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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