GDB/MI Data Manipulation (Debugging with GDB)

27.16 GDB/MI Data Manipulation

This section describes the GDB/MI commands that manipulate data: examine memory and registers, evaluate expressions, etc.

For details about what an addressable memory unit is, see addressable memory unit.

The `-data-disassemble` Command

Synopsis

 -data-disassemble
    [ -s start-addr -e end-addr ]
  | [ -a addr ]
  | [ -f filename -l linenum [ -n lines ] ]
  -- mode

Where:

‘start-addr’

is the beginning address (or $pc)

‘end-addr’

is the end address

‘addr’

is an address anywhere within (or the name of) the function to disassemble. If an address is specified, the whole function surrounding that address will be disassembled. If a name is specified, the whole function with that name will be disassembled.

‘filename’

is the name of the file to disassemble

‘linenum’

is the line number to disassemble around

‘lines’

is the number of disassembly lines to be produced. If it is -1, the whole function will be disassembled, in case no end-addr is specified. If end-addr is specified as a non-zero value, and lines is lower than the number of disassembly lines between start-addr and end-addr, only lines lines are displayed; if lines is higher than the number of lines between start-addr and end-addr, only the lines up to end-addr are displayed.

‘mode’

is one of:

0 disassembly only
1 mixed source and disassembly (deprecated)
2 disassembly with raw opcodes
3 mixed source and disassembly with raw opcodes (deprecated)
4 mixed source and disassembly
5 mixed source and disassembly with raw opcodes

Modes 1 and 3 are deprecated. The output is “source centric” which hasn’t proved useful in practice. See Machine Code, for a discussion of the difference between /m and /s output of the disassemble command.

Result

The result of the -data-disassemble command will be a list named ‘asm_insns’, the contents of this list depend on the mode used with the -data-disassemble command.

For modes 0 and 2 the ‘asm_insns’ list contains tuples with the following fields:

address: The address at which this instruction was disassembled.
func-name: The name of the function this instruction is within.
offset: The decimal offset in bytes from the start of ‘func-name’.
inst: The text disassembly for this ‘address’.
opcodes: This field is only present for modes 2, 3 and 5. This contains the raw opcode bytes for the ‘inst’ field.

For modes 1, 3, 4 and 5 the ‘asm_insns’ list contains tuples named ‘src_and_asm_line’, each of which has the following fields:

line

The line number within ‘file’.

file

The file name from the compilation unit. This might be an absolute file name or a relative file name depending on the compile command used.

fullname

Absolute file name of ‘file’. It is converted to a canonical form using the source file search path (see Specifying Source Directories) and after resolving all the symbolic links.

If the source file is not found this field will contain the path as present in the debug information.

line_asm_insn

This is a list of tuples containing the disassembly for ‘line’ in ‘file’. The fields of each tuple are the same as for -data-disassemble in mode 0 and 2, so ‘address’, ‘func-name’, ‘offset’, ‘inst’, and optionally ‘opcodes’.

Note that whatever included in the ‘inst’ field, is not manipulated directly by GDB/MI, i.e., it is not possible to adjust its format.

GDB Command

The corresponding GDB command is ‘disassemble’.

Example

Disassemble from the current value of $pc to $pc + 20:

(gdb)
-data-disassemble -s $pc -e "$pc + 20" -- 0
^done,
asm_insns=[
{address="0x000107c0",func-name="main",offset="4",
inst="mov  2, %o0"},
{address="0x000107c4",func-name="main",offset="8",
inst="sethi  %hi(0x11800), %o2"},
{address="0x000107c8",func-name="main",offset="12",
inst="or  %o2, 0x140, %o1\t! 0x11940 <_lib_version+8>"},
{address="0x000107cc",func-name="main",offset="16",
inst="sethi  %hi(0x11800), %o2"},
{address="0x000107d0",func-name="main",offset="20",
inst="or  %o2, 0x168, %o4\t! 0x11968 <_lib_version+48>"}]
(gdb)

Disassemble the whole main function. Line 32 is part of main.

