The commands described in this chapter 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 (see Choosing Files), or by one of the file-management commands (see Commands to Specify 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 (see Program 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.
set case-sensitive onset case-sensitive offset case-sensitive autoset
case-sensitive lets you do that by specifying on for
case-sensitive matches or off for case-insensitive ones. If
you specify auto, case sensitivity is reset to the default
suitable for the source language. The default is case-sensitive
matches for all languages except for Fortran, for which the default is
case-insensitive matches.
show case-sensitiveset print type methodsset print type methods onset print type methods offptype, or using set
print type methods. Specifying on will cause gdb to
display the methods; this is the default. Specifying off will
cause gdb to omit the methods.
show print type methodsset print type nested-type-limit limitset print type nested-type-limit unlimitedunlimited or -1 will show all
nested definitions. By default, the type printer will not show any nested
types defined in classes.
show print type nested-type-limitset print type typedefsset print type typedefs onset print type typedefs offptype, or using set
print type typedefs. Specifying on will cause gdb to
display the typedef definitions; this is the default. Specifying
off will cause gdb to omit the typedef definitions.
Note that this controls whether the typedef definition itself is
printed, not whether typedef names are substituted when printing other
types.
show print type typedefsinfo address symbolNote the contrast with ‘print &symbol’, which does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.
info symbol addr (gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the info address command. You can use
it to find out the name of a variable or a function given its address.
For dynamically linked executables, the name of executable or shared library containing the symbol is also printed:
(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out
(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
demangle [-l language] [–] nameThe ‘--’ option specifies the end of options, and is useful when name begins with a dash.
The parameter demangle-style specifies how to interpret the kind
of mangling used. See Print Settings.
whatis[/flags] [arg]$, the last value in the value history.
If arg is an expression (see Expressions), it is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place.
If arg is a variable or an expression, whatis prints its
literal type as it is used in the source code. If the type was
defined using a typedef, whatis will not print
the data type underlying the typedef. If the type of the
variable or the expression is a compound data type, such as
struct or class, whatis never prints their
fields or methods. It just prints the struct/class
name (a.k.a. its tag). If you want to see the members of
such a compound data type, use ptype.
If arg is a type name that was defined using typedef,
whatis unrolls only one level of that typedef.
Unrolling means that whatis will show the underlying type used
in the typedef declaration of arg. However, if that
underlying type is also a typedef, whatis will not
unroll it.
For C code, the type names may also have the form ‘class class-name’, ‘struct struct-tag’, ‘union union-tag’ or ‘enum enum-tag’.
flags can be used to modify how the type is displayed. Available flags are:
r/r flag disables this.
mMtTo/tm flags.
For example, given the following declarations:
struct tuv
{
int a1;
char *a2;
int a3;
};
struct xyz
{
int f1;
char f2;
void *f3;
struct tuv f4;
};
union qwe
{
struct tuv fff1;
struct xyz fff2;
};
struct tyu
{
int a1 : 1;
int a2 : 3;
int a3 : 23;
char a4 : 2;
int64_t a5;
int a6 : 5;
int64_t a7 : 3;
};
Issuing a ptype /o struct tuv command would print:
(gdb) ptype /o struct tuv
/* offset | size */ type = struct tuv {
/* 0 | 4 */ int a1;
/* XXX 4-byte hole */
/* 8 | 8 */ char *a2;
/* 16 | 4 */ int a3;
/* total size (bytes): 24 */
}
Notice the format of the first column of comments. There, you can find two parts separated by the ‘|’ character: the offset, which indicates where the field is located inside the struct, in bytes, and the size of the field. Another interesting line is the marker of a hole in the struct, indicating that it may be possible to pack the struct and make it use less space by reorganizing its fields.
It is also possible to print offsets inside an union:
(gdb) ptype /o union qwe
/* offset | size */ type = union qwe {
/* 24 */ struct tuv {
/* 0 | 4 */ int a1;
/* XXX 4-byte hole */
/* 8 | 8 */ char *a2;
/* 16 | 4 */ int a3;
/* total size (bytes): 24 */
} fff1;
/* 40 */ struct xyz {
/* 0 | 4 */ int f1;
/* 4 | 1 */ char f2;
/* XXX 3-byte hole */
/* 8 | 8 */ void *f3;
/* 16 | 24 */ struct tuv {
/* 16 | 4 */ int a1;
/* XXX 4-byte hole */
/* 24 | 8 */ char *a2;
/* 32 | 4 */ int a3;
/* total size (bytes): 24 */
} f4;
/* total size (bytes): 40 */
} fff2;
/* total size (bytes): 40 */
}
In this case, since struct tuv and struct xyz occupy the
same space (because we are dealing with an union), the offset is not
printed for them. However, you can still examine the offset of each
of these structures' fields.
