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This is ./gdb.info, produced by Makeinfo version 3.12f 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 Eighth Edition, March 2000, of `Debugging with GDB: the
GNU Source-Level Debugger' for GDB Version 5.0.
Copyright (C) 1988-2000 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.

File: gdb.info, Node: Expressions, Next: Variables, Up: Data
Expressions
===========
`print' and many other GDB commands accept an expression and compute
its value. Any kind of constant, variable or operator defined by the
programming language you are using is valid in an expression in GDB.
This includes conditional expressions, function calls, casts and string
constants. It unfortunately does not include symbols defined by
preprocessor `#define' commands.
GDB supports array constants in expressions input by the user. The
syntax is {ELEMENT, ELEMENT...}. For example, you can use the command
`print {1, 2, 3}' to build up an array in memory that is `malloc'ed in
the target program.
Because C is so widespread, most of the expressions shown in
examples in this manual are in C. *Note Using GDB with Different
Languages: Languages, for information on how to use expressions in other
languages.
In this section, we discuss operators that you can use in GDB
expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer in order to examine a structure
at that address in memory.
GDB supports these operators, in addition to those common to
programming languages:
`@'
`@' is a binary operator for treating parts of memory as arrays.
*Note Artificial arrays: Arrays, for more information.
`::'
`::' allows you to specify a variable in terms of the file or
function where it is defined. *Note Program variables: Variables.
`{TYPE} ADDR'
Refers to an object of type TYPE stored at address ADDR in memory.
ADDR may be any expression whose value is an integer or pointer
(but parentheses are required around binary operators, just as in
a cast). This construct is allowed regardless of what kind of
data is normally supposed to reside at ADDR.

File: gdb.info, Node: Variables, Next: Arrays, Prev: Expressions, Up: Data
Program variables
=================
The most common kind of expression to use is the name of a variable
in your program.
Variables in expressions are understood in the selected stack frame
(*note Selecting a frame: Selection.); they must be either:
* global (or file-static)
or
* visible according to the scope rules of the programming language
from the point of execution in that frame
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable `a' whenever your program is
executing within the function `foo', but you can only use or examine
the variable `b' while your program is executing inside the block where
`b' is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file,
using the colon-colon notation:
FILE::VARIABLE
FUNCTION::VARIABLE
Here FILE or FUNCTION is the name of the context for the static
VARIABLE. In the case of file names, you can use quotes to make sure
GDB parses the file name as a single word--for example, to print a
global value of `x' defined in `f2.c':
(gdb) p 'f2.c'::x
This use of `::' is very rarely in conflict with the very similar
use of the same notation in C++. GDB also supports use of the C++
scope resolution operator in GDB expressions.
_Warning:_ Occasionally, a local variable may appear to have the
wrong value at certain points in a function--just after entry to a
new scope, and just before exit.
You may see this problem when you are stepping by machine
instructions. This is because, on most machines, it takes more than
one instruction to set up a stack frame (including local variable
definitions); if you are stepping by machine instructions, variables
may appear to have the wrong values until the stack frame is completely
built. On exit, it usually also takes more than one machine
instruction to destroy a stack frame; after you begin stepping through
that group of instructions, local variable definitions may be gone.
This may also happen when the compiler does significant
optimizations. To be sure of always seeing accurate values, turn off
all optimization when compiling.
Another possible effect of compiler optimizations is to optimize
unused variables out of existence, or assign variables to registers (as
opposed to memory addresses). Depending on the support for such cases
offered by the debug info format used by the compiler, GDB might not be
able to display values for such local variables. If that happens, GDB
will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or
use a different debug info format, if the compiler supports several such
formats. For example, GCC, the GNU C/C++ compiler usually supports the
`-gstabs' option. `-gstabs' produces debug info in a format that is
superior to formats such as COFF. You may be able to use DWARF-2
(`-gdwarf-2'), which is also an effective form for debug info. See
*Note Options for Debugging Your Program or GNU CC: (gcc.info)Debugging
Options, for more information.

