modes>' is a list of modes according to the parameter
specification of the procedure and `<address>' shows the address of the
entry point.

   Substructures of string mode-, array mode- or structure mode-values
(e.g. array slices, fields of structure locations) are accessed using
certain operations which are described in the next section, *Note
Values and their Operations::.

   A location value may be interpreted as having a different mode using
the location conversion.  This mode conversion is written as `<mode
name>(<location>)'.  The user has to consider that the sizes of the
modes have to be equal otherwise an error occurs.  Furthermore, no range
checking of the location against the destination mode is performed, and
therefore the result can be quite confusing.

     (gdb) print int (s(3 up 4)) XXX TO be filled in !! XXX


File: gdb.info,  Node: Values and their Operations,  Next: Chill type and range checks,  Prev: Locations,  Up: Chill

Values and their Operations
...........................

   Values are used to alter locations, to investigate complex
structures in more detail or to filter relevant information out of a
large amount of data.  There are several (mode dependent) operations
defined which enable such investigations.  These operations are not
only applicable to constant values but also to locations, which can
become quite useful when debugging complex structures.  During parsing
the command line (e.g. evaluating an expression) GDB treats location
names as the values behind these locations.

   This section describes how values have to be specified and which
operations are legal to be used with such values.

`Literal Values'
     Literal values are specified in the same manner as in GNU Chill
     programs.  For detailed specification refer to the GNU Chill
     implementation Manual chapter 1.5.

`Tuple Values'
     A tuple is specified by `<mode name>[<tuple>]', where `<mode
     name>' can be omitted if the mode of the tuple is unambiguous.
     This unambiguity is derived from the context of a evaluated
     expression.  `<tuple>' can be one of the following:

        * _Powerset Tuple_

        * _Array Tuple_

        * _Structure Tuple_ Powerset tuples, array tuples and structure
          tuples are specified in the same manner as in Chill programs
          refer to z200/88 chpt 5.2.5.

`String Element Value'
     A string element value is specified by
          `<string value>(<index>)'
     where `<index>' is a integer expression.  It delivers a character
     value which is equivalent to the character indexed by `<index>' in
     the string.

`String Slice Value'
     A string slice value is specified by `<string value>(<slice
     spec>)', where `<slice spec>' can be either a range of integer
     expressions or specified by `<start expr> up <size>'.  `<size>'
     denotes the number of elements which the slice contains.  The
     delivered value is a string value, which is part of the specified
     string.

`Array Element Values'
     An array element value is specified by `<array value>(<expr>)' and
     delivers a array element value of the mode of the specified array.

`Array Slice Values'
     An array slice is specified by `<array value>(<slice spec>)', where
     `<slice spec>' can be either a range specified by expressions or by
     `<start expr> up <size>'.  `<size>' denotes the number of
     arrayelements the slice contains.  The delivered value is an array
     value which is part of the specified array.

`Structure Field Values'
     A structure field value is derived by `<structure value>.<field
     name>', where `<field name>' indicates the name of a field
     specified in the mode definition of the structure.  The mode of
     the delivered value corresponds to this mode definition in the
     structure definition.

`Procedure Call Value'
     The procedure call value is derived from the return value of the
     procedure(1).

     Values of duration mode locations are represented by `ULONG'
     literals.

     Values of time mode locations appear as
          `TIME(<secs>:<nsecs>)'

`Zero-adic Operator Value'
     The zero-adic operator value is derived from the instance value
     for the current active process.

`Expression Values'
     The value delivered by an expression is the result of the
     evaluation of the specified expression.  If there are error
     conditions (mode incompatibility, etc.) the evaluation of
     expressions is aborted with a corresponding error message.
     Expressions may be parenthesised which causes the evaluation of
     this expression before any other expression which uses the result
     of the parenthesised expression.  The following operators are
     supported by GDB:

    ``OR, ORIF, XOR''
    ``AND, ANDIF''
    ``NOT''
          Logical operators defined over operands of boolean mode.

    ``=, /=''
          Equality and inequality operators defined over all modes.

    ``>, >=''
    ``<, <=''
          Relational operators defined over predefined modes.

    ``+, -''
    ``*, /, MOD, REM''
          Arithmetic operators defined over predefined modes.

    ``-''
          Change sign operator.

    ``//''
          String concatenation operator.

    ``()''
          String repetition operator.

    ``->''
          Referenced location operator which can be used either to take
          the address of a location (`->loc'), or to dereference a
          reference location (`loc->').

    ``OR, XOR''
    ``AND''
    ``NOT''
          Powerset and bitstring operators.

    ``>, >=''
    ``<, <=''
          Powerset inclusion operators.

    ``IN''
          Membership operator.

   ---------- Footnotes ----------

   (1) If a procedure call is used for instance in an expression, then
this procedure is called with all its side effects.  This can lead to
confusing results if used carelessly.


File: gdb.info,  Node: Chill type and range checks,  Next: Chill defaults,  Prev: Values and their Operations,  Up: Chill

Chill type and range checks
...........................

   GDB considers two Chill variables mode equivalent if the sizes of
the two modes are equal.  This rule applies recursively to more complex
datatypes which means that complex modes are treated equivalent if all
element modes (which also can be complex modes like structures, arrays,
etc.) have the same size.

   Range checking is done on all mathematical operations, assignment,
array index bounds and all built in procedures.

   Strong type checks are forced using the GDB command `set check
strong'.  This enforces strong type and range checks on all operations
where Chill constructs are used (expressions, built in functions, etc.)
in respect to the semantics as defined in the z.200 language
specification.

   All checks can be disabled by the GDB command `set check off'.


File: gdb.info,  Node: Chill defaults,  Prev: Chill type and range checks,  Up: Chill

Chill defaults
..............

   If type and range checking are set automatically by GDB, they both
default to `on' whenever the working language changes to Chill.  This
happens regardless of whether you or GDB selected the working language.

   If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with `.ch' sets the working
language to Chill.  *Note Having GDB set the language automatically:
Automatically, for further details.