Another useful scenario is printing the offsets of a struct containing bitfields:
(gdb) ptype /o struct tyu
/* offset | size */ type = struct tyu {
/* 0:31 | 4 */ int a1 : 1;
/* 0:28 | 4 */ int a2 : 3;
/* 0: 5 | 4 */ int a3 : 23;
/* 3: 3 | 1 */ signed char a4 : 2;
/* XXX 3-bit hole */
/* XXX 4-byte hole */
/* 8 | 8 */ int64_t a5;
/* 16:27 | 4 */ int a6 : 5;
/* 16:56 | 8 */ int64_t a7 : 3;
/* total size (bytes): 24 */
}
Note how the offset information is now extended to also include how many bits are left to be used in each bitfield.
ptype[/flags] [arg]ptype accepts the same arguments as whatis, but prints a
detailed description of the type, instead of just the name of the type.
See Expressions.
Contrary to whatis, ptype always unrolls any
typedefs in its argument declaration, whether the argument is
a variable, expression, or a data type. This means that ptype
of a variable or an expression will not print literally its type as
present in the source code—use whatis for that. typedefs at
the pointer or reference targets are also unrolled. Only typedefs of
fields, methods and inner class typedefs of structs,
classes and unions are not unrolled even with ptype.
For example, for this variable declaration:
typedef double real_t;
struct complex { real_t real; double imag; };
typedef struct complex complex_t;
complex_t var;
real_t *real_pointer_var;
the two commands give this output:
(gdb) whatis var
type = complex_t
(gdb) ptype var
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis complex_t
type = struct complex
(gdb) whatis struct complex
type = struct complex
(gdb) ptype struct complex
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis real_pointer_var
type = real_t *
(gdb) ptype real_pointer_var
type = double *
As with whatis, using ptype without an argument refers to
the type of $, the last value in the value history.
Sometimes, programs use opaque data types or incomplete specifications of complex data structure. If the debug information included in the program does not allow gdb to display a full declaration of the data type, it will say ‘<incomplete type>’. For example, given these declarations:
struct foo;
struct foo *fooptr;
but no definition for struct foo itself, gdb will say:
(gdb) ptype foo
$1 = <incomplete type>
“Incomplete type” is C terminology for data types that are not completely specified.
Othertimes, information about a variable's type is completely absent from the debug information included in the program. This most often happens when the program or library where the variable is defined includes no debug information at all. gdb knows the variable exists from inspecting the linker/loader symbol table (e.g., the ELF dynamic symbol table), but such symbols do not contain type information. Inspecting the type of a (global) variable for which gdb has no type information shows:
(gdb) ptype var
type = <data variable, no debug info>
See no debug info variables, for how to print the values of such variables.
info types regexpinfo typesvalue, but
‘i type ^value$’ gives information only on types whose complete
name is value.
In programs using different languages, gdb chooses the syntax to print the type description according to the ‘set language’ value: using ‘set language auto’ (see Set Language Automatically) means to use the language of the type, other values mean to use the manually specified language (see Set Language Manually).
This command differs from ptype in two ways: first, like
whatis, it does not print a detailed description; second, it
lists all source files and line numbers where a type is defined.
info type-printersinfo type-printers displays all the available type printers.
enable type-printer name...disable type-printer name...info scope location (gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to collect during a trace experiment, see collect.
info sourceinfo sourcesinfo functions [-q]In programs using different languages, gdb chooses the syntax to print the function name and type according to the ‘set language’ value: using ‘set language auto’ (see Set Language Automatically) means to use the language of the function, other values mean to use the manually specified language (see Set Language Manually).
The optional flag ‘-q’, which stands for ‘quiet’, disables
printing header information and messages explaining why no functions
have been printed.
info functions [-q] [-t type_regexp] [regexp]If regexp is provided, print only the functions whose names
match the regular expression regexp.
Thus, ‘info fun step’ finds all functions whose
names include step; ‘info fun ^step’ finds those whose names
start with step. If a function name contains characters that
conflict with the regular expression language (e.g.
‘operator*()’), they may be quoted with a backslash.
