4.4BSD/usr/src/contrib/gdb-4.7.lbl/gdb/gdb.info-4
This is Info file ./gdb.info, produced by Makeinfo-1.47 from the input
file gdb-all.texi.
START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
END-INFO-DIR-ENTRY
This file documents the GNU debugger GDB.
This is Edition 4.06, October 1992, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB Version 4.7.
Copyright (C) 1988, 1989, 1990, 1991, 1992 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 section entitled "GNU General Public License" is included
exactly as in the original, and provided 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, except that the section entitled "GNU General Public License"
may be included in a translation approved by the Free Software
Foundation instead of in the original English.
File: gdb.info, Node: Languages, Next: Symbols, Prev: Data, Up: Top
Using GDB with Different Languages
**********************************
Although programming languages generally have common aspects, they
are rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer `p' is accomplished by `*p', but in Modula-2,
it is accomplished by `p^'. Values can also be represented (and
displayed) differently. Hex numbers in C are written like `0x1ae',
while in Modula-2 they appear as `1AEH'.
Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program's
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program's native language. The
language you use to build expressions, called the "working language",
can be selected manually, or GDB can set it automatically.
* Menu:
* Setting:: Switching between source languages
* Show:: Displaying the language
* Checks:: Type and Range checks
* Support:: Supported languages
File: gdb.info, Node: Setting, Next: Show, Up: Languages
Switching between source languages
==================================
There are two ways to control the working language--either have GDB
set it automatically, or select it manually yourself. You can use the
`set language' command for either purpose. On startup, GDB defaults to
setting the language automatically.
* Menu:
* Manually:: Setting the working language manually
* Automatically:: Having GDB infer the source language
File: gdb.info, Node: Manually, Next: Automatically, Up: Setting
Setting the working language
----------------------------
To set the language, issue the command `set language LANG', where
LANG is the name of a language: `c' or `modula-2'. For a list of the
supported languages, type `set language'.
Setting the language manually prevents GDB from updating the working
language automatically. This can lead to confusion if you try to debug
a program when the working language is not the same as the source
language, when an expression is acceptable to both languages--but means
different things. For instance, if the current source file were
written in C, and GDB was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add `b'
and `c' and place the result in `a'. The result printed would be the
value of `a'. In Modula-2, this means to compare `a' to the result of
`b+c', yielding a `BOOLEAN' value.
If you allow GDB to set the language automatically, then you can
count on expressions evaluating the same way in your debugging session
and in your program.
File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting
Having GDB infer the source language
------------------------------------
To have GDB set the working language automatically, use `set
language local' or `set language auto'. GDB then infers the language
that a program was written in by looking at the name of its source
files, and examining their extensions:
`*.mod'
Modula-2 source file
`*.c'
C source file
`*.C'
`*.cc'
C++ source file
This information is recorded for each function or procedure in a
source file. When your program stops in a frame (usually by
encountering a breakpoint), GDB sets the working language to the
language recorded for the function in that frame. If the language for
a frame is unknown (that is, if the function or block corresponding to
the frame was defined in a source file that does not have a recognized
extension), the current working language is not changed, and GDB issues
a warning.
This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using `set language auto' in this case
frees you from having to set the working language manually.
File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages
Displaying the language
=======================
The following commands will help you find out which language is the
working language, and also what language source files were written in.
`show language'
Display the current working language. This is the language you
can use with commands such as `print' to build and compute
expressions that may involve variables in your program.
`info frame'
Among the other information listed here (*note Information about a
Frame: Frame Info.) is the source language for this frame. This
is the language that will become the working language if you ever
use an identifier that is in this frame.
`info source'
Among the other information listed here (*note Examining the
Symbol Table: Symbols.) is the source language of this source file.
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 will 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 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 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 though you may turn type checking off, other type-based reasons
may 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's default. If any type mismatches occur in
evaluating an expression while typechecking 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 ignore
range errors; to always treat them as errors and abandon the
expression; or to issue warnings when a range error occurs but evaluate
the expression anyway.
