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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, the following 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, the following 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 instance, the following 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. 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.

set print pretty on
Cause GDB to print structures in an indented format with one member per line, like the following example:

$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 the following example:

$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 instance, consider the following example’s 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}};

The example has set print union on in effect having ‘p foo’ printing the following result.

$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}

With set print union off in effect, it would print the following result.

$1 = {it = Tree, form = {...}}

The following 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.

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 functionality.

foo
Show the list of 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 vtbl

set print vtbl on
Pretty print C++ virtual function tables. The default is off.

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.