Re: [Pgubook-readers] toupper and command line args

[Emmet Ford]

> Well, when a program begins, aren't the stack pointer and the base pointer
> the same?

Yes, ebp gets set to the value in esp at the top of the program, setting up the 
initial stack frame. So, when I break at the second instruction and do an 'info 
registers', it shows esp and ebp holding the same value:


esp            0xbffffa70       0xbffffa70
ebp            0xbffffa70       0xbffffa70

> argc is at %esp (same as %ebp at this point), argv[0] is at 4(%esp), 
> arg[1] at 8(%esp) and so on...


You are correct. Examining the address stored in ebp (or esp, at this 
point) shows the address count. ebp + 4 contains the starting address of 
the arg0 string (the program name), ebp + 8 contains the starting 
address of the arg1 string, etc:

(gdb) x/1xw 0xbffffa70
0xbffffa70:     0x00000003
(gdb)
0xbffffa74:     0xbffffba4
(gdb)
0xbffffa78:     0xbffffbbb
(gdb)
0xbffffa7c:     0xbffffbc7

Examining the addresses of the argv strings reveals them to be where 
they should be, followed by the environmental variables:

(gdb) x/1sb 0xbffffba4
0xbffffba4:      "/home/eford/as/toupper"
(gdb)
0xbffffbbb:      "./toupper.s"
(gdb)
0xbffffbc7:      "./toupper.out"
(gdb)
0xbffffbd5:      "CPLUS_INCLUDE_PATH=/usr/lib/qt-3.2.1/include"

> But I keep thinking I'm missing something, because page 91 makes it clear
> that the first arg should be at 16(%esp), not 8(%esp).

Yes, it is curious. I suspect that you have happened upon a documentational 
anomaly. Time to hit the bug tracking database!

--Emmet Ford