FAQ: GDB commands for newbie.

Q.

Can you provide some quick GDB commands for a newbie?

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A.

This is a quick FAQ written in a typical debug sequence and is only meant for beginners. I just extracted the contents of the uploaded file in Doc Site:

http://docs.blackfin.uclinux.org/lib/exe/fetch.php?media=presentations:toolchain:gdb_for_dummies.pdf

Fact Sheet

What is GDB? http://shop.fsf.org/product/debugging-gdb-gnu-source-level-debugger/

“GDB, the GNU Project debugger, allows you to see what is going on `inside' another program while it executes -- or what another program was doing at the moment it crashed. GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

  • Start your program, specifying anything that might affect its behavior.
  • Make your program stop on specified conditions.
  • Examine what has happened, when your program has stopped.
  • Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another. ”

What is GDB Server? http://en.wikipedia.org/wiki/Gdbserver

“gdb-server is a computer program that makes it possible to remotely debug other programs.Running on the same system as the program to be debugged, it allows the GNU Debugger to connect from another system; that is, only the executable to be debugged needs to be resident on the target system, while the source code and a copy of the binary file to be debugged reside on the developer’s local computer. The connection can be either TCP or a serial line”

What is GDB Proxy? https://docs.blackfin.uclinux.org/doku.php?id=toolchain:debug:gdbproxy

When debugging under the gdb environment, the debugger needs to know what target to connect to. This target is a tiny server daemon, typically running on the host computer. It is equivalent to a gdbserver, although it does not run on the target hardware. This server is called gdbproxy and is derived from rproxy-0.7.

What is Eclipse? http://en.wikipedia.org/wiki/Eclipse_(software)

Eclipse is a multi-language software development environment comprising an integrated development environment (IDE) and an extensible plug-in system. It is written mostly in Java and can be used to develop applications in Java and, by means of various plug-ins, other programming languages including Ada, C, C++, COBOL, Perl, PHP, Python, R. Ruby (including Ruby on Rails framework), Scala, Clojure, and Scheme.

What is GDB/mi Interface? http://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI.html#GDB_002fMI

GDB/MI is a line based machine oriented text interface to GDB and is activated by specifying using the --interpreter command line option (see Mode Options). It is specifically intended to support the development of systems which use the debugger as just one small component as a larger system {Eclipse}.

Lets learn from a simple no-hardware code as below. All following gdb commands are sequentially tested…

Sample code for debugging

int global_data = 1111;

void func1();

void func2();

int main(void);

int main(void)

{

      func1();

      return 0;

}

void func1()

{

      int local_data = 1234;

      global_data = 5678;

      func2();

}

void func2()

{

}

Start the GDB first:

C:\Program Files\Analog Devices\GNU Toolchain\2010R1\elf\bin>bfin-elf-gdb.exe gdb_debugging

GNU gdb 6.6

Copyright (C) 2006 Free Software Foundation, Inc.

GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions.

Type "show copying" to see the conditions.

There is absolutely no warranty for GDB.  Type "show warranty" for details.

This GDB was configured as "--host=i386-mingw32msvc --target=bfin-elf"...

(gdb) target remote localhost:2000

Remote debugging using localhost:2000

main () at ../src/gdb_debugging.c:9

9               func1();

(gdb) load

Loading section .rodata, size 0x8 lma 0xff800000

Loading section .eh_frame, size 0x4 lma 0xff800008

Loading section .ctors, size 0x8 lma 0xff80000c

Loading section .dtors, size 0x8 lma 0xff800014

Loading section .jcr, size 0x4 lma 0xff80001c

Loading section .data, size 0x40c lma 0xff800020

Loading section .text, size 0x510 lma 0xffa00000

Loading section .init, size 0x12 lma 0xffa00510

Loading section .fini, size 0xe lma 0xffa00522

Start address 0xffa00000, load size 2396

Transfer rate: 140941 bits/sec, 266 bytes/write.

Insert some break points – at the entry of each function:

(gdb) break main

Breakpoint 1 at 0xffa00274: file ../src/gdb_debugging.c, line 9.

(gdb) break func1

Breakpoint 2 at 0xffa00284: file ../src/gdb_debugging.c, line 17.

(gdb) break func2

Breakpoint 3 at 0xffa002ac: file ../src/gdb_debugging.c, line 28.

