Debugging with GDB: Symbols

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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 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`.

`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 `typedef`s 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. `typedef`s at the pointer or reference targets are also unrolled. Only `typedef`s of fields, methods and inner `class typedef`s of `struct`s, `class`es and `union`s 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.

`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 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:

  - 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,
  - if the debug information provides it, the program that compiled the file (which may include, e.g., the compiler version and command line arguments),
  - 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 defined 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.

`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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