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16 Examining the Symbol Table

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 on
set case-sensitive off
set case-sensitive auto

Normally, when GDB looks up symbols, it matches their names with case sensitivity determined by the current source language. Occasionally, you may wish to control that. The command set 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-sensitive

This command shows the current setting of case sensitivity for symbols lookups.

set print type methods
set print type methods on
set print type methods off

Normally, when GDB prints a class, it displays any methods declared in that class. You can control this behavior either by passing the appropriate flag to ptype, 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 methods

This command shows the current setting of method display when printing classes.

set print type nested-type-limit limit
set print type nested-type-limit unlimited

Set the limit of displayed nested types that the type printer will show. A limit of unlimited 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-limit

This command shows the current display limit of nested types when printing classes.

set print type typedefs
set print type typedefs on
set print type typedefs off

Normally, when GDB prints a class, it displays any typedefs defined in that class. You can control this behavior either by passing the appropriate flag to ptype, 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 typedefs

This command shows the current setting of typedef display when printing classes.

info address symbol

Describe where the data for symbol is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.

Note 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

Print the name of a symbol which is stored at the address addr. If no symbol is stored exactly at addr, GDB prints the nearest symbol and an offset from it:

(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] [--] name

Demangle name. If language is provided it is the name of the language to demangle name in. Otherwise name is demangled in the current language.

The ‘--’ 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]

Print the data type of arg, which can be either an expression or a name of a data type. With no argument, print the data type of $, 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

Display in “raw” form. Normally, GDB substitutes template parameters and typedefs defined in a class when printing the class’ members. The /r flag disables this.

m

Do not print methods defined in the class.

M

Print methods defined in the class. This is the default, but the flag exists in case you change the default with set print type methods.

t

Do not print typedefs defined in the class. Note that this controls whether the typedef definition itself is printed, not whether typedef names are substituted when printing other types.

T

Print typedefs defined in the class. This is the default, but the flag exists in case you change the default with set print type typedefs.

o

Print the offsets and sizes of fields in a struct, similar to what the pahole tool does. This option implies the /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 regexp
info types

Print a brief description of all types whose names match the regular expression regexp (or all types in your program, if you supply no argument). Each complete typename is matched as though it were a complete line; thus, ‘i type value’ gives information on all types in your program whose names include the string value, but ‘i type ^value$’ gives information only on types whose complete name is value.

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-printers

Versions of GDB that ship with Python scripting enabled may have “type printers” available. When using ptype or whatis, these printers are consulted when the name of a type is needed. See Type Printing API, for more information on writing type printers.

info type-printers displays all the available type printers.

enable type-printer name
disable type-printer name

These commands can be used to enable or disable type printers.

info scope location

List all the variables local to a particular scope. This command accepts a location argument—a function name, a source line, or an address preceded by a ‘*’, and prints all the variables local to the scope defined by that location. (See Specify Location, for details about supported forms of location.) For example:

(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 source

Show information about the current source file—that is, the source file for the function containing the current point of execution:

info sources

Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.

info functions [-q]

Print the names and data types of all defined functions. Similarly to ‘info types’, this command groups its output by source files and annotates each function definition with its source line number.

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]

Like ‘info functions’, but only print the names and data types of the functions selected with the provided regexp(s).

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]

Print the names and data types of all variables that are defined outside of functions (i.e. excluding local variables). The printed variables are grouped by source files and annotated with their respective source line numbers.

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]

Like info variables, but only print the variables selected with the provided regexp(s).

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 classes
info classes regexp

Display all Objective-C classes in your program, or (with the regexp argument) all those matching a particular regular expression.

info selectors
info selectors regexp

Display all Objective-C selectors in your program, or (with the regexp argument) all those matching a particular regular expression.

set opaque-type-resolution on

Tell GDB to resolve opaque types. An opaque type is a type declared as a pointer to a struct, 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

Tell GDB not to resolve opaque types. In this case, the type is printed as follows:

{<no data fields>}
show opaque-type-resolution

Show whether opaque types are resolved or not.

set print symbol-loading
set print symbol-loading full
set print symbol-loading brief
set print symbol-loading off

The set 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-loading

Show whether messages will be printed when a GDB command entered from the keyboard causes symbol information to be loaded.

maint 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]

Write a dump of debugging symbol data into the file filename or the terminal if filename is unspecified. If -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 ]

List the 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 ]

List the 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 size

Set the size of the symbol cache to size. The default size is intended to be good enough for debugging most applications. This option exists to allow for experimenting with different sizes.

maint show symbol-cache-size

Show the size of the symbol cache.

maint print symbol-cache

Print the contents of the symbol cache. This is useful when debugging symbol cache issues.

maint print symbol-cache-statistics

Print symbol cache usage statistics. This helps determine how well the cache is being utilized.

maint flush-symbol-cache

Flush the contents of the symbol cache, all entries are removed. This command is useful when debugging the symbol cache. It is also useful when collecting performance data.


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