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INFO-DIR-SECTION Programming & development tools.
START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Eighth Edition, March 2000, of `Debugging with GDB: the
GNU Source-Level Debugger' for GDB Version 5.0.

   Copyright (C) 1988-2000 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.


File: gdb.info,  Node: Symbols,  Next: Altering,  Prev: Languages,  Up: Top

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
(*note Choosing files: File Options.), or by one of the file-management
commands (*note Commands to specify files: 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
(*note Program variables: 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.  *Note
     Expressions: 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.  *Note Commands to specify files:
     Files, for a discussion of how GDB reads symbols (in the
     description of `symbol-file').


File: gdb.info,  Node: Altering,  Next: GDB Files,  Prev: Symbols,  Up: Top

Altering Execution
******************

   Once you think you have found an error in your program, you might
want to find out for certain whether correcting the apparent error
would lead to correct results in the rest of the run.  You can find the
answer by experiment, using the GDB features for altering execution of
the program.

   For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.

* Menu:

* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
* Signaling::                   Giving your program a signal
* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program


File: gdb.info,  Node: Assignment,  Next: Jumping,  Up: Altering

Assignment to variables
=======================

   To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions.  For example,

     print x=4

stores the value 4 into the variable `x', and then prints the value of
the assignment expression (which is 4).  *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.

   If you are not interested in seeing the value of the assignment, use
the `set' command instead of the `print' command.  `set' is really the
same as `print' except that the expression's value is not printed and
is not put in the value history (*note Value history: Value History.).
The expression is evaluated only for its effects.

   If the beginning of the argument string of the `set' command appears
identical to a `set' subcommand, use the `set variable' command instead
of just `set'.  This command is identical to `set' except for its lack
of subcommands.  For example, if your program has a variable `width',
you get an error if you try to set a new value with just `set
width=13', because GDB has the command `set width':

     (gdb) whatis width
     type = double
     (gdb) p width
     $4 = 13
     (gdb) set width=47
     Invalid syntax in expression.

The invalid expression, of course, is `=47'.  In order to actually set
the program's variable `width', use

     (gdb) set var width=47

   Because the `set' command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the `set
variable' command instead of just `set'.  For example, if your program
has a variable `g', you run into problems if you try to set a new value
with just `set g=4', because GDB has the command `set gnutarget',
abbreviated `set g':

     (gdb) whatis g
     type = double
     (gdb) p g
     $1 = 1
     (gdb) set g=4
     (gdb) p g
     $2 = 1
     (gdb) r
     The program being debugged has been started already.
     Start it from the beginning? (y or n) y
     Starting program: /home/smith/cc_progs/a.out
     "/home/smith/cc_progs/a.out": can't open to read symbols:
                                      Invalid bfd target.
     (gdb) show g
     The current BFD target is "=4".

The program variable `g' did not change, and you silently set the
`gnutarget' to an invalid value.  In order to set the variable `g', use

     (gdb) set var g=4

   GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and you can convert any structure to any other structure that is the
same length or shorter.

   To store values into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.).  For example, `{int}0x83040' refers
to memory location `0x83040' as an integer (which implies a certain size
and representation in memory), and

     set {int}0x83040 = 4

stores the value 4 into that memory location.


File: gdb.info,  Node: Jumping,  Next: Signaling,  Prev: Assignment,  Up: Altering

Continuing at a different address
=================================

   Ordinarily, when you continue your program, you do so at the place
where it stopped, with the `continue' command.  You can instead
continue at an address of your own choosing, with the following
commands:

`jump LINESPEC'
     Resume execution at line LINESPEC.  Execution stops again
     immediately if there is a breakpoint there.  *Note Printing source
     lines: List, for a description of the different forms of LINESPEC.
     It is common practice to use the `tbreak' command in conjunction
     with `jump'.  *Note Setting breakpoints: Set Breaks.

