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This is Info file ./gdb.info, produced by Makeinfo-1.52 from the input
file gdb.texinfo.
START-INFO-DIR-ENTRY
* Gdb:: The GNU debugger.
END-INFO-DIR-ENTRY
This file documents the GNU debugger GDB.
This is Edition 4.09, August 1993, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB Version 4.11.
Copyright (C) 1988, '89, '90, '91, '92, '93 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: 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 to its caller.
* 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
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 will stop
immediately if there is a breakpoint there. *Note Printing source
lines: List, for a description of the different forms of LINESPEC.
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.
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 where it
*will* run when it is continued. For example,
set $pc = 0x485
causes the next `continue' command or stepping command to 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. The result is printed and saved in the
value history, if it is not void.

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 will open executable and core
files for both reading and writing; if you specify `set write off'
(the default), GDB will open 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 will be 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
=========================
The usual way to specify executable and core dump file names is with
the command arguments given when you start GDB (*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 symbol table file
`FILENAME.syms' may be available for FILENAME. If it is, GDB will
map in the symbol table from `FILENAME.syms', starting up more
quickly. See the descriptions of the options `-mapped' and
`-readnow' (available on the command line, and with the commands
`file', `symbol-file', or `add-symbol-file'), 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 will search 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' will not repeat if you press RET again after
executing it once.
When GDB is configured for a particular environment, it will
understand 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.
On some kinds of object files, 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.
`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 will 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 will always attempt 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.).
`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, like a.out, the object
file format fixes the address and so it won't necessarily match
the address you gave to the linker.
On VxWorks, `load' will dynamically link FILENAME on the current
target system as well as adding its symbols in GDB.
With the Nindy interface to an Intel 960 board, `load' will
download FILENAME to the 960 as well as adding its symbols in GDB.
When you select remote debugging to a Hitachi SH, H8/300, or
H8/500 board (*note GDB and Hitachi Microprocessors: Hitachi
Remote.), the `load' command downloads your program to the Hitachi
board and also opens it as the current executable target for GDB
on your host (like the `file' command).
`load' will not repeat if you press RET again after using it.
`add-symbol-file FILENAME ADDRESS'
`add-symbol-file FILENAME ADDRESS [ -readnow ] [ -mapped ]'
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 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.
`add-symbol-file' will 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.
`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 targets' 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
path name and remembers it that way.
GDB supports SunOS, SVR4, 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 will not understand references to a
function in a shared library, however--unless you are debugging a core
file).
`info share'
`info sharedlibrary'
Print the names of the shared libraries which are currently loaded.
`sharedlibrary REGEX'
`share REGEX'
This is an obsolescent command; you can use it to explicitly load
shared object library symbols for files matching a Unix regular
expression, but as with files loaded automatically, it will only
load 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.

File: gdb.info, Node: Symbol Errors, Prev: Files, Up: GDB Files
Errors reading symbol files
===========================
While reading a symbol file, GDB will occasionally encounter
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 will have 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
misunderstood information, in hexadecimal.
GDB circumvents the error by ignoring this symbol information.
This will usually allow your program to be debugged, though
certain symbols will not be 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: Controlling GDB, 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
* Remote:: Remote debugging

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 will use 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: Remote, 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 will 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.
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 sim'
CPU simulator. *Note Simulated CPU Target: Simulator.
`target udi KEYWORD'
Remote AMD29K target, using the AMD UDI protocol. The KEYWORD
argument specifies which 29K board or simulator to use. *Note GDB
and the UDI protocol for AMD29K: UDI29K Remote.
`target amd-eb DEV SPEED PROG'
Remote PC-resident AMD EB29K board, attached over serial lines.
dEV is the serial device, as for `target remote'; SPEED allows you
to specify the linespeed; and PROG is the name of the program to
be debugged, as it appears to DOS on the PC. *Note GDB with a
remote EB29K: EB29K Remote.
`target hms'
A Hitachi SH, H8/300, or H8/500 board, attached via serial line to
your host. Use special commands `device' and `speed' to control
the serial line and the communications speed used. *Note GDB and
Hitachi Microprocessors: Hitachi Remote.
`target nindy DEVICENAME'
An Intel 960 board controlled by a Nindy Monitor. DEVICENAME is
the name of the serial device to use for the connection, e.g.
`/dev/ttya'. *Note GDB with a remote i960 (Nindy): i960-Nindy
Remote.
`target st2000 DEV SPEED'
A Tandem ST2000 phone switch, running Tandem's STDBUG protocol.
dEV is the name of the device attached to the ST2000 serial line;
SPEED is the communication line speed. The arguments are not used
if GDB is configured to connect to the ST2000 using TCP or Telnet.
*Note GDB with a Tandem ST2000: ST2000 Remote.
`target vxworks MACHINENAME'
A VxWorks system, attached via TCP/IP. The argument MACHINENAME
is the target system's machine name or IP address. *Note GDB and
VxWorks: VxWorks Remote.
Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.