File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data
Artificial arrays
=================
It is often useful to print out several successive objects of the
same type in memory; a section of an array, or an array of dynamically
determined size for which only a pointer exists in the program.
You can do this by referring to a contiguous span of memory as an
"artificial array", using the binary operator `@'. The left operand of
`@' should be the first element of the desired array and be an
individual object. The right operand should be the desired length of
the array. The result is an array value whose elements are all of the
type of the left argument. The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on. Here is an
example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of `array' with
p *array@len
The left operand of `@' must reside in memory. Array values made
with `@' in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(*note Value history: Value History.), after printing one out.
Another way to create an artificial array is to use a cast. This
re-interprets a value as if it were an array. The value need not be in
memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in
`(TYPE[])VALUE') GDB calculates the size to fill the value (as
`sizeof(VALUE)/sizeof(TYPE)':
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent--for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is
to use a convenience variable (*note Convenience variables: Convenience
Vars.) as a counter in an expression that prints the first interesting
value, and then repeat that expression via <RET>. For instance,
suppose you have an array `dtab' of pointers to structures, and you are
interested in the values of a field `fv' in each structure. Here is an
example of what you might type:
set $i = 0
p dtab[$i++]->fv
<RET>
<RET>
...

File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data
Output formats
==============
By default, GDB prints a value according to its data type. Sometimes
this is not what you want. For example, you might want to print a
number in hex, or a pointer in decimal. Or you might want to view data
in memory at a certain address as a character string or as an
instruction. To do these things, specify an "output format" when you
print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
`print' command with a slash and a format letter. The format letters
supported are:
`x'
Regard the bits of the value as an integer, and print the integer
in hexadecimal.
`d'
Print as integer in signed decimal.
`u'
Print as integer in unsigned decimal.
`o'
Print as integer in octal.
`t'
Print as integer in binary. The letter `t' stands for "two". (1)
`a'
Print as an address, both absolute in hexadecimal and as an offset
from the nearest preceding symbol. You can use this format used
to discover where (in what function) an unknown address is located:
(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
`c'
Regard as an integer and print it as a character constant.
`f'
Regard the bits of the value as a floating point number and print
using typical floating point syntax.
For example, to print the program counter in hex (*note
Registers::.), type
p/x $pc
Note that no space is required before the slash; this is because command
names in GDB cannot contain a slash.
To reprint the last value in the value history with a different
format, you can use the `print' command with just a format and no
expression. For example, `p/x' reprints the last value in hex.
---------- Footnotes ----------
(1) `b' cannot be used because these format letters are also used
with the `x' command, where `b' stands for "byte"; see *Note Examining
memory: Memory.