     The `jump' command does not change the current stack frame, or the
     stack pointer, or the contents of any memory location or any
     register other than the program counter.  If line LINESPEC is in a
     different function from the one currently executing, the results
     may be bizarre if the two functions expect different patterns of
     arguments or of local variables.  For this reason, the `jump'
     command requests confirmation if the specified line is not in the
     function currently executing.  However, even bizarre results are
     predictable if you are well acquainted with the machine-language
     code of your program.

`jump *ADDRESS'
     Resume execution at the instruction at address ADDRESS.

   On many systems, you can get much the same effect as the `jump'
command by storing a new value into the register `$pc'.  The difference
is that this does not start your program running; it only changes the
address of where it _will_ run when you continue.  For example,

     set $pc = 0x485

makes the next `continue' command or stepping command execute at
address `0x485', rather than at the address where your program stopped.
*Note Continuing and stepping: Continuing and Stepping.

   The most common occasion to use the `jump' command is to back
up--perhaps with more breakpoints set--over a portion of a program that
has already executed, in order to examine its execution in more detail.


File: gdb.info,  Node: Signaling,  Next: Returning,  Prev: Jumping,  Up: Altering

Giving your program a signal
============================

`signal SIGNAL'
     Resume execution where your program stopped, but immediately give
     it the signal SIGNAL.  SIGNAL can be the name or the number of a
     signal.  For example, on many systems `signal 2' and `signal
     SIGINT' are both ways of sending an interrupt signal.

     Alternatively, if SIGNAL is zero, continue execution without
     giving a signal.  This is useful when your program stopped on
     account of a signal and would ordinary see the signal when resumed
     with the `continue' command; `signal 0' causes it to resume
     without a signal.

     `signal' does not repeat when you press <RET> a second time after
     executing the command.

   Invoking the `signal' command is not the same as invoking the `kill'
utility from the shell.  Sending a signal with `kill' causes GDB to
decide what to do with the signal depending on the signal handling
tables (*note Signals::).  The `signal' command passes the signal
directly to your program.


File: gdb.info,  Node: Returning,  Next: Calling,  Prev: Signaling,  Up: Altering

Returning from a function
=========================

`return'
`return EXPRESSION'
     You can cancel execution of a function call with the `return'
     command.  If you give an EXPRESSION argument, its value is used as
     the function's return value.

   When you use `return', GDB discards the selected stack frame (and
all frames within it).  You can think of this as making the discarded
frame return prematurely.  If you wish to specify a value to be
returned, give that value as the argument to `return'.

   This pops the selected stack frame (*note Selecting a frame:
Selection.), and any other frames inside of it, leaving its caller as
the innermost remaining frame.  That frame becomes selected.  The
specified value is stored in the registers used for returning values of
functions.

   The `return' command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned.  In contrast, the `finish' command (*note Continuing and
stepping: Continuing and Stepping.) resumes execution until the
selected stack frame returns naturally.


File: gdb.info,  Node: Calling,  Next: Patching,  Prev: Returning,  Up: Altering

Calling program functions
=========================

`call EXPR'
     Evaluate the expression EXPR without displaying `void' returned
     values.

   You can use this variant of the `print' command if you want to
execute a function from your program, but without cluttering the output
with `void' returned values.  If the result is not void, it is printed
and saved in the value history.

   For the A29K, a user-controlled variable `call_scratch_address',
specifies the location of a scratch area to be used when GDB calls a
function in the target.  This is necessary because the usual method of
putting the scratch area on the stack does not work in systems that
have separate instruction and data spaces.


File: gdb.info,  Node: Patching,  Prev: Calling,  Up: Altering

Patching programs
=================

   By default, GDB opens the file containing your program's executable
code (or the corefile) read-only.  This prevents accidental alterations
to machine code; but it also prevents you from intentionally patching
your program's binary.

   If you'd like to be able to patch the binary, you can specify that
explicitly with the `set write' command.  For example, you might want
to turn on internal debugging flags, or even to make emergency repairs.

`set write on'
`set write off'
     If you specify `set write on', GDB opens executable and core files
     for both reading and writing; if you specify `set write off' (the
     default), GDB opens them read-only.