-data-disassemble -f basics.c -l 32 -- 0
^done,asm_insns=[
{address="0x000107bc",func-name="main",offset="0",
inst="save  %sp, -112, %sp"},
{address="0x000107c0",func-name="main",offset="4",
inst="mov   2, %o0"},
{address="0x000107c4",func-name="main",offset="8",
inst="sethi %hi(0x11800), %o2"},
[…]
{address="0x0001081c",func-name="main",offset="96",inst="ret "},
{address="0x00010820",func-name="main",offset="100",inst="restore "}]
(gdb)

Disassemble 3 instructions from the start of main:

(gdb)
-data-disassemble -f basics.c -l 32 -n 3 -- 0
^done,asm_insns=[
{address="0x000107bc",func-name="main",offset="0",
inst="save  %sp, -112, %sp"},
{address="0x000107c0",func-name="main",offset="4",
inst="mov  2, %o0"},
{address="0x000107c4",func-name="main",offset="8",
inst="sethi  %hi(0x11800), %o2"}]
(gdb)

Disassemble 3 instructions from the start of main in mixed mode:

(gdb)
-data-disassemble -f basics.c -l 32 -n 3 -- 1
^done,asm_insns=[
src_and_asm_line={line="31",
file="../../../src/gdb/testsuite/gdb.mi/basics.c",
fullname="/absolute/path/to/src/gdb/testsuite/gdb.mi/basics.c",
line_asm_insn=[{address="0x000107bc",
func-name="main",offset="0",inst="save  %sp, -112, %sp"}]},
src_and_asm_line={line="32",
file="../../../src/gdb/testsuite/gdb.mi/basics.c",
fullname="/absolute/path/to/src/gdb/testsuite/gdb.mi/basics.c",
line_asm_insn=[{address="0x000107c0",
func-name="main",offset="4",inst="mov  2, %o0"},
{address="0x000107c4",func-name="main",offset="8",
inst="sethi  %hi(0x11800), %o2"}]}]
(gdb)

The `-data-evaluate-expression` Command

Synopsis

 -data-evaluate-expression expr

Evaluate expr as an expression. The expression could contain an inferior function call. The function call will execute synchronously. If the expression contains spaces, it must be enclosed in double quotes.

GDB Command

The corresponding GDB commands are ‘print’, ‘output’, and ‘call’. In gdbtk only, there’s a corresponding ‘gdb_eval’ command.

Example

In the following example, the numbers that precede the commands are the tokens described in GDB/MI Command Syntax. Notice how GDB/MI returns the same tokens in its output.

211-data-evaluate-expression A
211^done,value="1"
(gdb)
311-data-evaluate-expression &A
311^done,value="0xefffeb7c"
(gdb)
411-data-evaluate-expression A+3
411^done,value="4"
(gdb)
511-data-evaluate-expression "A + 3"
511^done,value="4"
(gdb)

The `-data-list-changed-registers` Command

Synopsis

 -data-list-changed-registers

Display a list of the registers that have changed.

GDB Command

GDB doesn’t have a direct analog for this command; gdbtk has the corresponding command ‘gdb_changed_register_list’.

Example

On a PPC MBX board:

(gdb)
-exec-continue
^running

(gdb)
*stopped,reason="breakpoint-hit",disp="keep",bkptno="1",frame={
func="main",args=[],file="try.c",fullname="/home/foo/bar/try.c",
line="5",arch="powerpc"}
(gdb)
-data-list-changed-registers
^done,changed-registers=["0","1","2","4","5","6","7","8","9",
"10","11","13","14","15","16","17","18","19","20","21","22","23",
"24","25","26","27","28","30","31","64","65","66","67","69"]
(gdb)

The `-data-list-register-names` Command

Synopsis

 -data-list-register-names [ ( regno )+ ]

Show a list of register names for the current target. If no arguments are given, it shows a list of the names of all the registers. If integer numbers are given as arguments, it will print a list of the names of the registers corresponding to the arguments. To ensure consistency between a register name and its number, the output list may include empty register names.

