Re: Why does gdb implement 'next' command with a series of "vCont;s"?

congli
2007-07-30T06:40:05Z

There is an error in my example. When I issue a 'next' command at
line 6, the step_range_end should be the first instruction of line 7,
0x8048372, not 804836f.

> 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.
>