     If you have already loaded a file, you must load it again (using
     the `exec-file' or `core-file' command) after changing `set
     write', for your new setting to take effect.

`show write'
     Display whether executable files and core files are opened for
     writing as well as reading.


File: gdb.info,  Node: GDB Files,  Next: Targets,  Prev: Altering,  Up: Top

GDB Files
*********

   GDB needs to know the file name of the program to be debugged, both
in order to read its symbol table and in order to start your program.
To debug a core dump of a previous run, you must also tell GDB the name
of the core dump file.

* Menu:

* Files::                       Commands to specify files
* Symbol Errors::               Errors reading symbol files


File: gdb.info,  Node: Files,  Next: Symbol Errors,  Up: GDB Files

Commands to specify files
=========================

   You may want to specify executable and core dump file names.  The
usual way to do this is at start-up time, using the arguments to GDB's
start-up commands (*note Getting In and Out of GDB: Invocation.).

   Occasionally it is necessary to change to a different file during a
GDB session.  Or you may run GDB and forget to specify a file you want
to use.  In these situations the GDB commands to specify new files are
useful.

`file FILENAME'
     Use FILENAME as the program to be debugged.  It is read for its
     symbols and for the contents of pure memory.  It is also the
     program executed when you use the `run' command.  If you do not
     specify a directory and the file is not found in the GDB working
     directory, GDB uses the environment variable `PATH' as a list of
     directories to search, just as the shell does when looking for a
     program to run.  You can change the value of this variable, for
     both GDB and your program, using the `path' command.

     On systems with memory-mapped files, an auxiliary file named
     `FILENAME.syms' may hold symbol table information for FILENAME.
     If so, GDB maps in the symbol table from `FILENAME.syms', starting
     up more quickly.  See the descriptions of the file options
     `-mapped' and `-readnow' (available on the command line, and with
     the commands `file', `symbol-file', or `add-symbol-file',
     described below), for more information.

`file'
     `file' with no argument makes GDB discard any information it has
     on both executable file and the symbol table.

`exec-file [ FILENAME ]'
     Specify that the program to be run (but not the symbol table) is
     found in FILENAME.  GDB searches the environment variable `PATH'
     if necessary to locate your program.  Omitting FILENAME means to
     discard information on the executable file.

`symbol-file [ FILENAME ]'
     Read symbol table information from file FILENAME.  `PATH' is
     searched when necessary.  Use the `file' command to get both symbol
     table and program to run from the same file.

     `symbol-file' with no argument clears out GDB information on your
     program's symbol table.

     The `symbol-file' command causes GDB to forget the contents of its
     convenience variables, the value history, and all breakpoints and
     auto-display expressions.  This is because they may contain
     pointers to the internal data recording symbols and data types,
     which are part of the old symbol table data being discarded inside
     GDB.

     `symbol-file' does not repeat if you press <RET> again after
     executing it once.

     When GDB is configured for a particular environment, it
     understands debugging information in whatever format is the
     standard generated for that environment; you may use either a GNU
     compiler, or other compilers that adhere to the local conventions.
     Best results are usually obtained from GNU compilers; for example,
     using `gcc' you can generate debugging information for optimized
     code.

     For most kinds of object files, with the exception of old SVR3
     systems using COFF, the `symbol-file' command does not normally
     read the symbol table in full right away.  Instead, it scans the
     symbol table quickly to find which source files and which symbols
     are present.  The details are read later, one source file at a
     time, as they are needed.

     The purpose of this two-stage reading strategy is to make GDB
     start up faster.  For the most part, it is invisible except for
     occasional pauses while the symbol table details for a particular
     source file are being read.  (The `set verbose' command can turn
     these pauses into messages if desired.  *Note Optional warnings
     and messages: Messages/Warnings.)

