15. 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 (see section Commands for managing targets).

15.1 Active targets
15.2 Commands for managing targets
15.3 Choosing target byte order
15.4 Remote debugging
15.5 Kernel Object Display

15.1 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 (see section Commands to specify files). To specify as a target a process that is already running, use the attach command (see section Debugging an already-running process).

15.2 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 (see section Commands to specify 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.

See section Commands to specify 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'). See section Remote debugging. 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 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.

15.3 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.

15.4 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.

15.5 Kernel Object Display

Some targets support kernel object display. Using this facility, GDB communicates specially with the underlying operating system and can display information about operating system-level objects such as mutexes and other synchronization objects. Exactly which objects can be displayed is determined on a per-OS basis.

Use the set os command to set the operating system. This tells GDB which kernel object display module to initialize:

(gdb) set os cisco

If set os succeeds, GDB will display some information about the operating system, and will create a new info command which can be used to query the target. The info command is named after the operating system:

(gdb) info cisco List of Cisco Kernel Objects Object Description any Any and all objects

Further subcommands can be used to query about particular objects known by the kernel.

There is currently no way to determine whether a given operating system is supported other than to try it.