File: gdb.info, Node: Remote, Prev: Target Commands, 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 will run on the remote system to
communicate with GDB.
Other remote targets may be available in your configuration of GDB;
use `help targets' to list them.
* Menu:
* Remote Serial:: GDB remote serial protocol
* i960-Nindy Remote:: GDB with a remote i960 (Nindy)
* UDI29K Remote:: GDB and the UDI protocol for AMD29K
* EB29K Remote:: GDB with a remote EB29K
* VxWorks Remote:: GDB and VxWorks
* ST2000 Remote:: GDB with a Tandem ST2000
* Hitachi Remote:: GDB and Hitachi Microprocessors
* MIPS Remote:: GDB and MIPS boards
* Simulator:: Simulated CPU target

File: gdb.info, Node: Remote Serial, Next: i960-Nindy Remote, 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. You probably need 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:
`sparc-stub.c'
For SPARC architectures.
`m68k-stub.c'
For Motorola 680x0 architectures.
`i386-stub.c'
For Intel 386 and compatible 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:: Outline of the communication protocol
* Server:: Using the `gdbserver' 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 will transfer 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 will get 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. To allow the stub to work, you must supply
these special low-level subroutines:
`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.
`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,
etc.) 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 themself
without help from `exceptionHandler'.
`void flush_i_cache()'
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 will vary 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 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 used for this 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.
Then 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 C-C), 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.

File: gdb.info, Node: Protocol, Next: Server, Prev: Debug Session, Up: Remote Serial
Outline of the communication protocol
-------------------------------------
The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file `remote.c'. Normally, you can simply allow these
subroutines to communicate, and ignore the details. (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files. `sparc-stub.c' is the best
organized, and therefore the easiest to read.)
However, there may be occasions when you need to know something about
the protocol--for example, if there is only one serial port to your
target machine, you might want your program to do something special if
it recognizes a packet meant for GDB.
All GDB commands and responses (other than acknowledgements, which
are single characters) are sent as a packet which includes a checksum.
A packet is introduced with the character `$', and ends with the
character `#' followed by a two-digit checksum:
$PACKET INFO#CHECKSUM
CHECKSUM is computed as the modulo 256 sum of the PACKET INFO
characters.
When either the host or the target machine receives a packet, the
first response expected is an acknowledgement: a single character,
either `+' (to indicate the package was received correctly) or `-' (to
request retransmission).
The host (GDB) sends commands, and the target (the debugging stub
incorporated in your program) sends data in response. The target also
sends data when your program stops.
Command packets are distinguished by their first character, which
identifies the kind of command.
These are the commands currently supported:
`g'
Requests the values of CPU registers.
`G'
Sets the values of CPU registers.
`mADDR,COUNT'
Read COUNT bytes at location ADDR.
`MADDR,COUNT:...'
Write COUNT bytes at location ADDR.
`c'
`cADDR'
Resume execution at the current address (or at ADDR if supplied).
`s'
`sADDR'
Step the target program for one instruction, from either the
current program counter or from ADDR if supplied.
`k'
Kill the target program.
`?'
Report the most recent signal. To allow you to take advantage of
the GDB signal handling commands, one of the functions of the
debugging stub is to report CPU traps as the corresponding POSIX
signal values.
If you have trouble with the serial connection, you can use the
command `set remotedebug'. This makes GDB report on all packets sent
back and forth across the serial line to the remote machine. The
packet-debugging information is printed on the GDB standard output
stream. `set remotedebug off' turns it off, and `show remotedebug'
will show you its current state.

