This is gdb.info, produced by makeinfo version 4.1 from ./gdb.texinfo.

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 Ninth Edition, December 2001, of `Debugging with GDB:
the GNU Source-Level Debugger' for GDB Version 5.3.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998,
1999, 2000, 2001, 2002 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.

   (a) The Free Software Foundation's Back-Cover Text is: "You have
freedom to copy and modify this GNU Manual, like GNU software.  Copies
published by the Free Software Foundation raise funds for GNU
development."


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

Type and range checking
=======================

     _Warning:_ In this release, the GDB commands for type and range
     checking are included, but they do not yet have any effect.  This
     section documents the intended facilities.

   Some languages are designed to guard you against making seemingly
common errors through a series of compile- and run-time checks.  These
include checking the type of arguments to functions and operators, and
making sure mathematical overflows are caught at run time.  Checks such
as these help to ensure a program's correctness once it has been
compiled by eliminating type mismatches, and providing active checks
for range errors when your program is running.

   GDB can check for conditions like the above if you wish.  Although
GDB does not check the statements in your program, it can check
expressions entered directly into GDB for evaluation via the `print'
command, for example.  As with the working language, GDB can also
decide whether or not to check automatically based on your program's
source language.  *Note Supported languages: Support, for the default
settings of supported languages.

* Menu:

* Type Checking::               An overview of type checking
* Range Checking::              An overview of range checking


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

An overview of type checking
----------------------------

   Some languages, such as Modula-2, are strongly typed, meaning that
the arguments to operators and functions have to be of the correct type,
otherwise an error occurs.  These checks prevent type mismatch errors
from ever causing any run-time problems.  For example,

     1 + 2 => 3
but
     error--> 1 + 2.3

   The second example fails because the `CARDINAL' 1 is not
type-compatible with the `REAL' 2.3.

   For the expressions you use in GDB commands, you can tell the GDB
type checker to skip checking; to treat any mismatches as errors and
abandon the expression; or to only issue warnings when type mismatches
occur, but evaluate the expression anyway.  When you choose the last of
these, GDB evaluates expressions like the second example above, but
also issues a warning.

   Even if you turn type checking off, there may be other reasons
related to type that prevent GDB from evaluating an expression.  For
instance, GDB does not know how to add an `int' and a `struct foo'.
These particular type errors have nothing to do with the language in
use, and usually arise from expressions, such as the one described
above, which make little sense to evaluate anyway.

   Each language defines to what degree it is strict about type.  For
instance, both Modula-2 and C require the arguments to arithmetical
operators to be numbers.  In C, enumerated types and pointers can be
represented as numbers, so that they are valid arguments to mathematical
operators.  *Note Supported languages: Support, for further details on
specific languages.

   GDB provides some additional commands for controlling the type
checker:

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

`set check type on'
`set check type off'
     Set type checking on or off, overriding the default setting for the
     current working language.  Issue a warning if the setting does not
     match the language default.  If any type mismatches occur in
     evaluating an expression while type checking is on, GDB prints a
     message and aborts evaluation of the expression.

`set check type warn'
     Cause the type checker to issue warnings, but to always attempt to
     evaluate the expression.  Evaluating the expression may still be
     impossible for other reasons.  For example, GDB cannot add numbers
     and structures.

`show type'
     Show the current setting of the type checker, and whether or not
     GDB is setting it automatically.


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, 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


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,  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: Symbols,  Next: Altering,  Prev: Languages,  Up: Top

Examining the Symbol Table
**************************

   The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program.  This information is inherent in the text of your program and
does not change as your program executes.  GDB finds it in your
program's symbol table, in the file indicated when you started GDB
(*note Choosing files: File Options.), or by one of the file-management
commands (*note Commands to specify files: Files.).

   Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters.  The most
frequent case is in referring to static variables in other source files
(*note Program variables: Variables.).  File names are recorded in
object files as debugging symbols, but GDB would ordinarily parse a
typical file name, like `foo.c', as the three words `foo' `.' `c'.  To
allow GDB to recognize `foo.c' as a single symbol, enclose it in single
quotes; for example,

     p 'foo.c'::x

looks up the value of `x' in the scope of the file `foo.c'.

`info address SYMBOL'
     Describe where the data for SYMBOL is stored.  For a register
     variable, this says which register it is kept in.  For a
     non-register local variable, this prints the stack-frame offset at
     which the variable is always stored.

     Note the contrast with `print &SYMBOL', which does not work at all
     for a register variable, and for a stack local variable prints the
     exact address of the current instantiation of the variable.

`info symbol ADDR'
     Print the name of a symbol which is stored at the address ADDR.
     If no symbol is stored exactly at ADDR, GDB prints the nearest
     symbol and an offset from it:

          (gdb) info symbol 0x54320
          _initialize_vx + 396 in section .text

     This is the opposite of the `info address' command.  You can use
     it to find out the name of a variable or a function given its
     address.

`whatis EXPR'
     Print the data type of expression EXPR.  EXPR is not actually
     evaluated, and any side-effecting operations (such as assignments
     or function calls) inside it do not take place.  *Note
     Expressions: Expressions.

`whatis'
     Print the data type of `$', the last value in the value history.

`ptype TYPENAME'
     Print a description of data type TYPENAME.  TYPENAME may be the
     name of a type, or for C code it may have the form `class
     CLASS-NAME', `struct STRUCT-TAG', `union UNION-TAG' or `enum
     ENUM-TAG'.

`ptype EXPR'
`ptype'
     Print a description of the type of expression EXPR.  `ptype'
     differs from `whatis' by printing a detailed description, instead
     of just the name of the type.