Run the code and it should hit breakpoints.

(gdb) continue

Continuing.

Breakpoint 1, main () at ../src/gdb_debugging.c:9

9               func1();

(gdb) continue

Continuing.

Breakpoint 2, func1 () at ../src/gdb_debugging.c:17

17              int local_data = 1234;

Let’s play with breakpoints. See the information of the breakpoints, how it can be disabled.

(gdb) info breakpoints

Num Type           Disp Enb Address    What

1   breakpoint     keep y   0xffa00274 in main at ../src/gdb_debugging.c:9

        breakpoint already hit 1 time

2   breakpoint     keep y   0xffa00284 in func1 at ../src/gdb_debugging.c:17

        breakpoint already hit 1 time

3   breakpoint     keep y   0xffa002ac in func2 at ../src/gdb_debugging.c:28

(gdb) disable 2

(gdb) info breakpoints

Num Type           Disp Enb Address    What

1   breakpoint     keep y   0xffa00274 in main at ../src/gdb_debugging.c:9

        breakpoint already hit 1 time

2   breakpoint     keep n   0xffa00284 in func1 at ../src/gdb_debugging.c:17

        breakpoint already hit 1 time

3   breakpoint     keep y   0xffa002ac in func2 at ../src/gdb_debugging.c:28
Where are we now?
gdb) where

#0  func1 () at ../src/gdb_debugging.c:17

#1  0xffa00278 in main () at ../src/gdb_debugging.c:9

Single stepping the code…

(gdb) step

19              global_data = 5678;

Let’s print the local variable..

(gdb) info local

local_data = 1234

And the global variable…

(gdb) print/x global_data

$1 = 0x457

Check out the dis-assembly

(gdb) disassemble 0xffa00280 0xffa002a0

Dump of assembler code from 0xffa00280 to 0xffa002a0:

0xffa00280 <func1+0>:   LINK 0x10;                 /* (16) */

0xffa00284 <func1+4>:   R0 = 0x4d2 (X);         /*              R0=0x0x4d2(1234) */

0xffa00288 <func1+8>:   [FP -0x4] = R0;

0xffa0028a <func1+10>:  P2.H = 0xff80;          /* (-128)       P2=0x0xff800000<_global_impure_ptr> */

0xffa0028e <func1+14>:  P2.L = 0x24;             /* ( 36)        P2=0x0xff800024<global_data> */

0xffa00292 <func1+18>:  R0 = 0x162e (X);     /*              R0=0x0x162e(5678) */

0xffa00296 <func1+22>:  [P2] = R0;

0xffa00298 <func1+24>:  CALL 0x0xffa002a4 <func2>;

0xffa0029c <func1+28>:  UNLINK;

End of assembler dump.

We run again…

(gdb) continue

Continuing.

Breakpoint 3, func2 () at ../src/gdb_debugging.c:28

28      }

Look at the back-trace info of function calls and frame info.

(gdb) bt

#0  func2 () at ../src/gdb_debugging.c:28

#1  0xffa0029c in func1 () at ../src/gdb_debugging.c:21

#2  0xffa00278 in main () at ../src/gdb_debugging.c:9

(gdb) backtrace full

#0  func2 () at ../src/gdb_debugging.c:28

No locals.

#1  0xffa0029c in func1 () at ../src/gdb_debugging.c:21

        local_data = 1234

#2  0xffa00278 in main () at ../src/gdb_debugging.c:9

No locals.

(gdb) frame

#0  func2 () at ../src/gdb_debugging.c:28

28      }

Core register information...

