Debugging a Program With dbx

Examining the Contents of Memory

Using addresses and the examine or x command, you can examine the content of memory locations as well as print the assembly language instruction at each address. Using a command derived from adb(1), the assembly language debugger, you can query for:

The address, using the = (equal sign) character; or,
The contents stored at an address, using the / (slash) character.

You can print the assembly commands using the dis and listi commands.

Using the `examine` or `x` Command

Use the examine command, or its alias x, to display memory contents or addresses.

Use the following syntax to display the contents of memory starting at addr for count items in format fmt. The default addr is the next one after the last address previously displayed. The default count is 1. The default fmt is the same as was used in the previous examine command, or X if this is the first command given.

The syntax for the examine command is:

examine [addr]
[/ [count] [fmt
]]

To display the contents of memory from addr1 through addr2 inclusive, in format fmt:

examine addr1, addr2
 [/ [fmt]]

Display the address, instead of the contents of the address in the given format:

examine addr =
[fmt]

To print the value stored at the next address after the one last displayed by examine:

examine +/ i

To print the value of an expression, enter the expression as an address:

examine addr=format
examine addr=

Addresses

The addr is any expression resulting in or usable as an address. The addr may be replaced with a + (plus sign) which displays the contents of the next address in the default format.

Example addresses are:

`0xff99`	An absolute address
`main`	Address of a function
`main+20`	Offset from a function address
`&errno`	Address of a variable
`str`	A pointer-value variable pointing to a string

Symbolic addresses used to display memory are specified by preceding a name with an ampersand (&). Function names can be used without the ampersand; &main is equal to main. Registers are denoted by preceding a name with a dollar sign ($).

Formats

The fmt is the address display format in which dbx displays the results of a query. The output produced depends on the current displayfmt. To change the display format, supply a different fmt code.

Set the fmt specifier to tell dbx how to display information associated with the addresses specified.

The default format set at the start of each dbx session is X, which displays an address/value as a 32-bit word in hexadecimal. The following memory display formats are legal.

i	Display as an assembly instruction
d	Display as 16 bits (2 bytes) in decimal
D	Display as 32 bits (4 bytes) in decimal
o	Display as 16 bits (2 bytes) in octal.
O	Display as 32 bits (4 bytes) in octal.
x	Display as 16 bits (2 bytes) in hexadecimal.
X	Display as 32 bits (4 bytes) in hexadecimal. (default format)
b	Display as a byte in octal
c	Display as a character
w	Display as a wide character.
s	Display as a string of characters terminated by a null byte.
W	Display as a wide character.
f	Display as a single-precision floating point number.
F, g	Display as a double-precision floating point number.
E	Display as an extended-precision floating point number.
ld, lD	Display 32 bits (4 bytes) in decimal (same as D)
lo, lO	Display 32 bits (4 bytes) in octal (same as O)
lx, LX	Display 32 bits (4 bytes) in hexadecimal (same as X)
Ld, LD	Display 64 bits (8 bytes) in decimal
Lo, LO	Display 64 bits (8 bytes) in octal
Lx, LX	Display 64 bits (8 bytes) in hexadecimal

Count

The count is a repetition count in decimal. The increment size depends on the memory display format.

Examples

The following examples show how to use an address with count and fmt options to display five successive disassembled instructions starting from the current stopping point.

For SPARC:

(dbx) stepi
stopped in main at 0x108bc
0x000108bc: main+0x000c: st    %l0, [%fp - 0x14]
(dbx) x 0x108bc/5i
0x000108bc: main+0x000c: st    %l0, [%fp - 0x14]
0x000108c0: main+0x0010: mov   0x1,%l0
0x000108c4: main+0x0014: or    %l0,%g0, %o0
0x000108c8: main+0x0018: call  0x00020b90 [unresolved PLT 8: malloc]
0x000108cc: main+0x001c: nop

For Intel:

(dbx) x &main/5i
0x08048988: main       :  pushl  %ebp
0x08048989: main+0x0001:  movl   %esp,%ebp
0x0804898b: main+0x0003:  subl   $0x28,%esp
0x0804898e: main+0x0006:  movl   0x8048ac0,%eax
0x08048993: main+0x000b:  movl   %eax,-8(%ebp)

Using the `dis` Command

The dis command is equivalent to the examine command with i as the default display format.

Here is the syntax for the dis command:

dis [addr][/count
]

The dis command without arguments displays 10 instructions starting at the address +. With only a count, the dis command displays count instructions starting at the address +.

Using the `listi` Command

To display source lines along with their corresponding assembly instructions, use listi, which is equivalent to list --i. See the discussion of list --i in Chapter 3, Viewing and Visiting Code".