     We have not implemented the two-stage strategy for COFF yet.  When
     the symbol table is stored in COFF format, `symbol-file' reads the
     symbol table data in full right away.  Note that "stabs-in-COFF"
     still does the two-stage strategy, since the debug info is actually
     in stabs format.

`symbol-file FILENAME [ -readnow ] [ -mapped ]'
`file FILENAME [ -readnow ] [ -mapped ]'
     You can override the GDB two-stage strategy for reading symbol
     tables by using the `-readnow' option with any of the commands that
     load symbol table information, if you want to be sure GDB has the
     entire symbol table available.

     If memory-mapped files are available on your system through the
     `mmap' system call, you can use another option, `-mapped', to
     cause GDB to write the symbols for your program into a reusable
     file.  Future GDB debugging sessions map in symbol information
     from this auxiliary symbol file (if the program has not changed),
     rather than spending time reading the symbol table from the
     executable program.  Using the `-mapped' option has the same
     effect as starting GDB with the `-mapped' command-line option.

     You can use both options together, to make sure the auxiliary
     symbol file has all the symbol information for your program.

     The auxiliary symbol file for a program called MYPROG is called
     `MYPROG.syms'.  Once this file exists (so long as it is newer than
     the corresponding executable), GDB always attempts to use it when
     you debug MYPROG; no special options or commands are needed.

     The `.syms' file is specific to the host machine where you run
     GDB.  It holds an exact image of the internal GDB symbol table.
     It cannot be shared across multiple host platforms.

`core-file [ FILENAME ]'
     Specify the whereabouts of a core dump file to be used as the
     "contents of memory".  Traditionally, core files contain only some
     parts of the address space of the process that generated them; GDB
     can access the executable file itself for other parts.

     `core-file' with no argument specifies that no core file is to be
     used.

     Note that the core file is ignored when your program is actually
     running under GDB.  So, if you have been running your program and
     you wish to debug a core file instead, you must kill the
     subprocess in which the program is running.  To do this, use the
     `kill' command (*note Killing the child process: Kill Process.).

`add-symbol-file FILENAME ADDRESS'
`add-symbol-file FILENAME ADDRESS [ -readnow ] [ -mapped ]'
`add-symbol-file FILENAME ADDRESS DATA_ADDRESS BSS_ADDRESS'
`add-symbol-file FILENAME -TSECTION ADDRESS'
     The `add-symbol-file' command reads additional symbol table
     information from the file FILENAME.  You would use this command
     when FILENAME has been dynamically loaded (by some other means)
     into the program that is running.  ADDRESS should be the memory
     address at which the file has been loaded; GDB cannot figure this
     out for itself.  You can specify up to three addresses, in which
     case they are taken to be the addresses of the text, data, and bss
     segments respectively.  For complicated cases, you can specify an
     arbitrary number of `-TSECTION ADDRESS' pairs, to give an explicit
     section name and base address for that section.  You can specify
     any ADDRESS as an expression.

     The symbol table of the file FILENAME is added to the symbol table
     originally read with the `symbol-file' command.  You can use the
     `add-symbol-file' command any number of times; the new symbol data
     thus read keeps adding to the old.  To discard all old symbol data
     instead, use the `symbol-file' command without any arguments.

     `add-symbol-file' does not repeat if you press <RET> after using
     it.

     You can use the `-mapped' and `-readnow' options just as with the
     `symbol-file' command, to change how GDB manages the symbol table
     information for FILENAME.

`add-shared-symbol-file'
     The `add-shared-symbol-file' command can be used only under
     Harris' CXUX operating system for the Motorola 88k.  GDB
     automatically looks for shared libraries, however if GDB does not
     find yours, you can run `add-shared-symbol-file'.  It takes no
     arguments.

`section'
     The `section' command changes the base address of section SECTION
     of the exec file to ADDR.  This can be used if the exec file does
     not contain section addresses, (such as in the a.out format), or
     when the addresses specified in the file itself are wrong.  Each
     section must be changed separately.  The `info files' command,
     described below, lists all the sections and their addresses.