File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output Formats, Up: Data
Examining memory
================
You can use the command `x' (for "examine") to examine memory in any
of several formats, independently of your program's data types.
`x/NFU ADDR'
`x ADDR'
`x'
Use the `x' command to examine memory.
N, F, and U are all optional parameters that specify how much memory
to display and how to format it; ADDR is an expression giving the
address where you want to start displaying memory. If you use defaults
for NFU, you need not type the slash `/'. Several commands set
convenient defaults for ADDR.
N, the repeat count
The repeat count is a decimal integer; the default is 1. It
specifies how much memory (counting by units U) to display.
F, the display format
The display format is one of the formats used by `print', `s'
(null-terminated string), or `i' (machine instruction). The
default is `x' (hexadecimal) initially. The default changes each
time you use either `x' or `print'.
U, the unit size
The unit size is any of
`b'
Bytes.
`h'
Halfwords (two bytes).
`w'
Words (four bytes). This is the initial default.
`g'
Giant words (eight bytes).
Each time you specify a unit size with `x', that size becomes the
default unit the next time you use `x'. (For the `s' and `i'
formats, the unit size is ignored and is normally not written.)
ADDR, starting display address
ADDR is the address where you want GDB to begin displaying memory.
The expression need not have a pointer value (though it may); it
is always interpreted as an integer address of a byte of memory.
*Note Expressions: Expressions, for more information on
expressions. The default for ADDR is usually just after the last
address examined--but several other commands also set the default
address: `info breakpoints' (to the address of the last breakpoint
listed), `info line' (to the starting address of a line), and
`print' (if you use it to display a value from memory).
For example, `x/3uh 0x54320' is a request to display three halfwords
(`h') of memory, formatted as unsigned decimal integers (`u'), starting
at address `0x54320'. `x/4xw $sp' prints the four words (`w') of
memory above the stack pointer (here, `$sp'; *note Registers:
Registers.) in hexadecimal (`x').
Since the letters indicating unit sizes are all distinct from the
letters specifying output formats, you do not have to remember whether
unit size or format comes first; either order works. The output
specifications `4xw' and `4wx' mean exactly the same thing. (However,
the count N must come first; `wx4' does not work.)
Even though the unit size U is ignored for the formats `s' and `i',
you might still want to use a count N; for example, `3i' specifies that
you want to see three machine instructions, including any operands.
The command `disassemble' gives an alternative way of inspecting
machine instructions; see *Note Source and machine code: Machine Code.
All the defaults for the arguments to `x' are designed to make it
easy to continue scanning memory with minimal specifications each time
you use `x'. For example, after you have inspected three machine
instructions with `x/3i ADDR', you can inspect the next seven with just
`x/7'. If you use <RET> to repeat the `x' command, the repeat count N
is used again; the other arguments default as for successive uses of
`x'.
The addresses and contents printed by the `x' command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
`$_' and `$__'. After an `x' command, the last address examined is
available for use in expressions in the convenience variable `$_'. The
contents of that address, as examined, are available in the convenience
variable `$__'.
If the `x' command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of
output.

File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data
Automatic display
=================
If you find that you want to print the value of an expression
frequently (to see how it changes), you might want to add it to the
"automatic display list" so that GDB prints its value each time your
program stops. Each expression added to the list is given a number to
identify it; to remove an expression from the list, you specify that
number. The automatic display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values.
As with displays you request manually using `x' or `print', you can
specify the output format you prefer; in fact, `display' decides
whether to use `print' or `x' depending on how elaborate your format
specification is--it uses `x' if you specify a unit size, or one of the
two formats (`i' and `s') that are only supported by `x'; otherwise it
uses `print'.
`display EXPR'
Add the expression EXPR to the list of expressions to display each
time your program stops. *Note Expressions: Expressions.
`display' does not repeat if you press <RET> again after using it.
`display/FMT EXPR'
For FMT specifying only a display format and not a size or count,
add the expression EXPR to the auto-display list but arrange to
display it each time in the specified format FMT. *Note Output
formats: Output Formats.
`display/FMT ADDR'
For FMT `i' or `s', or including a unit-size or a number of units,
add the expression ADDR as a memory address to be examined each
time your program stops. Examining means in effect doing `x/FMT
ADDR'. *Note Examining memory: Memory.
For example, `display/i $pc' can be helpful, to see the machine
instruction about to be executed each time execution stops (`$pc' is a
common name for the program counter; *note Registers: Registers.).
`undisplay DNUMS...'
`delete display DNUMS...'
Remove item numbers DNUMS from the list of expressions to display.
`undisplay' does not repeat if you press <RET> after using it.
(Otherwise you would just get the error `No display number ...'.)
`disable display DNUMS...'
Disable the display of item numbers DNUMS. A disabled display
item is not printed automatically, but is not forgotten. It may be
enabled again later.
`enable display DNUMS...'
Enable display of item numbers DNUMS. It becomes effective once
again in auto display of its expression, until you specify
otherwise.
`display'
Display the current values of the expressions on the list, just as
is done when your program stops.
`info display'
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing
the values. This includes disabled expressions, which are marked
as such. It also includes expressions which would not be
displayed right now because they refer to automatic variables not
currently available.
If a display expression refers to local variables, then it does not
make sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
`display last_char' while inside a function with an argument
`last_char', GDB displays this argument while your program continues to
stop inside that function. When it stops elsewhere--where there is no
variable `last_char'--the display is disabled automatically. The next
time your program stops where `last_char' is meaningful, you can enable
the display expression once again.

