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Why does gdb implement 'next' command with a series of "vCont;s"?
- From: congli <congli dot cn at gmail dot com>
- To: gdb at sourceware dot org
- Date: Mon, 30 Jul 2007 13:37:33 +0800
- Subject: Why does gdb implement 'next' command with a series of "vCont;s"?
I have a simple test program (x86 platform), debug it from
a remote machine. First, let the program stop at line 6,
then issue a 'next' command. I have set the 'debug remote'
option, and the command line output is:
6 j = i + 1;
(gdb) n
Sending packet: $m45e2ca,1#8e...Ack
Packet received: 55
Sending packet: $M45e2ca,1:cc#6e...Ack
Packet received: OK
Sending packet: $m8048364,1#3b...Ack
Packet received: c7
Sending packet: $M8048364,1:cc#1b...Ack
Packet received: OK
Sending packet: $m464005,1#fd...Ack
Packet received: 55
Sending packet: $M464005,1:cc#dd...Ack
Packet received: OK
Sending packet: $m496014,1#02...Ack
Packet received: 55
Sending packet: $M496014,1:cc#e2...Ack
Packet received: OK
Sending packet: $vCont;s#b8...Ack
Packet received: T0505:887a80bf;04:607a80bf;08:6e830408;
Sending packet: $vCont;s#b8...Ack
Packet received: T0505:887a80bf;04:607a80bf;08:6f830408;
Sending packet: $vCont;s#b8...Ack
Packet received: T0505:887a80bf;04:607a80bf;08:72830408;
...
this is the objdump of line 6 and line 7 of my program:
j = i + 1;
804836b: 8b 45 f4 mov 0xfffffff4(%ebp),%eax
804836e: 40 inc %eax
804836f: 89 45 f8 mov %eax,0xfffffff8(%ebp)
k = j + 1;
8048372: 8b 45 f8 mov 0xfffffff8(%ebp),%eax
8048375: 40 inc %eax
8048376: 89 45 fc mov %eax,0xfffffffc(%ebp)
As you can see, line 6 of my program consists of three assembly
instructions. gdb implements the 'next' command by three "vCont;s",
and stop at the first instruction of line 7, which address is
0x8048372.
My question is, when I issue the 'next' command, gdb already
know the 'step_range_end' is 0x804836f, why not implement the
'next' command by set a breakpoint at 0x804836f and then issue
"vCont;c"? When the program meet the breakpoint at 0x804836f,
we can do a single "vCont;s" to the first instruction of
line 7, 0x8048372.
This implementation of 'next' command will decrease network
traffic and make the command more efficient, especially when
there is a lot of assembly instructions within one C souce line.
But why does gdb take the "vCont;s" method?
Thanks.