GDB Command

GDB does not have a command which corresponds to ‘-data-list-register-names’. In gdbtk there is a corresponding command ‘gdb_regnames’.

Example

For the PPC MBX board:

(gdb)
-data-list-register-names
^done,register-names=["r0","r1","r2","r3","r4","r5","r6","r7",
"r8","r9","r10","r11","r12","r13","r14","r15","r16","r17","r18",
"r19","r20","r21","r22","r23","r24","r25","r26","r27","r28","r29",
"r30","r31","f0","f1","f2","f3","f4","f5","f6","f7","f8","f9",
"f10","f11","f12","f13","f14","f15","f16","f17","f18","f19","f20",
"f21","f22","f23","f24","f25","f26","f27","f28","f29","f30","f31",
"", "pc","ps","cr","lr","ctr","xer"]
(gdb)
-data-list-register-names 1 2 3
^done,register-names=["r1","r2","r3"]
(gdb)

The `-data-list-register-values` Command

Synopsis

 -data-list-register-values
    [ --skip-unavailable ] fmt [ ( regno )*]

Display the registers’ contents. The format according to which the registers’ contents are to be returned is given by fmt, followed by an optional list of numbers specifying the registers to display. A missing list of numbers indicates that the contents of all the registers must be returned. The --skip-unavailable option indicates that only the available registers are to be returned.

Allowed formats for fmt are:

x: Hexadecimal
o: Octal
t: Binary
d: Decimal
r: Raw
N: Natural

GDB Command

The corresponding GDB commands are ‘info reg’, ‘info all-reg’, and (in gdbtk) ‘gdb_fetch_registers’.

Example

For a PPC MBX board (note: line breaks are for readability only, they don’t appear in the actual output):

(gdb)
-data-list-register-values r 64 65
^done,register-values=[{number="64",value="0xfe00a300"},
{number="65",value="0x00029002"}]
(gdb)
-data-list-register-values x
^done,register-values=[{number="0",value="0xfe0043c8"},
{number="1",value="0x3fff88"},{number="2",value="0xfffffffe"},
{number="3",value="0x0"},{number="4",value="0xa"},
{number="5",value="0x3fff68"},{number="6",value="0x3fff58"},
{number="7",value="0xfe011e98"},{number="8",value="0x2"},
{number="9",value="0xfa202820"},{number="10",value="0xfa202808"},
{number="11",value="0x1"},{number="12",value="0x0"},
{number="13",value="0x4544"},{number="14",value="0xffdfffff"},
{number="15",value="0xffffffff"},{number="16",value="0xfffffeff"},
{number="17",value="0xefffffed"},{number="18",value="0xfffffffe"},
{number="19",value="0xffffffff"},{number="20",value="0xffffffff"},
{number="21",value="0xffffffff"},{number="22",value="0xfffffff7"},
{number="23",value="0xffffffff"},{number="24",value="0xffffffff"},
{number="25",value="0xffffffff"},{number="26",value="0xfffffffb"},
{number="27",value="0xffffffff"},{number="28",value="0xf7bfffff"},
{number="29",value="0x0"},{number="30",value="0xfe010000"},
{number="31",value="0x0"},{number="32",value="0x0"},
{number="33",value="0x0"},{number="34",value="0x0"},
{number="35",value="0x0"},{number="36",value="0x0"},
{number="37",value="0x0"},{number="38",value="0x0"},
{number="39",value="0x0"},{number="40",value="0x0"},
{number="41",value="0x0"},{number="42",value="0x0"},
{number="43",value="0x0"},{number="44",value="0x0"},
{number="45",value="0x0"},{number="46",value="0x0"},
{number="47",value="0x0"},{number="48",value="0x0"},
{number="49",value="0x0"},{number="50",value="0x0"},
{number="51",value="0x0"},{number="52",value="0x0"},
{number="53",value="0x0"},{number="54",value="0x0"},
{number="55",value="0x0"},{number="56",value="0x0"},
{number="57",value="0x0"},{number="58",value="0x0"},
{number="59",value="0x0"},{number="60",value="0x0"},
{number="61",value="0x0"},{number="62",value="0x0"},
{number="63",value="0x0"},{number="64",value="0xfe00a300"},
{number="65",value="0x29002"},{number="66",value="0x202f04b5"},
{number="67",value="0xfe0043b0"},{number="68",value="0xfe00b3e4"},
{number="69",value="0x20002b03"}]
(gdb)

