Yolinux.com Tutorial

GNU GDB Debugger Command Cheat Sheet

GDB Command cheat sheet: Command summaries.

Related YoLinux Tutorials:

°C++ Info, links

°C++ String Class

°C++ STL vector, list

°Emacs and C/C++

°Advanced VI

°CGI in C++

°Clearcase Commands

°MS/Visual C++ Practices

°C++ Memory corruption and leaks

°YoLinux Tutorials Index




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GDB Command Line Arguments:

Starting GDB:

  • gdb name-of-executable
  • gdb -e name-of-executable -c name-of-core-file
  • gdb name-of-executable -pid process-id
    (Use ps -auxw to list process id's.)

Command line options: (version 6. Older versions use a single "-")

Option Description
--help
-h
List command line arguments
--exec=file-name
-e file-name
Identify executable associated with core file.
--core=name-of-core-file
-c name-of-core-file
Specify core file.
--command=command-file
-x command-file
File listing GDB commands to perform. Good for automating set-up.
--directory=directory
-d directory
Add directory to the path to search for source files.
--cd=directory Run GDB using specified directory as the current working directory.
--nx
-n
Do not execute commands from ~/.gdbinit initialization file. Default is to look at this file and execute the list of commands.
--batch -x command-file Run in batch (not interactive) mode. Execute commands from file. Requires -x option.
--symbols=file-name
-s file-name
Read symbol table from file file.
--write Enable writing into executable and core files.
--quiet
-q
Do not print the introductory and copyright messages.
--tty=device Specify device for running program's standard input and output.
--pid=process-id
-p process-id
-c process-id
Specify process ID number to attach to.


GDB Commands:

Commands used within GDB:

Command Description
help List gdb command topics.
help topic-classes List gdb command within class.
help command Command description.
apropos search-word Search for commands and command topics containing search-word.
info args
i args
List program command line arguments
info breakpoints List breakpoints
info break List breakpoint numbers.
info break breakpoint-number List info about specific breakpoint.
info watchpoints List breakpoints
info registers List registers in use
info threads List threads in use
info set List set-able option
Break and Watch
break funtion-name
break line-number
break ClassName::functionName
Suspend program at specified function of line number.
break +offset
break -offset
Set a breakpoint specified number of lines forward or back from the position at which execution stopped.
break filename:function Don't specify path, just the file name and function name.
break filename:line-number Don't specify path, just the file name and line number.
break Directory/Path/filename.cpp:62
break *address Suspend processing at an instruction address. Used when you do not have source.
break line-number if condition Where condition is an expression. i.e. x > 5
Suspend when boolean expression is true.
break line thread thread-number Break in thread at specified line number. Use info threads to display thread numbers.
tbreak Temporary break. Break once only. Break is then removed. See "break" above for options.
watch condition Suspend processing when condition is met. i.e. x > 5
clear
clear function
clear line-number
Delete breakpoints as identified by command option.
delete
d
Delete all breakpoints, watchpoints, or catchpoints.
delete breakpoint-number
delete range
Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments.
disable breakpoint-number-or-range
enable breakpoint-number-or-range
Does not delete breakpoints. Just enables/disables them.
Example:
Show breakpoints: info break
Disable: disable 2-9
enable breakpoint-number once Enables once
continue
c
Continue executing until next break point/watchpoint.
continue number Continue but ignore current breakpoint number times. Usefull for breakpoints within a loop.
finish Continue to end of function.
Line Execution
step
s
step number-of-steps-to-perform
Step to next line of code. Will step into a function.
next
n
next number
Execute next line of code. Will not enter functions.
until
until line-number
Continue processing until you reach a specified line number. Also: function name, address, filename:function or filename:line-number.
info signals
info handle
handle SIGNAL-NAME option
Perform the following option when signal recieved: nostop, stop, print, noprint, pass/noignore or nopass/ignore
where Shows current line number and which function you are in.
Stack
backtrace
bt
bt inner-function-nesting-depth
bt -outer-function-nesting-depth
Show trace of where you are currently. Which functions you are in. Prints stack backtrace.
backtrace full Print values of local variables.
frame
frame number
f number
Show current stack frame (function where you are stopped)
Select frame number. (can also user up/down to navigate frames)
up
down
up number
down number
Move up a single frame (element in the call stack)
Move down a single frame
Move up/down the specified number of frames in the stack.
info frame List address, language, address of arguments/local variables and which registers were saved in frame.
info args
info locals
info catch
Info arguments of selected frame, local variables and exception handlers.
Source Code
list
l
list line-number
list function
list -
list start#,end#
list filename:function
List source code.
set listsize count
show listsize
Number of lines listed when list command given.
directory directory-name
dir directory-name
show directories
Add specified directory to front of source code path.
directory Clear sourcepath when nothing specified.
Machine Language
info line
info line number
Displays the start and end position in object code for the current line in source.
Display position in object code for a specified line in source.
disassemble 0xstart 0xend Displays machine code for positions in object code specified (can use start and end hex memory values given by the info line command.
stepi
si
nexti
ni
step/next assembly/processor instruction.
x 0xaddress
x/nfu 0xaddress
Examine the contents of memory.
Examine the contents of memory and specify formatting.
  • n: number of display items to print
  • f: specify the format for the output
  • u: specify the size of the data unit (eg. byte, word, ...)
Example: x/4dw var
Examine Variables
print variable-name
p variable-name
p file-name::variable-name
p 'file-name'::variable-name
Print value stored in variable.
p *array-variable@length Print first # values of array specified by length. Good for pointers to dynamicaly allocated memory.
p/x variable Print as integer variable in hex.
p/d variable Print variable as a signed integer.
p/u variable Print variable as a un-signed integer.
p/o variable Print variable as a octal.
p/t variable
x/b address
x/b &variable
Print as integer value in binary. (1 byte/8bits)
p/c variable Print integer as character.
p/f variable Print variable as floating point number.
p/a variable Print as a hex address.
x/w address
x/4b &variable
Print binary representation of 4 bytes (1 32 bit word) of memory pointed to by address.
GDB Modes
set gdb-option value Set a GDB option
set logging on
set logging off
show logging
set logging file log-file
Turn on/off logging. Default name of file is gdb.txt
set print array on
set print array off
show print array
Default is off. Convient readable format for arrays turned on/off.
set print array-indexes on
set print array-indexes off
show print array-indexes
Default off. Print index of array elements.
set print pretty on
set print pretty off
show print pretty
Format printing of C structures.
set print union on
set print union off
show print union
Default is on. Print C unions.
set print demangle on
set print demangle off
show print demangle
Default on. Controls printing of C++ names.
Start and Stop
run
r
run command-line-arguments
run < infile > outfile
Start program execution from the beginning of the program. The command break main will get you started. Also allows basic I/O redirection.
continue
c
Continue execution to next break point.
kill Stop program execution.
quit
q
Exit GDB debugger.