File: gdb.info, Node: Server, Prev: Protocol, Up: Remote Serial
Using the `gdbserver' program
-----------------------------
`gdbserver' is a control program for Unix-like systems, which allows
you to connect your program with a remote GDB via `target remote'--but
without linking in the usual debugging stub.
`gdbserver' is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run `gdbserver' to
connect to a remote GDB could also run GDBN locally! `gdbserver' is
sometimes useful nevertheless, because it is a much smaller program
than GDB itself. It is also easier to port than all of GDBN, so you
may be able to get started more quickly on a new system by using
`gdbserver'.
GDB and `gdbserver' communicate via either a serial line or a TCP
connection, using the standard GDB remote serial protocol.
*On the target,*
you need to have a copy of the program you want to debug.
`gdbserver' does not need your program's symbol table, so you can
strip the program if necessary to save space. GDB on the host
system does all the symbol handling.
To use the server, you must tell it how to communicate with {No
Value For "GDB"}; the name of your program; and the arguments for
your program. The syntax is:
target> gdbserver COMM PROGRAM [ ARGS ... ]
COMM is either a device name (to use a serial line) or a TCP
hostname and portnumber. For example, to debug emacs with the
argument `foo.txt' and communicate with GDB over the serial port
`/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
`gdbserver' waits passively for the host GDB to communicate with
it.
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first
argument, specifying that you are communicating with the host GDB
via TCP. The `host:2345' argument means that `gdbserver' is to
expect a TCP connection from machine `host' to local TCP port 2345.
(Currently, the `host' part is ignored.) You can choose any number
you want for the port number as long as it does not conflict with
any TCP ports already in use on the target system.(1) You must use
the same port number with the host GDB `target remote' command.
*On the host,*
you need an unstripped copy of your program, since GDB needs
symbols and debugging information. Start up GDB as usual, using
the name of the local copy of your program as the first argument.
(You may also need the `--baud' option if the serial line is
running at anything other than 9600 bps.) After that, use `target
remote' to establish communications with `gdbserver'. Its
argument is either a device name (usually a serial device, like
`/dev/ttyb'), or a TCP port descriptof in the form `HOST:PORT'.
For example:
(gdb) target remote /dev/ttyb
communicates with the server via serial line `/dev/ttyb', and
(gdb) target remote the-target:2345
communicates via a TCP connection to port 2345 on host
`the-target'. For TCP connections, you must start up `gdbserver'
prior to using the `target remote' command. Otherwise you may get
an error whose text depends on the host system, but which usually
looks something like `Connection refused'.
---------- Footnotes ----------
(1) If you choose a port number that conflicts with another
service, `gdbserver' prints an error message and exits.

File: gdb.info, Node: i960-Nindy Remote, Next: UDI29K Remote, Prev: Remote Serial, Up: Remote
GDB with a remote i960 (Nindy)
------------------------------
"Nindy" is a ROM Monitor program for Intel 960 target systems. When
GDB is configured to control a remote Intel 960 using Nindy, you can
tell GDB how to connect to the 960 in several ways:
* Through command line options specifying serial port, version of the
Nindy protocol, and communications speed;
* By responding to a prompt on startup;
* By using the `target' command at any point during your GDB
session. *Note Commands for managing targets: Target Commands.
* Menu:
* Nindy Startup:: Startup with Nindy
* Nindy Options:: Options for Nindy
* Nindy Reset:: Nindy reset command

File: gdb.info, Node: Nindy Startup, Next: Nindy Options, Up: i960-Nindy Remote
Startup with Nindy
------------------
If you simply start `gdb' without using any command-line options,
you are prompted for what serial port to use, *before* you reach the
ordinary GDB prompt:
Attach /dev/ttyNN -- specify NN, or "quit" to quit:
Respond to the prompt with whatever suffix (after `/dev/tty')
identifies the serial port you want to use. You can, if you choose,
simply start up with no Nindy connection by responding to the prompt
with an empty line. If you do this and later wish to attach to Nindy,
use `target' (*note Commands for managing targets: Target Commands.).
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