If type_regexp is provided, print only the functions whose
types, as printed by the whatis command, match
the regular expression type_regexp.
If type_regexp contains space(s), it should be enclosed in
quote characters. If needed, use backslash to escape the meaning
of special characters or quotes.
Thus, ‘info fun -t '^int ('’ finds the functions that return
an integer; ‘info fun -t '(.*int.*'’ finds the functions that
have an argument type containing int; ‘info fun -t '^int (' ^step’
finds the functions whose names start with step and that return
int.
If both regexp and type_regexp are provided, a function is printed only if its name matches regexp and its type matches type_regexp.
info variables [-q]In programs using different languages, gdb chooses the syntax to print the variable name and type according to the ‘set language’ value: using ‘set language auto’ (see Set Language Automatically) means to use the language of the variable, other values mean to use the manually specified language (see Set Language Manually).
The optional flag ‘-q’, which stands for ‘quiet’, disables
printing header information and messages explaining why no variables
have been printed.
info variables [-q] [-t type_regexp] [regexp]If regexp is provided, print only the variables whose names match the regular expression regexp.
If type_regexp is provided, print only the variables whose
types, as printed by the whatis command, match
the regular expression type_regexp.
If type_regexp contains space(s), it should be enclosed in
quote characters. If needed, use backslash to escape the meaning
of special characters or quotes.
If both regexp and type_regexp are provided, an argument is printed only if its name matches regexp and its type matches type_regexp.
info classesinfo classes regexpinfo selectorsinfo selectors regexpset opaque-type-resolution onstruct, class, or
union—for example, struct MyType *—that is used in one
source file although the full declaration of struct MyType is in
another source file. The default is on.
A change in the setting of this subcommand will not take effect until
the next time symbols for a file are loaded.
set opaque-type-resolution off {<no data fields>}
show opaque-type-resolutionset print symbol-loadingset print symbol-loading fullset print symbol-loading briefset print symbol-loading offset print symbol-loading command allows you to control the
printing of messages when gdb loads symbol information.
By default a message is printed for the executable and one for each
shared library, and normally this is what you want. However, when
debugging apps with large numbers of shared libraries these messages
can be annoying.
When set to brief a message is printed for each executable,
and when gdb loads a collection of shared libraries at once
it will only print one message regardless of the number of shared
libraries. When set to off no messages are printed.
show print symbol-loadingmaint print symbols [-pc address] [filename]maint print symbols [-objfile objfile] [-source source] [--] [filename]maint print psymbols [-objfile objfile] [-pc address] [--] [filename]maint print psymbols [-objfile objfile] [-source source] [--] [filename]maint print msymbols [-objfile objfile] [--] [filename]-objfile objfile is specified, only dump symbols for
that objfile.
If -pc address is specified, only dump symbols for the file
with code at that address. Note that address may be a symbol like
main.
If -source source is specified, only dump symbols for that
source file.
These commands are used to debug the gdb symbol-reading code.
These commands do not modify internal gdb state, therefore
‘maint print symbols’ will only print symbols for already expanded symbol
tables.
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’ just dumps “minimal symbols”, e.g.,
“ELF symbols”.
See Commands to Specify Files, for a discussion of how
gdb reads symbols (in the description of symbol-file).
maint info symtabs [ regexp ]maint info psymtabs [ regexp ]struct symtab or struct partial_symtab
structures whose names match regexp. If regexp is not
given, list them all. The output includes expressions which you can
copy into a gdb debugging this one to examine a particular
structure in more detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
dependencies (none)
}
}
(gdb) maint info symtabs
(gdb)
We see that there is one partial symbol table whose filename contains the string ‘dwarf2read’, belonging to the ‘gdb’ executable; and we see that gdb has not read in any symtabs yet at all. If we set a breakpoint on a function, that will cause gdb to read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ symtab /home/gnu/src/gdb/dwarf2read.c
((struct symtab *) 0x86c1f38)
dirname (null)
fullname (null)
blockvector ((struct blockvector *) 0x86c1bd0) (primary)
linetable ((struct linetable *) 0x8370fa0)
debugformat DWARF 2
}
}
(gdb)
maint info line-table [ regexp ]struct linetable from all struct symtab
instances whose name matches regexp. If regexp is not
given, list the struct linetable from all struct symtab.
maint set symbol-cache-size sizemaint show symbol-cache-sizemaint print symbol-cachemaint print symbol-cache-statisticsmaint flush-symbol-cache