`info files'
`info target'
     `info files' and `info target' are synonymous; both print the
     current target (*note Specifying a Debugging Target: Targets.),
     including the names of the executable and core dump files
     currently in use by GDB, and the files from which symbols were
     loaded.  The command `help target' lists all possible targets
     rather than current ones.

   All file-specifying commands allow both absolute and relative file
names as arguments.  GDB always converts the file name to an absolute
file name and remembers it that way.

   GDB supports HP-UX, SunOS, SVr4, Irix 5, and IBM RS/6000 shared
libraries.

   GDB automatically loads symbol definitions from shared libraries
when you use the `run' command, or when you examine a core file.
(Before you issue the `run' command, GDB does not understand references
to a function in a shared library, however--unless you are debugging a
core file).

   On HP-UX, if the program loads a library explicitly, GDB
automatically loads the symbols at the time of the `shl_load' call.

`info share'
`info sharedlibrary'
     Print the names of the shared libraries which are currently loaded.

`sharedlibrary REGEX'
`share REGEX'
     Load shared object library symbols for files matching a Unix
     regular expression.  As with files loaded automatically, it only
     loads shared libraries required by your program for a core file or
     after typing `run'.  If REGEX is omitted all shared libraries
     required by your program are loaded.

   On HP-UX systems, GDB detects the loading of a shared library and
automatically reads in symbols from the newly loaded library, up to a
threshold that is initially set but that you can modify if you wish.

   Beyond that threshold, symbols from shared libraries must be
explicitly loaded.  To load these symbols, use the command
`sharedlibrary FILENAME'.  The base address of the shared library is
determined automatically by GDB and need not be specified.

   To display or set the threshold, use the commands:

`set auto-solib-add THRESHOLD'
     Set the autoloading size threshold, in megabytes.  If THRESHOLD is
     nonzero, symbols from all shared object libraries will be loaded
     automatically when the inferior begins execution or when the
     dynamic linker informs GDB that a new library has been loaded,
     until the symbol table of the program and libraries exceeds this
     threshold.  Otherwise, symbols must be loaded manually, using the
     `sharedlibrary' command.  The default threshold is 100 megabytes.

`show auto-solib-add'
     Display the current autoloading size threshold, in megabytes.


File: gdb.info,  Node: Symbol Errors,  Prev: Files,  Up: GDB Files

Errors reading symbol files
===========================

   While reading a symbol file, GDB occasionally encounters problems,
such as symbol types it does not recognize, or known bugs in compiler
output.  By default, GDB does not notify you of such problems, since
they are relatively common and primarily of interest to people
debugging compilers.  If you are interested in seeing information about
ill-constructed symbol tables, you can either ask GDB to print only one
message about each such type of problem, no matter how many times the
problem occurs; or you can ask GDB to print more messages, to see how
many times the problems occur, with the `set complaints' command (*note
Optional warnings and messages: Messages/Warnings.).

   The messages currently printed, and their meanings, include:

`inner block not inside outer block in SYMBOL'
     The symbol information shows where symbol scopes begin and end
     (such as at the start of a function or a block of statements).
     This error indicates that an inner scope block is not fully
     contained in its outer scope blocks.

     GDB circumvents the problem by treating the inner block as if it
     had the same scope as the outer block.  In the error message,
     SYMBOL may be shown as "`(don't know)'" if the outer block is not a
     function.

`block at ADDRESS out of order'
     The symbol information for symbol scope blocks should occur in
     order of increasing addresses.  This error indicates that it does
     not do so.

     GDB does not circumvent this problem, and has trouble locating
     symbols in the source file whose symbols it is reading.  (You can
     often determine what source file is affected by specifying `set
     verbose on'.  *Note Optional warnings and messages:
     Messages/Warnings.)

`bad block start address patched'
     The symbol information for a symbol scope block has a start address
     smaller than the address of the preceding source line.  This is
     known to occur in the SunOS 4.1.1 (and earlier) C compiler.

     GDB circumvents the problem by treating the symbol scope block as
     starting on the previous source line.

