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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 Ninth Edition, April 2001, of `Debugging with GDB: the

GNU Source-Level Debugger' for GDB Version 20010707.

   Copyright (C)

1988,1989,1990,1991,1992,1993,1994,1995,1996,1998,1999,2000,2001

Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being "A Sample GDB Session" and "Free Software",

with the Front-Cover texts being "A GNU Manual," and with the

Back-Cover Texts as in (a) below.

   (a) The FSF's Back-Cover Text is: "You have freedom to copy and

modify this GNU Manual, like GNU software.  Copies published by the Free

Software Foundation raise funds for GNU development."

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  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 -sSECTION 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 additionally specify an arbitrary number

     of `-sSECTION 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  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  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  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.

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

What you must do for the stub

.............................

   The debugging stubs that come with GDB are set up for a particular

chip architecture, but they have no information about the rest of your

debugging target machine.

   First of all you need to tell the stub how to communicate with the

serial port.

`int getDebugChar()'

     Write this subroutine to read a single character from the serial

     port.  It may be identical to `getchar' for your target system; a

     different name is used to allow you to distinguish the two if you

     wish.

`void putDebugChar(int)'

     Write this subroutine to write a single character to the serial

     port.  It may be identical to `putchar' for your target system; a

     different name is used to allow you to distinguish the two if you

     wish.

   If you want GDB to be able to stop your program while it is running,

you need to use an interrupt-driven serial driver, and arrange for it

to stop when it receives a `^C' (`\003', the control-C character).

That is the character which GDB uses to tell the remote system to stop.

   Getting the debugging target to return the proper status to GDB

probably requires changes to the standard stub; one quick and dirty way

is to just execute a breakpoint instruction (the "dirty" part is that

GDB reports a `SIGTRAP' instead of a `SIGINT').

   Other routines you need to supply are:

`void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)'

     Write this function to install EXCEPTION_ADDRESS in the exception

     handling tables.  You need to do this because the stub does not

     have any way of knowing what the exception handling tables on your

     target system are like (for example, the processor's table might

     be in ROM, containing entries which point to a table in RAM).

     EXCEPTION_NUMBER is the exception number which should be changed;

     its meaning is architecture-dependent (for example, different

     numbers might represent divide by zero, misaligned access, etc).

     When this exception occurs, control should be transferred directly

     to EXCEPTION_ADDRESS, and the processor state (stack, registers,

     and so on) should be just as it is when a processor exception

     occurs.  So if you want to use a jump instruction to reach

     EXCEPTION_ADDRESS, it should be a simple jump, not a jump to

     subroutine.

     For the 386, EXCEPTION_ADDRESS should be installed as an interrupt

     gate so that interrupts are masked while the handler runs.  The

     gate should be at privilege level 0 (the most privileged level).

     The SPARC and 68k stubs are able to mask interrupts themselves

     without help from `exceptionHandler'.

`void flush_i_cache()'

     On SPARC and SPARCLITE only, write this subroutine to flush the

     instruction cache, if any, on your target machine.  If there is no

     instruction cache, this subroutine may be a no-op.

     On target machines that have instruction caches, GDB requires this

     function to make certain that the state of your program is stable.

You must also make sure this library routine is available:

`void *memset(void *, int, int)'

     This is the standard library function `memset' that sets an area of

     memory to a known value.  If you have one of the free versions of

     `libc.a', `memset' can be found there; otherwise, you must either

     obtain it from your hardware manufacturer, or write your own.

   If you do not use the GNU C compiler, you may need other standard

library subroutines as well; this varies from one stub to another, but

in general the stubs are likely to use any of the common library

subroutines which `gcc' generates as inline code.

File: gdb.info,  Node: Debug Session,  Next: Protocol,  Prev: Bootstrapping,  Up: Remote Serial

Putting it all together

.......................

   In summary, when your program is ready to debug, you must follow

these steps.

  1. Make sure you have defined the supporting low-level routines

     (*note What you must do for the stub: Bootstrapping.):

          `getDebugChar', `putDebugChar',

          `flush_i_cache', `memset', `exceptionHandler'.

  2. Insert these lines near the top of your program:

          set_debug_traps();

          breakpoint();

  3. For the 680x0 stub only, you need to provide a variable called

     `exceptionHook'.  Normally you just use:

          void (*exceptionHook)() = 0;

     but if before calling `set_debug_traps', you set it to point to a

     function in your program, that function is called when `GDB'

     continues after stopping on a trap (for example, bus error).  The

     function indicated by `exceptionHook' is called with one

     parameter: an `int' which is the exception number.

  4. Compile and link together: your program, the GDB debugging stub for

     your target architecture, and the supporting subroutines.

  5. Make sure you have a serial connection between your target machine

     and the GDB host, and identify the serial port on the host.

  6. Download your program to your target machine (or get it there by

     whatever means the manufacturer provides), and start it.

  7. To start remote debugging, run GDB on the host machine, and specify

     as an executable file the program that is running in the remote

     machine.  This tells GDB how to find your program's symbols and

     the contents of its pure text.

  8. Establish communication using the `target remote' command.  Its

     argument specifies how to communicate with the target

     machine--either via a devicename attached to a direct serial line,

     or a TCP port (usually to a terminal server which in turn has a

     serial line to the target).  For example, to use a serial line

     connected to the device named `/dev/ttyb':

          target remote /dev/ttyb

     To use a TCP connection, use an argument of the form `HOST:port'.

     For example, to connect to port 2828 on a terminal server named

     `manyfarms':

          target remote manyfarms:2828

   Now you can use all the usual commands to examine and change data

and to step and continue the remote program.

   To resume the remote program and stop debugging it, use the `detach'

command.

   Whenever GDB is waiting for the remote program, if you type the

interrupt character (often ), GDB attempts to stop the program.

This may or may not succeed, depending in part on the hardware and the

serial drivers the remote system uses.  If you type the interrupt

character once again, GDB displays this prompt:

     Interrupted while waiting for the program.

     Give up (and stop debugging it)?  (y or n)

   If you type `y', GDB abandons the remote debugging session.  (If you

decide you want to try again later, you can use `target remote' again

to connect once more.)  If you type `n', GDB goes back to waiting.