This printf is same with the simple gdb command, it can do formatted
output. But it will happen in gdbserver part when it break by a
tracepoint.
Then the user can get the format value that he want in tracepint. It
will be more easy and clear to handle the bug sometimes.
About why I add printf to the tracepoint, I have 2 reasons:
1. The gdb and gdbserver connect through a low speed net. Sometimes,
the debug target that I use is in the other side of the earth.
The breakpoint commands "printf" is too slow for that issue, because
each time the inferior is break by the breakpoint, gdbserver need send
the rsp package to gdb, and gdb will get the data that "printf" need
though low speed net from gdbsever. And sometime, it will disconnect.
But if through tracepoint, I will not have this trouble. I can "set
disconnected-tracing on" to handle the network disconnect issue. I
still need to get the value from inferior through tfind and other
commands. It is still be affect by the low speed network. So I make
the tracepoint "printf" to handle it.
2. KGTP(https://code.google.com/p/kgtp/) just support the gdb
tracepoint rsp commands. For not stop the Linux the Kernel. It
doesn't support the breakpoint.
So if it want directly show the Kernel val value, it need "printf".
This printf will be very powerful that can set most part of Kernel and
we can set condition for it.
And in https://code.google.com/p/kgtp/wiki/HOWTO#Offline_debug, we
can dump the gdbrsp package to a file and send to Kernel. Then kernel
can be debug without a local gdb or a remote connect. But user still
need copy the trace file to pc that have GDB. But if support
tracepoint "printf", we will not need do that.
BTW, the kgtp have supported the agent expression "printf".
About the command part, I use the "printf" instead add a new commands
because the behavior of this command is same with printf. They will
use the same function string_printf(update from ui_printf) to parse
the command arg.
To support the printf command, I add a new agent expression 0x31
printf, the format for it is:
0x31(op_printf) + arg(1/0) + format string with end by 0x0.
The arg is the number of argument of printf. It will only be 1 (one
argument) or 0 (no argument). I make it cannot have more than one
argument because I cannot found a good way to handle va_list that send
arguments to vprintf.
The format string with end by 0x0 is the simple format string. It end
by 0x0 then the gdbserver can use it directly.
Example:
(gdb) trace 16
During symbol reading, DW_AT_name missing from DW_TAG_base_type.
Tracepoint 1 at 0x4004c3: file 1.c, line 16.
(gdb) tvariable $c
Trace state variable $c created, with initial value 0.
(gdb) actions
Enter actions for tracepoint 1, one per line.
End with a line saying just "end".
printf "%d 0x%lx %d\n",$c=$c+1,$rax,argc
end
(gdb) target remote localhost:1234
(gdb) tstart
(gdb) c
gdbserver/gdbserver localhost:1234 ./a.out
Process ./a.out created; pid = 25804
Listening on port 1234
Remote debugging from host 127.0.0.1
1 0x7f2cb8711ec8 1
2 0x7f2cb8711ec8 2
3 0x7f2cb8711ec8 3
4 0x7f2cb8711ec8 4
5 0x7f2cb8711ec8 5
6 0x7f2cb8711ec8 6
7 0x7f2cb8711ec8 7
Please help me review the patches.
Thanks,
Hui