`bad string table offset in symbol N'
     Symbol number N contains a pointer into the string table which is
     larger than the size of the string table.

     GDB circumvents the problem by considering the symbol to have the
     name `foo', which may cause other problems if many symbols end up
     with this name.

`unknown symbol type `0xNN''
     The symbol information contains new data types that GDB does not
     yet know how to read.  `0xNN' is the symbol type of the
     uncomprehended information, in hexadecimal.

     GDB circumvents the error by ignoring this symbol information.
     This usually allows you to debug your program, though certain
     symbols are not accessible.  If you encounter such a problem and
     feel like debugging it, you can debug `gdb' with itself, breakpoint
     on `complain', then go up to the function `read_dbx_symtab' and
     examine `*bufp' to see the symbol.

`stub type has NULL name'
     GDB could not find the full definition for a struct or class.

`const/volatile indicator missing (ok if using g++ v1.x), got...'
     The symbol information for a C++ member function is missing some
     information that recent versions of the compiler should have
     output for it.

`info mismatch between compiler and debugger'
     GDB could not parse a type specification output by the compiler.


File: gdb.info,  Node: Targets,  Next: Configurations,  Prev: GDB Files,  Up: Top

Specifying a Debugging Target
*****************************

   A "target" is the execution environment occupied by your program.

   Often, GDB runs in the same host environment as your program; in
that case, the debugging target is specified as a side effect when you
use the `file' or `core' commands.  When you need more flexibility--for
example, running GDB on a physically separate host, or controlling a
standalone system over a serial port or a realtime system over a TCP/IP
connection--you can use the `target' command to specify one of the
target types configured for GDB (*note Commands for managing targets:
Target Commands.).

* Menu:

* Active Targets::              Active targets
* Target Commands::             Commands for managing targets
* Byte Order::                  Choosing target byte order
* Remote::                      Remote debugging
* KOD::                         Kernel Object Display


File: gdb.info,  Node: Active Targets,  Next: Target Commands,  Up: Targets

Active targets
==============

   There are three classes of targets: processes, core files, and
executable files.  GDB can work concurrently on up to three active
targets, one in each class.  This allows you to (for example) start a
process and inspect its activity without abandoning your work on a core
file.

   For example, if you execute `gdb a.out', then the executable file
`a.out' is the only active target.  If you designate a core file as
well--presumably from a prior run that crashed and coredumped--then GDB
has two active targets and uses them in tandem, looking first in the
corefile target, then in the executable file, to satisfy requests for
memory addresses.  (Typically, these two classes of target are
complementary, since core files contain only a program's read-write
memory--variables and so on--plus machine status, while executable
files contain only the program text and initialized data.)

   When you type `run', your executable file becomes an active process
target as well.  When a process target is active, all GDB commands
requesting memory addresses refer to that target; addresses in an
active core file or executable file target are obscured while the
process target is active.

   Use the `core-file' and `exec-file' commands to select a new core
file or executable target (*note Commands to specify files: Files.).
To specify as a target a process that is already running, use the
`attach' command (*note Debugging an already-running process: Attach.).


File: gdb.info,  Node: Target Commands,  Next: Byte Order,  Prev: Active Targets,  Up: Targets

Commands for managing targets
=============================

`target TYPE PARAMETERS'
     Connects the GDB host environment to a target machine or process.
     A target is typically a protocol for talking to debugging
     facilities.  You use the argument TYPE to specify the type or
     protocol of the target machine.

     Further PARAMETERS are interpreted by the target protocol, but
     typically include things like device names or host names to connect
     with, process numbers, and baud rates.

     The `target' command does not repeat if you press <RET> again
     after executing the command.

`help target'
     Displays the names of all targets available.  To display targets
     currently selected, use either `info target' or `info files'
     (*note Commands to specify files: Files.).

`help target NAME'
     Describe a particular target, including any parameters necessary to
     select it.

