10.5. Examining memory
You can use the command x (for "examine") to examine memory in
any of several formats, independently of your program's data types.
- x/nfu addr, x addr, x
Use the x command to examine memory.
n, f, and u are all optional parameters that specify how
much memory to display and how to format it; addr is an
expression giving the address where you want to start displaying memory.
If you use defaults for nfu, you need not type the slash /.
Several commands set convenient defaults for addr.
- n, the repeat count
The repeat count is a decimal integer; the default is 1. It specifies
how much memory (counting by units u) to display.
- f, the display format
The display format is one of the formats used by print,
s (null-terminated string), or i (machine instruction).
The default is x (hexadecimal) initially.
The default changes each time you use either x or print.
- u, the unit size
The unit size is any of
- b
Bytes.
- h
Halfwords (two bytes).
- w
Words (four bytes). This is the initial default.
- g
Giant words (eight bytes).
Each time you specify a unit size with x, that size becomes the
default unit the next time you use x. (For the s and
i formats, the unit size is ignored and is normally not written.)
- addr, starting display address
addr is the address where you want gdb to begin displaying
memory. The expression need not have a pointer value (though it may);
it is always interpreted as an integer address of a byte of memory.
Refer to Section 10.1 Expressions, for more information on expressions. The default for
addr is usually just after the last address examined--but several
other commands also set the default address: info breakpoints (to
the address of the last breakpoint listed), info line (to the
starting address of a line), and print (if you use it to display
a value from memory).
For example, x/3uh 0x54320 is a request to display three halfwords
(h) of memory, formatted as unsigned decimal integers (u),
starting at address 0x54320. x/4xw $sp prints the four
words (w) of memory above the stack pointer (here, $sp;
(refer to Section 10.10 Registers) in hexadecimal (x).
Since the letters indicating unit sizes are all distinct from the
letters specifying output formats, you do not have to remember whether
unit size or format comes first; either order works. The output
specifications 4xw and 4wx mean exactly the same thing.
(However, the count n must come first; wx4 does not work.)
Even though the unit size u is ignored for the formats s
and i, you might still want to use a count n; for example,
3i specifies that you want to see three machine instructions,
including any operands. The command disassemble gives an
alternative way of inspecting machine instructions; refer to Section 9.5 Source and machine code.
All the defaults for the arguments to x are designed to make it
easy to continue scanning memory with minimal specifications each time
you use x. For example, after you have inspected three machine
instructions with x/3i addr, you can inspect the next seven
with just x/7. If you use [RET] to repeat the x command,
the repeat count n is used again; the other arguments default as
for successive uses of x.
The addresses and contents printed by the x command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, gdb makes these values available for
subsequent use in expressions as values of the convenience variables
$_ and $__. After an x command, the last address
examined is available for use in expressions in the convenience variable
$_. The contents of that address, as examined, are available in
the convenience variable $__.
If the x command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.