File: gdb.info, Node: Print Settings, Next: Value History, Prev: Auto Display, Up: Data
Print settings
==============
GDB provides the following ways to control how arrays, structures,
and symbols are printed.
These settings are useful for debugging programs in any language:
`set print address'
`set print address on'
GDB prints memory addresses showing the location of stack traces,
structure values, pointer values, breakpoints, and so forth, even
when it also displays the contents of those addresses. The default
is `on'. For example, this is what a stack frame display looks
like with `set print address on':
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530 if (lquote != def_lquote)
`set print address off'
Do not print addresses when displaying their contents. For
example, this is the same stack frame displayed with `set print
address off':
(gdb) set print addr off
(gdb) f
#0 set_quotes (lq="<<", rq=">>") at input.c:530
530 if (lquote != def_lquote)
You can use `set print address off' to eliminate all machine
dependent displays from the GDB interface. For example, with
`print address off', you should get the same text for backtraces on
all machines--whether or not they involve pointer arguments.
`show print address'
Show whether or not addresses are to be printed.
When GDB prints a symbolic address, it normally prints the closest
earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
`info line', for example `info line *0x4537'. Alternately, you can set
GDB to print the source file and line number when it prints a symbolic
address:
`set print symbol-filename on'
Tell GDB to print the source file name and line number of a symbol
in the symbolic form of an address.
`set print symbol-filename off'
Do not print source file name and line number of a symbol. This
is the default.
`show print symbol-filename'
Show whether or not GDB will print the source file name and line
number of a symbol in the symbolic form of an address.
Another situation where it is helpful to show symbol filenames and
line numbers is when disassembling code; GDB shows you the line number
and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being
printed is reasonably close to the closest earlier symbol:
`set print max-symbolic-offset MAX-OFFSET'
Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less
than MAX-OFFSET. The default is 0, which tells GDB to always
print the symbolic form of an address if any symbol precedes it.
`show print max-symbolic-offset'
Ask how large the maximum offset is that GDB prints in a symbolic
address.
If you have a pointer and you are not sure where it points, try `set
print symbol-filename on'. Then you can determine the name and source
file location of the variable where it points, using `p/a POINTER'.
This interprets the address in symbolic form. For example, here GDB
shows that a variable `ptt' points at another variable `t', defined in
`hi2.c':
(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c>
_Warning:_ For pointers that point to a local variable, `p/a' does
not show the symbol name and filename of the referent, even with
the appropriate `set print' options turned on.
Other settings control how different kinds of objects are printed:
`set print array'
`set print array on'
Pretty print arrays. This format is more convenient to read, but
uses more space. The default is off.
`set print array off'
Return to compressed format for arrays.
`show print array'
Show whether compressed or pretty format is selected for displaying
arrays.
`set print elements NUMBER-OF-ELEMENTS'
Set a limit on how many elements of an array GDB will print. If
GDB is printing a large array, it stops printing after it has
printed the number of elements set by the `set print elements'
command. This limit also applies to the display of strings. When
GDB starts, this limit is set to 200. Setting NUMBER-OF-ELEMENTS
to zero means that the printing is unlimited.
`show print elements'
Display the number of elements of a large array that GDB will
print. If the number is 0, then the printing is unlimited.
`set print null-stop'
Cause GDB to stop printing the characters of an array when the
first NULL is encountered. This is useful when large arrays
actually contain only short strings. The default is off.
`set print pretty on'
Cause GDB to print structures in an indented format with one member
per line, like this:
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
`set print pretty off'
Cause GDB to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
`show print pretty'
Show which format GDB is using to print structures.
`set print sevenbit-strings on'
Print using only seven-bit characters; if this option is set, GDB
displays any eight-bit characters (in strings or character values)
using the notation `\'NNN. This setting is best if you are
working in English (ASCII) and you use the high-order bit of
characters as a marker or "meta" bit.
`set print sevenbit-strings off'
Print full eight-bit characters. This allows the use of more
international character sets, and is the default.
`show print sevenbit-strings'
Show whether or not GDB is printing only seven-bit characters.
`set print union on'
Tell GDB to print unions which are contained in structures. This
is the default setting.
`set print union off'
Tell GDB not to print unions which are contained in structures.
`show print union'
Ask GDB whether or not it will print unions which are contained in
structures.
For example, given the declarations
typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
Bug_forms;
struct thing {
Species it;
union {
Tree_forms tree;
Bug_forms bug;
} form;
};
struct thing foo = {Tree, {Acorn}};
with `set print union on' in effect `p foo' would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with `set print union off' in effect it would print
$1 = {it = Tree, form = {...}}
These settings are of interest when debugging C++ programs:
`set print demangle'
`set print demangle on'
Print C++ names in their source form rather than in the encoded
("mangled") form passed to the assembler and linker for type-safe
linkage. The default is on.
`show print demangle'
Show whether C++ names are printed in mangled or demangled form.
`set print asm-demangle'
`set print asm-demangle on'
Print C++ names in their source form rather than their mangled
form, even in assembler code printouts such as instruction
disassemblies. The default is off.
`show print asm-demangle'
Show whether C++ names in assembly listings are printed in mangled
or demangled form.
`set demangle-style STYLE'
Choose among several encoding schemes used by different compilers
to represent C++ names. The choices for STYLE are currently:
`auto'
Allow GDB to choose a decoding style by inspecting your
program.
`gnu'
Decode based on the GNU C++ compiler (`g++') encoding
algorithm. This is the default.
`hp'
Decode based on the HP ANSI C++ (`aCC') encoding algorithm.
`lucid'
Decode based on the Lucid C++ compiler (`lcc') encoding
algorithm.
`arm'
Decode using the algorithm in the `C++ Annotated Reference
Manual'. *Warning:* this setting alone is not sufficient to
allow debugging `cfront'-generated executables. GDB would
require further enhancement to permit that.
If you omit STYLE, you will see a list of possible formats.
`show demangle-style'
Display the encoding style currently in use for decoding C++
symbols.
`set print object'
`set print object on'
When displaying a pointer to an object, identify the _actual_
(derived) type of the object rather than the _declared_ type, using
the virtual function table.
`set print object off'
Display only the declared type of objects, without reference to the
virtual function table. This is the default setting.
`show print object'
Show whether actual, or declared, object types are displayed.
`set print static-members'
`set print static-members on'
Print static members when displaying a C++ object. The default is
on.
`set print static-members off'
Do not print static members when displaying a C++ object.
`show print static-members'
Show whether C++ static members are printed, or not.
`set print vtbl'
`set print vtbl on'
Pretty print C++ virtual function tables. The default is off.
(The `vtbl' commands do not work on programs compiled with the HP
ANSI C++ compiler (`aCC').)
`set print vtbl off'
Do not pretty print C++ virtual function tables.
`show print vtbl'
Show whether C++ virtual function tables are pretty printed, or
not.

