Debugging with GDB: Print Settings

10.8 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
set print max-symbolic-offset unlimited: 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 unlimited, which tells GDB to always print the symbolic form of an address if any symbol precedes it. Zero is equivalent to unlimited.
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.

You can also enable ‘/a’-like formatting all the time using ‘set print symbol on’:

set print symbol on: Tell GDB to print the symbol corresponding to an address, if one exists.
set print symbol off: Tell GDB not to print the symbol corresponding to an address. In this mode, GDB will still print the symbol corresponding to pointers to functions. This is the default.
show print symbol: Show whether GDB will display the symbol corresponding to an address.

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 array-indexes

set print array-indexes on

Print the index of each element when displaying arrays. May be more convenient to locate a given element in the array or quickly find the index of a given element in that printed array. The default is off.

set print array-indexes off

Stop printing element indexes when displaying arrays.

show print array-indexes

Show whether the index of each element is printed when displaying arrays.

set print elements number-of-elements

set print elements unlimited

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 unlimited or zero means that the number of elements to print 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 frame-arguments value

This command allows to control how the values of arguments are printed when the debugger prints a frame (see Frames). The possible values are:

all

The values of all arguments are printed.

scalars

Print the value of an argument only if it is a scalar. The value of more complex arguments such as arrays, structures, unions, etc, is replaced by …. This is the default. Here is an example where only scalar arguments are shown:

#1  0x08048361 in call_me (i=3, s=…, ss=0xbf8d508c, u=…, e=green)
  at frame-args.c:23

none

None of the argument values are printed. Instead, the value of each argument is replaced by …. In this case, the example above now becomes:

#1  0x08048361 in call_me (i=…, s=…, ss=…, u=…, e=…)
  at frame-args.c:23

presence

Only the presence of arguments is indicated by …. The … are not printed for function without any arguments. None of the argument names and values are printed. In this case, the example above now becomes:

#1  0x08048361 in call_me (…) at frame-args.c:23

By default, only scalar arguments are printed. This command can be used to configure the debugger to print the value of all arguments, regardless of their type. However, it is often advantageous to not print the value of more complex parameters. For instance, it reduces the amount of information printed in each frame, making the backtrace more readable. Also, it improves performance when displaying Ada frames, because the computation of large arguments can sometimes be CPU-intensive, especially in large applications. Setting print frame-arguments to scalars (the default), none or presence avoids this computation, thus speeding up the display of each Ada frame.

show print frame-arguments

Show how the value of arguments should be displayed when printing a frame.

set print raw-frame-arguments on

Print frame arguments in raw, non pretty-printed, form.

set print raw-frame-arguments off

Print frame arguments in pretty-printed form, if there is a pretty-printer for the value (see Pretty Printing), otherwise print the value in raw form. This is the default.

show print raw-frame-arguments

Show whether to print frame arguments in raw form.

set print entry-values value

Set printing of frame argument values at function entry. In some cases GDB can determine the value of function argument which was passed by the function caller, even if the value was modified inside the called function and therefore is different. With optimized code, the current value could be unavailable, but the entry value may still be known.

The default value is default (see below for its description). Older GDB behaved as with the setting no. Compilers not supporting this feature will behave in the default setting the same way as with the no setting.

This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce ‘DW_TAG_call_site’ tags. With GCC, you need to specify -O -g during compilation, to get this information.

The value parameter can be one of the following:

no

Print only actual parameter values, never print values from function entry point.

#0  equal (val=5)
#0  different (val=6)
#0  lost (val=<optimized out>)
#0  born (val=10)
#0  invalid (val=<optimized out>)

only

Print only parameter values from function entry point. The actual parameter values are never printed.

#0  equal (val@entry=5)
#0  different (val@entry=5)
#0  lost (val@entry=5)
#0  born (val@entry=<optimized out>)
#0  invalid (val@entry=<optimized out>)

preferred

Print only parameter values from function entry point. If value from function entry point is not known while the actual value is known, print the actual value for such parameter.

#0  equal (val@entry=5)
#0  different (val@entry=5)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val@entry=<optimized out>)

if-needed

Print actual parameter values. If actual parameter value is not known while value from function entry point is known, print the entry point value for such parameter.

#0  equal (val=5)
#0  different (val=6)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)

both

Always print both the actual parameter value and its value from function entry point, even if values of one or both are not available due to compiler optimizations.