     For example, for this variable declaration:

          struct complex {double real; double imag;} v;

     the two commands give this output:

          (gdb) whatis v
          type = struct complex
          (gdb) ptype v
          type = struct complex {
              double real;
              double imag;
          }

     As with `whatis', using `ptype' without an argument refers to the
     type of `$', the last value in the value history.

`info types REGEXP'
`info types'
     Print a brief description of all types whose names match REGEXP
     (or all types in your program, if you supply no argument).  Each
     complete typename is matched as though it were a complete line;
     thus, `i type value' gives information on all types in your
     program whose names include the string `value', but `i type
     ^value$' gives information only on types whose complete name is
     `value'.

     This command differs from `ptype' in two ways: first, like
     `whatis', it does not print a detailed description; second, it
     lists all source files where a type is defined.

`info scope ADDR'
     List all the variables local to a particular scope.  This command
     accepts a location--a function name, a source line, or an address
     preceded by a `*', and prints all the variables local to the scope
     defined by that location.  For example:

          (gdb) info scope command_line_handler
          Scope for command_line_handler:
          Symbol rl is an argument at stack/frame offset 8, length 4.
          Symbol linebuffer is in static storage at address 0x150a18, length 4.
          Symbol linelength is in static storage at address 0x150a1c, length 4.
          Symbol p is a local variable in register $esi, length 4.
          Symbol p1 is a local variable in register $ebx, length 4.
          Symbol nline is a local variable in register $edx, length 4.
          Symbol repeat is a local variable at frame offset -8, length 4.

     This command is especially useful for determining what data to
     collect during a "trace experiment", see *Note collect: Tracepoint
     Actions.

`info source'
     Show information about the current source file--that is, the
     source file for the function containing the current point of
     execution:
        * the name of the source file, and the directory containing it,

        * the directory it was compiled in,

        * its length, in lines,

        * which programming language it is written in,

        * whether the executable includes debugging information for
          that file, and if so, what format the information is in
          (e.g., STABS, Dwarf 2, etc.), and

        * whether the debugging information includes information about
          preprocessor macros.

`info sources'
     Print the names of all source files in your program for which
     there is debugging information, organized into two lists: files
     whose symbols have already been read, and files whose symbols will
     be read when needed.

`info functions'
     Print the names and data types of all defined functions.

`info functions REGEXP'
     Print the names and data types of all defined functions whose
     names contain a match for regular expression REGEXP.  Thus, `info
     fun step' finds all functions whose names include `step'; `info
     fun ^step' finds those whose names start with `step'.  If a
     function name contains characters that conflict with the regular
     expression language (eg.  `operator*()'), they may be quoted with
     a backslash.

`info variables'
     Print the names and data types of all variables that are declared
     outside of functions (i.e. excluding local variables).

`info variables REGEXP'
     Print the names and data types of all variables (except for local
     variables) whose names contain a match for regular expression
     REGEXP.

     Some systems allow individual object files that make up your
     program to be replaced without stopping and restarting your
     program.  For example, in VxWorks you can simply recompile a
     defective object file and keep on running.  If you are running on
     one of these systems, you can allow GDB to reload the symbols for
     automatically relinked modules:

    `set symbol-reloading on'
          Replace symbol definitions for the corresponding source file
          when an object file with a particular name is seen again.

    `set symbol-reloading off'
          Do not replace symbol definitions when encountering object
          files of the same name more than once.  This is the default
          state; if you are not running on a system that permits
          automatic relinking of modules, you should leave
          `symbol-reloading' off, since otherwise GDB may discard
          symbols when linking large programs, that may contain several
          modules (from different directories or libraries) with the
          same name.

    `show symbol-reloading'
          Show the current `on' or `off' setting.

`set opaque-type-resolution on'
     Tell GDB to resolve opaque types.  An opaque type is a type
     declared as a pointer to a `struct', `class', or `union'--for
     example, `struct MyType *'--that is used in one source file
     although the full declaration of `struct MyType' is in another
     source file.  The default is on.

     A change in the setting of this subcommand will not take effect
     until the next time symbols for a file are loaded.

`set opaque-type-resolution off'
     Tell GDB not to resolve opaque types.  In this case, the type is
     printed as follows:
          {<no data fields>}

`show opaque-type-resolution'
     Show whether opaque types are resolved or not.

`maint print symbols FILENAME'
`maint print psymbols FILENAME'
`maint print msymbols FILENAME'
     Write a dump of debugging symbol data into the file FILENAME.
     These commands are used to debug the GDB symbol-reading code.  Only
     symbols with debugging data are included.  If you use `maint print
     symbols', GDB includes all the symbols for which it has already
     collected full details: that is, FILENAME reflects symbols for
     only those files whose symbols GDB has read.  You can use the
     command `info sources' to find out which files these are.  If you
     use `maint print psymbols' instead, the dump shows information
     about symbols that GDB only knows partially--that is, symbols
     defined in files that GDB has skimmed, but not yet read
     completely.  Finally, `maint print msymbols' dumps just the
     minimal symbol information required for each object file from
     which GDB has read some symbols.  *Note Commands to specify files:
     Files, for a discussion of how GDB reads symbols (in the
     description of `symbol-file').


File: gdb.info,  Node: Altering,  Next: GDB Files,  Prev: Symbols,  Up: Top

Altering Execution
******************

   Once you think you have found an error in your program, you might
want to find out for certain whether correcting the apparent error
would lead to correct results in the rest of the run.  You can find the
answer by experiment, using the GDB features for altering execution of
the program.

   For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.

* Menu:

* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
* Signaling::                   Giving your program a signal
* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program