File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Print Settings, Up: Data
Value history
=============
Values printed by the `print' command are saved in the GDB "value
history". This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded (for
example with the `file' or `symbol-file' commands). When the symbol
table changes, the value history is discarded, since the values may
contain pointers back to the types defined in the symbol table.
The values printed are given "history numbers" by which you can
refer to them. These are successive integers starting with one.
`print' shows you the history number assigned to a value by printing
`$NUM = ' before the value; here NUM is the history number.
To refer to any previous value, use `$' followed by the value's
history number. The way `print' labels its output is designed to
remind you of this. Just `$' refers to the most recent value in the
history, and `$$' refers to the value before that. `$$N' refers to the
Nth value from the end; `$$2' is the value just prior to `$$', `$$1' is
equivalent to `$$', and `$$0' is equivalent to `$'.
For example, suppose you have just printed a pointer to a structure
and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component `next' points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this
command--which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value
of `x' is 4 and you type these commands:
print x
set x=5
then the value recorded in the value history by the `print' command
remains 4 even though the value of `x' has changed.
`show values'
Print the last ten values in the value history, with their item
numbers. This is like `p $$9' repeated ten times, except that
`show values' does not change the history.
`show values N'
Print ten history values centered on history item number N.
`show values +'
Print ten history values just after the values last printed. If
no more values are available, `show values +' produces no display.
Pressing <RET> to repeat `show values N' has exactly the same effect
as `show values +'.