#0  equal (val=5, val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val=<optimized out>, val@entry=5)
#0  born (val=10, val@entry=<optimized out>)
#0  invalid (val=<optimized out>, val@entry=<optimized out>)

compact

Print the actual parameter value if it is known and also its value from function entry point if it is known. If neither is known, print for the actual value <optimized out>. If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

#0  equal (val=val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)

default

Always print the actual parameter value. Print also its value from function entry point, but only if it is known. If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

#0  equal (val=val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val=<optimized out>, val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)

For analysis messages on possible failures of frame argument values at function entry resolution see set debug entry-values.

show print entry-values

Show the method being used for printing of frame argument values at function entry.

set print frame-info value

This command allows to control the information printed when the debugger prints a frame. See Frames, Backtrace, for a general explanation about frames and frame information. Note that some other settings (such as set print frame-arguments and set print address) are also influencing if and how some frame information is displayed. In particular, the frame program counter is never printed if set print address is off.

The possible values for set print frame-info are:

short-location: Print the frame level, the program counter (if not at the beginning of the location source line), the function, the function arguments.
location: Same as short-location but also print the source file and source line number.
location-and-address: Same as location but print the program counter even if located at the beginning of the location source line.
source-line: Print the program counter (if not at the beginning of the location source line), the line number and the source line.
source-and-location: Print what location and source-line are printing.
auto: The information printed for a frame is decided automatically by the GDB command that prints a frame. For example, frame prints the information printed by source-and-location while stepi will switch between source-line and source-and-location depending on the program counter. The default value is auto.

set print repeats number-of-repeats

set print repeats unlimited

Set the threshold for suppressing display of repeated array elements. When the number of consecutive identical elements of an array exceeds the threshold, GDB prints the string "<repeats n times>", where n is the number of identical repetitions, instead of displaying the identical elements themselves. Setting the threshold to unlimited or zero will cause all elements to be individually printed. The default threshold is 10.

show print repeats

Display the current threshold for printing repeated identical elements.

set print max-depth depth

set print max-depth unlimited

Set the threshold after which nested structures are replaced with ellipsis, this can make visualising deeply nested structures easier.

For example, given this C code

typedef struct s1 { int a; } s1;
typedef struct s2 { s1 b; } s2;
typedef struct s3 { s2 c; } s3;
typedef struct s4 { s3 d; } s4;

s4 var = { { { { 3 } } } };

The following table shows how different values of depth will effect how var is printed by GDB:

`depth` setting	Result of ‘`p var`’
unlimited	`$1 = {d = {c = {b = {a = 3}}}}`
`0`	`$1 = {...}`
`1`	`$1 = {d = {...}}`
`2`	`$1 = {d = {c = {...}}}`
`3`	`$1 = {d = {c = {b = {...}}}}`
`4`	`$1 = {d = {c = {b = {a = 3}}}}`

To see the contents of structures that have been hidden the user can either increase the print max-depth, or they can print the elements of the structure that are visible, for example

(gdb) set print max-depth 2
(gdb) p var
$1 = {d = {c = {...}}}
(gdb) p var.d
$2 = {c = {b = {...}}}
(gdb) p var.d.c
$3 = {b = {a = 3}}

The pattern used to replace nested structures varies based on language, for most languages {...} is used, but Fortran uses (...).

show print max-depth

Display the current threshold after which nested structures are replaces with ellipsis.

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.

show print null-stop

Show whether GDB stops printing an array on the first NULL character.

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 raw-values on

Print values in raw form, without applying the pretty printers for the value.

set print raw-values off

Print values in pretty-printed form, if there is a pretty-printer for the value (see Pretty Printing), otherwise print the value in raw form.

The default setting is “off”.

show print raw-values

Show whether to print values in raw form.

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 and other unions. This is the default setting.

set print union off

Tell GDB not to print unions which are contained in structures and other unions. GDB will print "{...}" instead.

show print union

Ask GDB whether or not it will print unions which are contained in structures and other unions.

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 = {...}}

set print union affects programs written in C-like languages and in Pascal.

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. If you omit style, you will see a list of possible formats. The default value is auto, which lets GDB choose a decoding style by inspecting your program.
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. Note that the virtual function table is required—this feature can only work for objects that have run-time type identification; a single virtual method in the object’s declared type is sufficient. Note that this setting is also taken into account when working with variable objects via MI (see GDB/MI).
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 pascal_static-members
set print pascal_static-members on: Print static members when displaying a Pascal object. The default is on.
set print pascal_static-members off: Do not print static members when displaying a Pascal object.
show print pascal_static-members: Show whether Pascal 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.