A range error can result from numerical overflow, from exceeding an
array index bound, or when you type in 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's default. If a range error occurs,
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 4 supports C, C++, 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 of the supported
languages.
The following sections detail to what degree each of these source
languages 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 will accept, 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 both languages
together.
The C++ debugging facilities are jointly implemented by the GNU C++
compiler and GDB. Therefore, to debug your C++ code effectively, you
must compile your C++ programs with the GNU C++ compiler, `g++'.
* Menu:
* C Operators:: C and C++ Operators
* C Constants:: C and C++ Constants
* Cplusplus 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:: Special 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', and `enum's.
* *Floating-point types* include `float' and `double'.
* *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 precendence. 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's
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's and `union's.
`[]'
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.
`::'
The GDB scope operator (*note Expressions: Expressions.). Same
precedence as `::', above.
File: gdb.info, Node: C Constants, Next: Cplusplus 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' (ie. 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.
* 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 (`"').
* Pointer constants are an integral value.
File: gdb.info, Node: Cplusplus expressions, Next: C Defaults, Prev: C Constants, Up: C
C++ Expressions
...............
GDB's expression handling has a number of extensions to interpret a
significant subset of C++ expressions.
*Warning:* Most of these extensions depend on the use of additional
debugging information in the symbol table, and thus require a rich,
extendable object code format. In particular, if your system uses
a.out, MIPS ECOFF, RS/6000 XCOFF, or Sun ELF with stabs extensions
to the symbol table, these facilities are all available. Where the
object code format is standard COFF, on the other hand, most of
the C++ support in GDB will *not* work, nor can it. For the
standard SVr4 debugging format, DWARF in ELF, the standard is
still evolving, so the C++ support in GDB is still fragile; when
this debugging format stabilizes, however, C++ support will also
be available on systems that use it.
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 will resolve the function
call to the right definition, with one restriction--you must use
arguments of the type required by the function that you want to
call. GDB will not perform conversions requiring constructors or
user-defined type operators.
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.).
File: gdb.info, Node: C Defaults, Next: C Checks, Prev: Cplusplus 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, selected the
working language.
If you allow GDB to set the language automatically, it sets the
working language to C or C++ on entering code compiled from a source
file whose name ends with `.c', `.C', or `.cc'. *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
...............................
*Warning:* in this release, GDB does not yet perform type or range
checking.
By default, when GDB parses C or C++ expressions, type checking is
not used. However, if you turn type checking on, GDB will consider two
variables type equivalent if:
* The two variables are structured and have the same structure,
union, or enumerated tag.
* Two 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' will also be printed. Otherwise, it will appear as `{...}'.
The `@' operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. (*note Expressions:
Expressions.)
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's breakpoint menus help you specify which function definition
you want. *Note 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 EXCEPTIONS'
`info catch'
Debug C++ exception handling using these commands. *Note
Breakpoints and Exceptions: Exception Handling.
`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.
`Overloaded symbol names'
You can specify a particular definition of an overloaded symbol,
using the same notation that's used to declare such symbols in
C++: type `SYMBOL(TYPES)' rather than just SYMBOL. You can also
use GDB's 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 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 will most likely result in 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's and `BITSET's.
* *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, greater than, 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 conjuction. 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's and `REAL's.
`^'
Pointer dereferencing. Defined on pointer types.
`NOT'
Boolean negation. Defined on boolean types. Same precedence as
`^'.
`.'
`RECORD' field selector. Defined on `RECORD's. Same precedence
as `^'.
`[]'
Array indexing. Defined on `ARRAY's. Same precedence as `^'.
`()'
Procedure argument list. Defined on `PROCEDURE's. Same precedence
as `^'.
`::, .'
GDB and Modula-2 scope operators.
*Warning:* Sets and their operations are not yet supported, so GDB
will treat 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. 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. 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 will treat 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' will set 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 will print 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 will search 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 C's `union' type, which
has no direct analogue in Modula-2.
The `@' operator (*note Expressions: Expressions.), while available
while using 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. (*note Expressions: Expressions.)
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 section 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 variables, and for a stack local variable prints
the exact address of the current instantiation of the variable.
