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# 3. Using with GDB

If you want to use gdb as a source-level debugger with simulavr running as a remote target, start simulavr with the ‘`--gdbserver`’ or ‘`-g`’ option. This will put simulavr into gdbserver mode. simulavr will then act as a TCP server program on the localhost listening for a connection from gdb.

Once simulavr has accepted a connection from gdb, the two programs communicate via gdb's remote serial protocol (see GDB Remote Serial Protocol: (gdb)Top section `Protocol' in Debugging with GDB).

Here's how you would start up simulavr in gdbserver mode:

    $ simulavr -d at90s8515 -g

Here's a sample gdb session showing what to do on the gdb side to get gdb to talk to simulavr:

    This GDB was configured as "--host=i686-pc-linux-gnu --target=avr".
    (gdb) file demo_kr.elf
    Reading symbols from demo_kr.elf...done.
    (gdb) target remote localhost:1212
    Remote debugging using localhost:1212
    0x0 in .__start_of_init__ ()
    (gdb) load
    Loading section .text, size 0x76 lma 0x0
    Start address 0x0 , load size 118
    Transfer rate: 944 bits in <1 sec, 29 bytes/write.
    (gdb) break main
    Breakpoint 1 at 0x6e: file demo_kr.c, line 17.
    (gdb) continue
    Continuing.

    Breakpoint 1, main () at demo_kr.c:17
    17          sbi(DDRC, (
    (gdb) quit
    The program is running.  Exit anyway? (y or n) y

Notice that simulavr knew nothing about the program to debug when it was started. Gdb was told which file to debug with the ‘`file`’ command. After gdb has read in the program and connected to simulavr, the program's instructions are downloaded into the simulator via the ‘`load`’ command. The ‘`load`’ command is not necessary if simulavr already has the program loaded into it's flash memory area. It is ok to issue multiple ‘`load`’ commands.

Also, notice that no ‘`run`’ command was given to gdb. Gdb assumes that the simulator has started and is ready to continue. Giving gdb the ‘`run`’ command, will cause it to stop the current debug session and start a new one, which is not likely to be what you want to do.

When specifying the remote target to connect to, it is sufficient to write “target remote :1212” instead of “target remote localhost:1212”.

Hitting `CTRL-c` in gdb can be used to interrupt the simulator while it is processing instructions and return control back to gdb. This is most useful when gdb is waiting for a response from the simulator and the program running in the simulator is in an infinite loop.

Issuing a ‘`signal SIGxxx`’ command from gdb will send the signal to the simulator via a continue with signal packet. The simulator will process and interpret the signal, but will not pass it on to the AVR program running in the simulator since it really makes no sense to do so. In some circumstances, it may make sense to use the gdb signal mechanism as a way to initiate some sort of external stimulus to be passed on to the virtual hardware system of the simulator. Signals from gdb which are processed have the following meanings:

`SIGHUP`

  Initiate a reset of the simulator. (Simulates a hardware reset).

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