Debugging with GDB: Memory

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### 10.6 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` (‘`x`’, ‘`d`’, ‘`u`’, ‘`o`’, ‘`t`’, ‘`a`’, ‘`c`’, ‘`f`’, ‘`s`’), and in addition ‘`i`’ (for machine instructions). 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 ‘`i`’ format, the unit size is ignored and is normally not written. For the ‘`s`’ format, the unit size defaults to ‘`b`’, unless it is explicitly given. Use `x /hs` to display 16-bit char strings and `x /ws` to display 32-bit strings. The next use of `x /s` will again display 8-bit strings. Note that the results depend on the programming language of the current compilation unit. If the language is C, the ‘`s`’ modifier will use the UTF-16 encoding while ‘`w`’ will use UTF-32. The encoding is set by the programming language and cannot be altered.

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. See 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`’; see 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. For convenience, especially when used with the `display` command, the ‘`i`’ format also prints branch delay slot instructions, if any, beyond the count specified, which immediately follow the last instruction that is within the count. The command `disassemble` gives an alternative way of inspecting machine instructions; see 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`.

When examining machine instructions, the instruction at current program counter is shown with a `=>` marker. For example:

    (gdb) x/5i $pc-6
       0x804837f <main+11>: mov    %esp,%ebp
       0x8048381 <main+13>: push   %ecx
       0x8048382 <main+14>: sub    $0x4,%esp
    => 0x8048385 <main+17>: movl   $0x8048460,(%esp)
       0x804838c <main+24>: call   0x80482d4 <puts@plt>

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.

Most targets have an addressable memory unit size of 8 bits. This means that to each memory address are associated 8 bits of data. Some targets, however, have other addressable memory unit sizes. Within GDB and this document, the term addressable memory unit (or memory unit for short) is used when explicitly referring to a chunk of data of that size. The word byte is used to refer to a chunk of data of 8 bits, regardless of the addressable memory unit size of the target. For most systems, addressable memory unit is a synonym of byte.

When you are debugging a program running on a remote target machine (see Remote Debugging), you may wish to verify the program’s image in the remote machine’s memory against the executable file you downloaded to the target. Or, on any target, you may want to check whether the program has corrupted its own read-only sections. The `compare-sections` command is provided for such situations.

`compare-sections [section-name|`-r`]`

  Compare the data of a loadable section section-name in the executable file of the program being debugged with the same section in the target machine’s memory, and report any mismatches. With no arguments, compares all loadable sections. With an argument of `-r`, compares all loadable read-only sections.

  Note: for remote targets, this command can be accelerated if the target supports computing the CRC checksum of a block of memory (see qCRC packet).

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