`set gnutarget ARGS'
     GDB uses its own library BFD to read your files.  GDB knows
     whether it is reading an "executable", a "core", or a ".o" file;
     however, you can specify the file format with the `set gnutarget'
     command.  Unlike most `target' commands, with `gnutarget' the
     `target' refers to a program, not a machine.

          _Warning:_ To specify a file format with `set gnutarget', you
          must know the actual BFD name.

     *Note Commands to specify files: Files.

`show gnutarget'
     Use the `show gnutarget' command to display what file format
     `gnutarget' is set to read.  If you have not set `gnutarget', GDB
     will determine the file format for each file automatically, and
     `show gnutarget' displays `The current BDF target is "auto"'.

   Here are some common targets (available, or not, depending on the GDB
configuration):

`target exec PROGRAM'
     An executable file.  `target exec PROGRAM' is the same as
     `exec-file PROGRAM'.

`target core FILENAME'
     A core dump file.  `target core FILENAME' is the same as
     `core-file FILENAME'.

`target remote DEV'
     Remote serial target in GDB-specific protocol.  The argument DEV
     specifies what serial device to use for the connection (e.g.
     `/dev/ttya'). *Note Remote debugging: Remote.  `target remote'
     supports the `load' command.  This is only useful if you have some
     other way of getting the stub to the target system, and you can put
     it somewhere in memory where it won't get clobbered by the
     download.

`target sim'
     Builtin CPU simulator.  GDB includes simulators for most
     architectures.  In general,
                  target sim
                  load
                  run

     works; however, you cannot assume that a specific memory map,
     device drivers, or even basic I/O is available, although some
     simulators do provide these.  For info about any
     processor-specific simulator details, see the appropriate section
     in *Note Embedded Processors: Embedded Processors.

   Some configurations may include these targets as well:

`target nrom DEV'
     NetROM ROM emulator.  This target only supports downloading.

   Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.

   Many remote targets require you to download the executable's code
once you've successfully established a connection.

`load FILENAME'
     Depending on what remote debugging facilities are configured into
     GDB, the `load' command may be available.  Where it exists, it is
     meant to make FILENAME (an executable) available for debugging on
     the remote system--by downloading, or dynamic linking, for example.
     `load' also records the FILENAME symbol table in GDB, like the
     `add-symbol-file' command.

     If your GDB does not have a `load' command, attempting to execute
     it gets the error message "`You can't do that when your target is
     ...'"

     The file is loaded at whatever address is specified in the
     executable.  For some object file formats, you can specify the
     load address when you link the program; for other formats, like
     a.out, the object file format specifies a fixed address.

     `load' does not repeat if you press <RET> again after using it.


File: gdb.info,  Node: Byte Order,  Next: Remote,  Prev: Target Commands,  Up: Targets

Choosing target byte order
==========================

   Some types of processors, such as the MIPS, PowerPC, and Hitachi SH,
offer the ability to run either big-endian or little-endian byte
orders.  Usually the executable or symbol will include a bit to
designate the endian-ness, and you will not need to worry about which
to use.  However, you may still find it useful to adjust GDB's idea of
processor endian-ness manually.

`set endian big'
     Instruct GDB to assume the target is big-endian.

`set endian little'
     Instruct GDB to assume the target is little-endian.

`set endian auto'
     Instruct GDB to use the byte order associated with the executable.

`show endian'
     Display GDB's current idea of the target byte order.

   Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the target
system.


File: gdb.info,  Node: Remote,  Next: KOD,  Prev: Byte Order,  Up: Targets

Remote debugging
================

   If you are trying to debug a program running on a machine that
cannot run GDB in the usual way, it is often useful to use remote
debugging.  For example, you might use remote debugging on an operating
system kernel, or on a small system which does not have a general
purpose operating system powerful enough to run a full-featured
debugger.

   Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets.  In addition, GDB
comes with a generic serial protocol (specific to GDB, but not specific
to any particular target system) which you can use if you write the
remote stubs--the code that runs on the remote system to communicate
with GDB.

