RTEMS GNU Tools On-Line Library

Symbols

Debugging with GDB

Chapter 10: 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 File Options), or by one of the file-management commands (see 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 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'.

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.

whatis expr

Print the data type of expression expr. expr is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. See Expressions.

whatis

Print the data type of $, the last value in the value history.

ptype typename

Print a description of data type typename. typename may be the name of a type, or for C code it may have the form `class class-name', `struct struct-tag', `union union-tag' or `enum enum-tag'.

ptype expr

ptype

Print a description of the type of expression expr. ptype differs from whatis by printing a detailed description, instead of just the name of the type.

For example, for this variable declaration:

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

the two commands give this output:

(gdb) whatis v
type = struct complex
(gdb) 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.

info types regexp

info types

Print a brief description of all types whose names match 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 source

Show the name of the current source file---that is, the source file for the function containing the current point of execution---and the language it was written in.

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.

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.

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 GDB 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 GDB 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 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.

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 GDB symbol-reading code. Only symbols with debugging data are included. If you use `maint print symbols', GDB includes all the symbols for which it has already collected full details: that is, filename reflects symbols for only those files whose symbols GDB 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 GDB only knows partially---that is, symbols defined in files that GDB has skimmed, but not yet read completely. Finally, `maint print msymbols' dumps just the minimal symbol information required for each object file from which GDB has read some symbols. See Files, for a discussion of how GDB reads symbols (in the description of symbol-file).

Debugging with GDB