File: gdb.info, Node: Convenience Vars, Next: Registers, Prev: Value History, Up: Data
Convenience variables
=====================
GDB provides "convenience variables" that you can use within GDB to
hold on to a value and refer to it later. These variables exist
entirely within GDB; they are not part of your program, and setting a
convenience variable has no direct effect on further execution of your
program. That is why you can use them freely.
Convenience variables are prefixed with `$'. Any name preceded by
`$' can be used for a convenience variable, unless it is one of the
predefined machine-specific register names (*note Registers:
Registers.). (Value history references, in contrast, are _numbers_
preceded by `$'. *Note Value history: Value History.)
You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program. For
example:
set $foo = *object_ptr
would save in `$foo' the value contained in the object pointed to by
`object_ptr'.
Using a convenience variable for the first time creates it, but its
value is `void' until you assign a new value. You can alter the value
with another assignment at any time.
Convenience variables have no fixed types. You can assign a
convenience variable any type of value, including structures and
arrays, even if that variable already has a value of a different type.
The convenience variable, when used as an expression, has the type of
its current value.
`show convenience'
Print a list of convenience variables used so far, and their
values. Abbreviated `show conv'.
One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced. For example, to print a field
from successive elements of an array of structures:
set $i = 0
print bar[$i++]->contents
Repeat that command by typing <RET>.
Some convenience variables are created automatically by GDB and given
values likely to be useful.
`$_'
The variable `$_' is automatically set by the `x' command to the
last address examined (*note Examining memory: Memory.). Other
commands which provide a default address for `x' to examine also
set `$_' to that address; these commands include `info line' and
`info breakpoint'. The type of `$_' is `void *' except when set
by the `x' command, in which case it is a pointer to the type of
`$__'.
`$__'
The variable `$__' is automatically set by the `x' command to the
value found in the last address examined. Its type is chosen to
match the format in which the data was printed.
`$_exitcode'
The variable `$_exitcode' is automatically set to the exit code
when the program being debugged terminates.
On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name
first, before it searches for a convenience variable.

