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13. 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. No value for GDBN finds it in your program's symbol table, in the file indicated when you started No value for GDBN (see section Choosing Files), or by one of the file-management commands (see section Commands to Specify Files).

Occasionally, you may need to refer to symbols that contain unusual characters, which No value for GDBN ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see section Program Variables). File names are recorded in object files as debugging symbols, but No value for GDBN would ordinarily parse a typical file name, like ‘foo.c’, as the three words ‘foo’ ‘.’ ‘c’. To allow No value for GDBN 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 No value for GDBN 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.

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, No value for GDBN prints the nearest symbol and an offset from it:

(No value for GDBP) 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.

whatis [arg]

Print the data type of arg, which can be either an expression or a data type. With no argument, print the data type of $, the last value in the value history. If arg is an expression, 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 type name, it may be the name of a type or typedef, or for C code it may have the form ‘class class-name’, ‘struct struct-tag’, ‘union union-tag’ or ‘enum enum-tag’. See section Expressions.

ptype [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 section Expressions.

For example, for this variable declaration:

struct complex {double real; double imag;} v;

the two commands give this output:

(No value for GDBP) whatis v
type = struct complex
(No value for GDBP) ptype v
type = struct complex {
    double real;
    double imag;

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 No value for GDBN 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, No value for GDBN will say:

  (No value for GDBP) ptype foo
  $1 = <incomplete type>

“Incomplete type” is C terminology for data types that are not completely specified.

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 where a type is defined.

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. For example:

(No value for GDBP) 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:

  • the name of the source file, and the directory containing it,
  • the directory it was compiled in,
  • its length, in lines,
  • which programming language it is written in,
  • whether the executable includes debugging information for that file, and if so, what format the information is in (e.g., STABS, Dwarf 2, etc.), and
  • whether the debugging information includes information about preprocessor macros.
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

Print the names and data types of all defined functions.

info functions regexp

Print the names and data types of all defined functions whose names contain a match for 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.

info variables

Print the names and data types of all variables that are declared outside of functions (i.e. excluding local variables).

info variables regexp

Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression 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.

Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program. For example, in VxWorks you can simply recompile a defective object file and keep on running. If you are running on one of these systems, you can allow No value for GDBN to reload the symbols for automatically relinked modules:

set symbol-reloading on

Replace symbol definitions for the corresponding source file when an object file with a particular name is seen again.

set symbol-reloading off

Do not replace symbol definitions when encountering object files of the same name more than once. This is the default state; if you are not running on a system that permits automatic relinking of modules, you should leave symbol-reloading off, since otherwise No value for GDBN may discard symbols when linking large programs, that may contain several modules (from different directories or libraries) with the same name.

show symbol-reloading

Show the current on or off setting.

set opaque-type-resolution on

Tell No value for GDBN 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 No value for GDBN 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.

maint print symbols filename
maint print psymbols filename
maint print msymbols filename

Write a dump of debugging symbol data into the file filename. These commands are used to debug the No value for GDBN symbol-reading code. Only symbols with debugging data are included. If you use ‘maint print symbols’, No value for GDBN includes all the symbols for which it has already collected full details: that is, filename reflects symbols for only those files whose symbols No value for GDBN has read. 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 No value for GDBN only knows partially—that is, symbols defined in files that No value for GDBN has skimmed, but not yet read completely. Finally, ‘maint print msymbols’ dumps just the minimal symbol information required for each object file from which No value for GDBN has read some symbols. See section Commands to Specify Files, for a discussion of how No value for GDBN 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 No value for GDBN debugging this one to examine a particular structure in more detail. For example:

(No value for GDBP) 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)
(No value for GDBP) maint info symtabs
(No value for GDBP)

We see that there is one partial symbol table whose filename contains the string ‘dwarf2read’, belonging to the ‘gdb’ executable; and we see that No value for GDBN has not read in any symtabs yet at all. If we set a breakpoint on a function, that will cause No value for GDBN to read the symtab for the compilation unit containing that function:

(No value for GDBP) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(No value for GDBP) 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)
    debugformat DWARF 2
(No value for GDBP)

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