The `-data-read-memory` Command

This command is deprecated, use -data-read-memory-bytes instead.

Synopsis

 -data-read-memory [ -o byte-offset ]
   address word-format word-size
   nr-rows nr-cols [ aschar ]

where:

‘address’: An expression specifying the address of the first memory word to be read. Complex expressions containing embedded white space should be quoted using the C convention.
‘word-format’: The format to be used to print the memory words. The notation is the same as for GDB’s print command (see Output Formats).
‘word-size’: The size of each memory word in bytes.
‘nr-rows’: The number of rows in the output table.
‘nr-cols’: The number of columns in the output table.
‘aschar’: If present, indicates that each row should include an ASCII dump. The value of aschar is used as a padding character when a byte is not a member of the printable ASCII character set (printable ASCII characters are those whose code is between 32 and 126, inclusively).
‘byte-offset’: An offset to add to the address before fetching memory.

This command displays memory contents as a table of nr-rows by nr-cols words, each word being word-size bytes. In total, nr-rows * nr-cols * word-size bytes are read (returned as ‘total-bytes’). Should less than the requested number of bytes be returned by the target, the missing words are identified using ‘N/A’. The number of bytes read from the target is returned in ‘nr-bytes’ and the starting address used to read memory in ‘addr’.

The address of the next/previous row or page is available in ‘next-row’ and ‘prev-row’, ‘next-page’ and ‘prev-page’.

GDB Command

The corresponding GDB command is ‘x’. gdbtk has ‘gdb_get_mem’ memory read command.

Example

Read six bytes of memory starting at bytes+6 but then offset by -6 bytes. Format as three rows of two columns. One byte per word. Display each word in hex.

(gdb)
9-data-read-memory -o -6 -- bytes+6 x 1 3 2
9^done,addr="0x00001390",nr-bytes="6",total-bytes="6",
next-row="0x00001396",prev-row="0x0000138e",next-page="0x00001396",
prev-page="0x0000138a",memory=[
{addr="0x00001390",data=["0x00","0x01"]},
{addr="0x00001392",data=["0x02","0x03"]},
{addr="0x00001394",data=["0x04","0x05"]}]
(gdb)

Read two bytes of memory starting at address shorts + 64 and display as a single word formatted in decimal.

(gdb)
5-data-read-memory shorts+64 d 2 1 1
5^done,addr="0x00001510",nr-bytes="2",total-bytes="2",
next-row="0x00001512",prev-row="0x0000150e",
next-page="0x00001512",prev-page="0x0000150e",memory=[
{addr="0x00001510",data=["128"]}]
(gdb)

Read thirty two bytes of memory starting at bytes+16 and format as eight rows of four columns. Include a string encoding with ‘x’ used as the non-printable character.

(gdb)
4-data-read-memory bytes+16 x 1 8 4 x
4^done,addr="0x000013a0",nr-bytes="32",total-bytes="32",
next-row="0x000013c0",prev-row="0x0000139c",
next-page="0x000013c0",prev-page="0x00001380",memory=[
{addr="0x000013a0",data=["0x10","0x11","0x12","0x13"],ascii="xxxx"},
{addr="0x000013a4",data=["0x14","0x15","0x16","0x17"],ascii="xxxx"},
{addr="0x000013a8",data=["0x18","0x19","0x1a","0x1b"],ascii="xxxx"},
{addr="0x000013ac",data=["0x1c","0x1d","0x1e","0x1f"],ascii="xxxx"},
{addr="0x000013b0",data=["0x20","0x21","0x22","0x23"],ascii=" !\"#"},
{addr="0x000013b4",data=["0x24","0x25","0x26","0x27"],ascii="$%&'"},
{addr="0x000013b8",data=["0x28","0x29","0x2a","0x2b"],ascii="()*+"},
{addr="0x000013bc",data=["0x2c","0x2d","0x2e","0x2f"],ascii=",-./"}]
(gdb)