File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Vars, Up: Data
Registers
=========
You can refer to machine register contents, in expressions, as
variables with names starting with `$'. The names of registers are
different for each machine; use `info registers' to see the names used
on your machine.
`info registers'
Print the names and values of all registers except floating-point
registers (in the selected stack frame).
`info all-registers'
Print the names and values of all registers, including
floating-point registers.
`info registers REGNAME ...'
Print the "relativized" value of each specified register REGNAME.
As discussed in detail below, register values are normally
relative to the selected stack frame. REGNAME may be any register
name valid on the machine you are using, with or without the
initial `$'.
GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
`$pc' and `$sp' are used for the program counter register and the stack
pointer. `$fp' is used for a register that contains a pointer to the
current stack frame, and `$ps' is used for a register that contains the
processor status. For example, you could print the program counter in
hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer(1) with
set $sp += 4
Whenever possible, these four standard register names are available
on your machine even though the machine has different canonical
mnemonics, so long as there is no conflict. The `info registers'
command shows the canonical names. For example, on the SPARC, `info
registers' displays the processor status register as `$psr' but you can
also refer to it as `$ps'; and on x86-based machines `$ps' is an alias
for the EFLAGS register.
GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way. Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value
(although you can _print_ it as a floating point value with `print/f
$REGNAME').
Some registers have distinct "raw" and "virtual" data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format. In such cases,
GDB normally works with the virtual format only (the format that makes
sense for your program), but the `info registers' command prints the
data in both formats.
Normally, register values are relative to the selected stack frame
(*note Selecting a frame: Selection.). This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost
frame (with `frame 0').
However, GDB must deduce where registers are saved, from the machine
code generated by your compiler. If some registers are not saved, or if
GDB is unable to locate the saved registers, the selected stack frame
makes no difference.
---------- Footnotes ----------
(1) This is a way of removing one word from the stack, on machines
where stacks grow downward in memory (most machines, nowadays). This
assumes that the innermost stack frame is selected; setting `$sp' is
not allowed when other stack frames are selected. To pop entire frames
off the stack, regardless of machine architecture, use `return'; see
*Note Returning from a function: Returning.

File: gdb.info, Node: Floating Point Hardware, Prev: Registers, Up: Data
Floating point hardware
=======================
Depending on the configuration, GDB may be able to give you more
information about the status of the floating point hardware.
`info float'
Display hardware-dependent information about the floating point
unit. The exact contents and layout vary depending on the
floating point chip. Currently, `info float' is supported on the
ARM and x86 machines.

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 appear as `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 is called the "working language".
* 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. The working language is used to
determine how expressions you type are interpreted, how values are
printed, etc.
In addition to the working language, every source file that GDB
knows about has its own working language. For some object file
formats, the compiler might indicate which language a particular source
file is in. However, most of the time GDB infers the language from the
name of the file. The language of a source file controls whether C++
names are demangled--this way `backtrace' can show each frame
appropriately for its own language. There is no way to set the
language of a source file from within GDB, but you can set the language
associated with a filename extension. *Note Displaying the language:
Show.
This is most commonly a problem when you use a program, such as
`cfront' or `f2c', that generates C but is written in another language.
In that case, make the program use `#line' directives in its C output;
that way GDB will know the correct language of the source code of the
original program, and will display that source code, not the generated
C code.
* Menu:
* Filenames:: Filename extensions and languages.
* Manually:: Setting the working language manually
* Automatically:: Having GDB infer the source language

File: gdb.info, Node: Filenames, Next: Manually, Up: Setting
List of filename extensions and languages
-----------------------------------------
If a source file name ends in one of the following extensions, then
GDB infers that its language is the one indicated.
`.c'
C source file
`.C'
`.cc'
`.cp'
`.cpp'
`.cxx'
`.c++'
C++ source file
`.f'
`.F'
Fortran source file
`.ch'
`.c186'
`.c286'
CHILL source file
`.mod'
Modula-2 source file
`.s'
`.S'
Assembler source file. This actually behaves almost like C, but
GDB does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename
extension. *Note Displaying the language: Show.

File: gdb.info, Node: Manually, Next: Automatically, Prev: Filenames, Up: Setting
Setting the working language
----------------------------
If you allow GDB to set the language automatically, expressions are
interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue
the command `set language LANG', where LANG is the name of a language,
such as `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.

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 working
language. That is, 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 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'
Display the source language for this frame. This language becomes
the working language if you use an identifier from this frame.
*Note Information about a frame: Frame Info, to identify the other
information listed here.
`info source'
Display the source language of this source file. *Note Examining
the Symbol Table: Symbols, to identify the other information
listed here.
In unusual circumstances, you may have source files with extensions
not in the standard list. You can then set the extension associated
with a language explicitly:
`set extension-language .EXT LANGUAGE'
Set source files with extension .EXT to be assumed to be in the
source language LANGUAGE.
`info extensions'
List all the filename extensions and the associated languages.

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.
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