(gdb) info registers

r0             0x162e   5678

r1             0xff80042c       -8387540

r2             0x0      0A

r3             0x0      0

r4             0x8000   32768

r5             0x989680 10000000

r6             0x0      0

r7             0x0      0

p0             0xff800178       0xff800178

p1             0x1      0x1

p2             0xff800024       0xff800024

p3             0x3f1ff80        0x3f1ff80

p4             0x3f22148        0x3f22148

p5             0xffc03060       0xffc03060

sp             0xffb00f9c       0xffb00f9c

fp             0xffb00f9c       0xffb00f9c

i0             0xffe01300       -2092288

i1             0xffe00300       -2096384

i2             0x7d801048       2105544776

i3             0x7f900840       2140145728

m0             0x0      0

m1             0x0      0

m2             0xff807ffc       -8355844

m3             0x0      0

b0             0x7fa7e73e       2141710142

b1             0xb5025ff6       -1258135562

b2             0x5b4597f7       1531287543

b3             0x4c3dac51       1279110225

l0             0x0      0

l1             0x0      0

l2             0x0      0

l3             0x0      0

a0x            0x0      0

a0w            0x3200000        52428800

a1x            0x0      0

a1w            0x2b53   11091

astat          0x2001025        33558565

rets           0xffa0029c       0xffa0029c <func1+28>

lc0            0x0      0

lt0            0xffa0005c       -6291364

lb0            0xffa0005d       -6291363

lc1            0x0      0

lt1            0xffa00366       -6290586

lb1            0xffa00367       -6290585

cycles         0xa07d46a4       -1602402652

cycles2        0x0      0

usp            0xffb00ff4       0xffb00ff4

seqstat        0x0      0

syscfg         0x32     50

reti           0xffa000e8       -6291224

retx           0xffa004d6       -6290218

retn           0x986b21f        159822367

rete           0xffa002ac       -6290772

pc             0xffa002ac       0xffa002ac <func2+8>

cc             0x1      1

text_addr      0x0      0

text_end_addr  0x0      0

data_addr      0x0      0

fdpic_exec     0x0      0

fdpic_interp   0x0      0

ipend          0x0      0

Set some global data

(gdb) set global_data = 9999

(gdb) print global_data

$2 = 9999

GDB/MI interface is meant for IDDE such as Eclipse. There are of course some simple useful commands. But beware – the console out seems terribly bad, when these commands are invoked directly.

Setting up as usual.

C:\Program Files\Analog Devices\GNU Toolchain\2010R1\elf\bin>bfin-elf-gdb.exe --interpreter=mi gdb_debugging

~"GNU gdb 6.6\n"

~"Copyright (C) 2006 Free Software Foundation, Inc.\n"

~"GDB is free software, covered by the GNU General Public License, and you are\n"

~"welcome to change it and/or distribute copies of it under certain conditions.\n"

~"Type \"show copying\" to see the conditions.\n"

~"There is absolutely no warranty for GDB.  Type \"show warranty\" for details.\n"

~"This GDB was configured as \"--host=i386-mingw32msvc --target=bfin-elf\"...\n"

(gdb)

target remote localhost:2000

&"target remote localhost:2000\n"

~"Remote debugging using localhost:2000\n"

~"main () at ../src/gdb_debugging.c:9\n"

~"9\t\tfunc1();\n"

^done

Dis-assembly

(gdb)

-data-disassemble -f gdb_debugging.c -l 15 -n 3 -- 0

^done,asm_insns=

[{address="0xffa00280",func-name="func1",offset="0",inst="LINK 0x10;\t\t/* (16) */"},

{address="0xffa00284",func-name="func1",offset="4",inst="R0 = 0x4d2 (X);\t\t/*\t\tR0=0x0x4d2(1234) */"},

{address="0xffa00288",func-name="func1",offset="8",inst="[FP -0x4] = R0;"}]

Reading and writing data

(gdb)

-data-read-memory 0xff800000 x 1 3 3

^done,addr="0xff800000",nr-bytes="9",total-bytes="9",next-row="0xff800003",prev-row="0xff7ffffd",next-page="0xff800009",prev-page="0xff7ffff7",

memory=[{addr="0xff800000",data=["0x00","0x00","0x00"]},

{addr="0xff800003",data=["0x00","0x43","0x00"]},

{addr="0xff800006",data=["0x00","0x00","0xff"]}]

(gdb)

-data-write-memory 0xff800000 x 1 0x11

^done

(gdb)

-data-write-memory 0xff800001 x 1 0x22

^done

(gdb)

-data-write-memory 0xff800001 x 1 0x33

^done

(gdb)

-data-read-memory 0xff800000 x 1 3 3

^done,addr="0xff800000",nr-bytes="9",total-bytes="9",next-row="0xff800003",prev-row="0xff7ffffd",next-page="0xff800009",prev-page="0xff7ffff7",

memory=[{addr="0xff800000",data=["0x11","0x33","0x00"]},

{addr="0xff800003",data=["0x00","0x43","0x00"]},

{addr="0xff800006",data=["0x00","0x00","0xff"]}]

(gdb)
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