   Other remote targets may be available in your configuration of GDB;
use `help target' to list them.

* Menu:

* Remote Serial::               GDB remote serial protocol


File: gdb.info,  Node: Remote Serial,  Up: Remote

The GDB remote serial protocol
------------------------------

   To debug a program running on another machine (the debugging
"target" machine), you must first arrange for all the usual
prerequisites for the program to run by itself.  For example, for a C
program, you need:

  1. A startup routine to set up the C runtime environment; these
     usually have a name like `crt0'.  The startup routine may be
     supplied by your hardware supplier, or you may have to write your
     own.

  2. A C subroutine library to support your program's subroutine calls,
     notably managing input and output.

  3. A way of getting your program to the other machine--for example, a
     download program.  These are often supplied by the hardware
     manufacturer, but you may have to write your own from hardware
     documentation.

   The next step is to arrange for your program to use a serial port to
communicate with the machine where GDB is running (the "host" machine).
In general terms, the scheme looks like this:

_On the host,_
     GDB already understands how to use this protocol; when everything
     else is set up, you can simply use the `target remote' command
     (*note Specifying a Debugging Target: Targets.).

_On the target,_
     you must link with your program a few special-purpose subroutines
     that implement the GDB remote serial protocol.  The file
     containing these subroutines is called  a "debugging stub".

     On certain remote targets, you can use an auxiliary program
     `gdbserver' instead of linking a stub into your program.  *Note
     Using the `gdbserver' program: Server, for details.

   The debugging stub is specific to the architecture of the remote
machine; for example, use `sparc-stub.c' to debug programs on SPARC
boards.

   These working remote stubs are distributed with GDB:

`i386-stub.c'
     For Intel 386 and compatible architectures.

`m68k-stub.c'
     For Motorola 680x0 architectures.

`sh-stub.c'
     For Hitachi SH architectures.

`sparc-stub.c'
     For SPARC architectures.

`sparcl-stub.c'
     For Fujitsu SPARCLITE architectures.

   The `README' file in the GDB distribution may list other recently
added stubs.

* Menu:

* Stub Contents::       What the stub can do for you
* Bootstrapping::       What you must do for the stub
* Debug Session::       Putting it all together
* Protocol::            Definition of the communication protocol
* Server::                Using the `gdbserver' program
* NetWare::                Using the `gdbserve.nlm' program


File: gdb.info,  Node: Stub Contents,  Next: Bootstrapping,  Up: Remote Serial

What the stub can do for you
............................

   The debugging stub for your architecture supplies these three
subroutines:

`set_debug_traps'
     This routine arranges for `handle_exception' to run when your
     program stops.  You must call this subroutine explicitly near the
     beginning of your program.

`handle_exception'
     This is the central workhorse, but your program never calls it
     explicitly--the setup code arranges for `handle_exception' to run
     when a trap is triggered.

     `handle_exception' takes control when your program stops during
     execution (for example, on a breakpoint), and mediates
     communications with GDB on the host machine.  This is where the
     communications protocol is implemented; `handle_exception' acts as
     the GDB representative on the target machine.  It begins by
     sending summary information on the state of your program, then
     continues to execute, retrieving and transmitting any information
     GDB needs, until you execute a GDB command that makes your program
     resume; at that point, `handle_exception' returns control to your
     own code on the target machine.

`breakpoint'
     Use this auxiliary subroutine to make your program contain a
     breakpoint.  Depending on the particular situation, this may be
     the only way for GDB to get control.  For instance, if your target
     machine has some sort of interrupt button, you won't need to call
     this; pressing the interrupt button transfers control to
     `handle_exception'--in effect, to GDB.  On some machines, simply
     receiving characters on the serial port may also trigger a trap;
     again, in that situation, you don't need to call `breakpoint' from
     your own program--simply running `target remote' from the host GDB
     session gets control.

     Call `breakpoint' if none of these is true, or if you simply want
     to make certain your program stops at a predetermined point for the
     start of your debugging session.