GNU Info
(gdb.info)Print Settings
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.
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