The `-data-read-memory-bytes` Command

Synopsis

 -data-read-memory-bytes [ -o offset ]
   address count

where:

‘address’: An expression specifying the address of the first addressable memory unit to be read. Complex expressions containing embedded white space should be quoted using the C convention.
‘count’: The number of addressable memory units to read. This should be an integer literal.
‘offset’: The offset relative to address at which to start reading. This should be an integer literal. This option is provided so that a frontend is not required to first evaluate address and then perform address arithmetics itself.

This command attempts to read all accessible memory regions in the specified range. First, all regions marked as unreadable in the memory map (if one is defined) will be skipped. See Memory Region Attributes. Second, GDB will attempt to read the remaining regions. For each one, if reading full region results in an errors, GDB will try to read a subset of the region.

In general, every single memory unit in the region may be readable or not, and the only way to read every readable unit is to try a read at every address, which is not practical. Therefore, GDB will attempt to read all accessible memory units at either beginning or the end of the region, using a binary division scheme. This heuristic works well for reading across a memory map boundary. Note that if a region has a readable range that is neither at the beginning or the end, GDB will not read it.

The result record (see GDB/MI Result Records) that is output of the command includes a field named ‘memory’ whose content is a list of tuples. Each tuple represent a successfully read memory block and has the following fields:

begin: The start address of the memory block, as hexadecimal literal.
end: The end address of the memory block, as hexadecimal literal.
offset: The offset of the memory block, as hexadecimal literal, relative to the start address passed to -data-read-memory-bytes.
contents: The contents of the memory block, in hex.

GDB Command

The corresponding GDB command is ‘x’.

Example

(gdb)
-data-read-memory-bytes &a 10
^done,memory=[{begin="0xbffff154",offset="0x00000000",
              end="0xbffff15e",
              contents="01000000020000000300"}]
(gdb)

The `-data-write-memory-bytes` Command

Synopsis

 -data-write-memory-bytes address contents
 -data-write-memory-bytes address contents [count]

where:

‘address’: An expression specifying the address of the first addressable memory unit to be written. Complex expressions containing embedded white space should be quoted using the C convention.
‘contents’: The hex-encoded data to write. It is an error if contents does not represent an integral number of addressable memory units.
‘count’: Optional argument indicating the number of addressable memory units to be written. If count is greater than contents’ length, GDB will repeatedly write contents until it fills count memory units.

GDB Command

There’s no corresponding GDB command.

Example

(gdb)
-data-write-memory-bytes &a "aabbccdd"
^done
(gdb)

(gdb)
-data-write-memory-bytes &a "aabbccdd" 16e
^done
(gdb)

27.16 GDB/MI Data Manipulation

The -data-disassemble Command

Synopsis

Result

GDB Command

Example

The -data-evaluate-expression Command

Synopsis

GDB Command

Example

The -data-list-changed-registers Command

Synopsis

GDB Command

Example

The -data-list-register-names Command

Synopsis

GDB Command

Example

The -data-list-register-values Command

Synopsis

GDB Command

Example

The -data-read-memory Command

Synopsis

GDB Command

Example

The -data-read-memory-bytes Command

Synopsis

GDB Command

Example

The -data-write-memory-bytes Command

Synopsis

GDB Command

Example

The `-data-disassemble` Command

The `-data-evaluate-expression` Command

The `-data-list-changed-registers` Command

The `-data-list-register-names` Command

The `-data-list-register-values` Command

The `-data-read-memory` Command

The `-data-read-memory-bytes` Command

The `-data-write-memory-bytes` Command