`whatis EXP'
Print the data type of expression EXP. EXP 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 `$', the last value in the value history.
`ptype TYPENAME'
Print a description of data type TYPENAME. TYPENAME may be the
name of a type, or for C code it may have the form `struct
STRUCT-TAG', `union UNION-TAG' or `enum ENUM-TAG'.
`ptype EXP'
`ptype'
Print a description of the type of expression EXP. `ptype'
differs from `whatis' by printing a detailed description, instead
of just the name of the type. For example, if your program
declares a variable as
struct complex {double real; double imag;} v;
compare the output of the two commands:
(gdb) whatis v
type = struct complex
(gdb) ptype v
type = struct complex {
double real;
double imag;
}
As with `whatis', using `ptype' without an argument refers to the
type of `$', the last value in the value history.
`info types REGEXP'
`info types'
Print a brief description of all types whose name matches 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 name includes 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 source'
Show the name of the current source file--that is, the source file
for the function containing the current point of execution--and
the language it was written in.
`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'.
`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.
`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. The description of `symbol-file'
explains how GDB reads symbols; both `info source' and
`symbol-file' are described in *Note Commands to Specify Files:
Files.
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 to its caller.
* 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
File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering
Assignment to Variables
=======================
To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions. For example,
print x=4
stores the value 4 into the variable `x', and then prints the value of
the assignment expression (which is 4). *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.
If you are not interested in seeing the value of the assignment, use
the `set' command instead of the `print' command. `set' is really the
same as `print' except that the expression's value is not printed and
is not put in the value history (*note Value History: Value History.).
The expression is evaluated only for its effects.
If the beginning of the argument string of the `set' command appears
identical to a `set' subcommand, use the `set variable' command instead
of just `set'. This command is identical to `set' except for its lack
of subcommands. For example, a program might well have a variable
`width'--which leads to an error if we try to set a new value with just
`set width=13', as we might if `set width' did not happen to be a GDB
command:
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is `=47'. What we can do in order
to actually set our program's variable `width' is
(gdb) set var width=47
GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and any structure can be converted to any other structure that is the
same length or shorter.
To store values into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.). For example, `{int}0x83040' refers
to memory location `0x83040' as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering
Continuing at a Different Address
=================================
Ordinarily, when you continue your program, you do so at the place
where it stopped, with the `continue' command. You can instead
continue at an address of your own choosing, with the following
commands:
`jump LINESPEC'
Resume execution at line LINESPEC. Execution will stop
immediately if there is a breakpoint there. *Note Printing Source
Lines: List, for a description of the different forms of LINESPEC.
The `jump' command does not change the current stack frame, or the
stack pointer, or the contents of any memory location or any
register other than the program counter. If line LINESPEC is in a
different function from the one currently executing, the results
may be bizarre if the two functions expect different patterns of
arguments or of local variables. For this reason, the `jump'
command requests confirmation if the specified line is not in the
function currently executing. However, even bizarre results are
predictable if you are well acquainted with the machine-language
code of your program.
`jump *ADDRESS'
Resume execution at the instruction at address ADDRESS.
You can get much the same effect as the `jump' command by storing a
new value into the register `$pc'. The difference is that this does
not start your program running; it only changes the address where it
*will* run when it is continued. For example,
set $pc = 0x485
causes the next `continue' command or stepping command to execute at
address `0x485', rather than at the address where your program stopped.
*Note Continuing and Stepping: Continuing and Stepping.
The most common occasion to use the `jump' command is to back up,
perhaps with more breakpoints set, over a portion of a program that has
already executed, in order to examine its execution in more detail.
File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering
Giving your program a Signal
============================
`signal SIGNALNUM'
Resume execution where your program stopped, but give it
immediately the signal number SIGNALNUM.
Alternatively, if SIGNALNUM is zero, continue execution without
giving a signal. This is useful when your program stopped on
account of a signal and would ordinary see the signal when resumed
with the `continue' command; `signal 0' causes it to resume
without a signal.
`signal' does not repeat when you press RET a second time after
executing the command.