GDB Operation:

  • Compile with the "-g" option (for most GNU and Intel compilers) which generates added information in the object code so the debugger can match a line of source code with the step of execution.
  • Do not use compiler optimization directive such as "-O" or "-O2" which rearrange computing operations to gain speed as this reordering will not match the order of execution in the source code and it may be impossible to follow.
  • control+c: Stop execution. It can stop program anywhere, in your source or a C library or anywhere.
  • To execute a shell command: ! command
    or shell command
  • GDB command completion: Use TAB key
    info bre + TAB will complete the command resulting in info breakpoints
    Press TAB twice to see all available options if more than one option is available or type "M-?" + RETURN.
  • GDB command abreviation:
    info bre + RETURN will work as bre is a valid abreviation for breakpoints


De-Referencing STL Containers:

Displaying STL container classes using the GDB "p variable-name" results in an cryptic display of template definitions and pointers. Use the following ~/.gdbinit file (V1.03 09/15/08). Now works with GDB 4.3+.
(Archived versions: [V1.01 GDB 6.4+ only])
Thanks to Dr. Eng. Dan C. Marinescu for permission to post this script.

Use the following commands provided by the script:

Data typeGDB command
std::vector<T>pvector stl_variable
std::list<T>plist stl_variable T
std::map<T,T>pmap stl_variable
std::multimap<T,T>pmap stl_variable
std::set<T>pset stl_variable T
std::multiset<T>pset stl_variable
std::deque<T>pdequeue stl_variable
std::stack<T>pstack stl_variable
std::queue<T>pqueue stl_variable
std::priority_queue<T>ppqueue stl_variable
std::bitset<n>td>pbitset stl_variable
std::stringpstring stl_variable
std::widestringpwstring stl_variable
Where T refers to native C++ data types. While classes and other STL data types will work with the STL container classes, this de-reference tool may not handle non-native types.

Also see the YoLinux.com STL string class tutorial and debugging with GDB.


De-Referencing a vector:

Example: STL_vector_int.cpp

#include <iostream>
#include <vector>
#include <string>

using namespace std;

main()
{
   vector<int> II;

   II.push_back(10);
   II.push_back(20);
   II.push_back(30);

   cout << II.size() << endl;

}
Compile: g++ -g STL_vector_int.cpp

Debug in GDB: gdb a.out

(gdb) l
1       #include <iostream>
2       #include <vector>
3       #include <string>
4
5       using namespace std;
6
7       main()
8       {
9          vector<int> II;
10
(gdb) l
11         II.push_back(10);
12         II.push_back(20);
13         II.push_back(30);
14
15         cout << II.size() << endl;
16
17      }
(gdb) break 15
Breakpoint 1 at 0x8048848: file STL_vector_int.cpp, line 15.
(gdb) r
Starting program: /home/userx/a.out

Breakpoint 1, main () at STL_vector_int.cpp:15
15         cout << II.size() << endl;
(gdb) p II
$1 = {
  <std::_Vector_base<int,std::allocator<int> >> = {
    _M_impl = {
      <std::allocator<int>> = {
        <__gnu_cxx::new_allocator<int>> = {<No data fields>}, <No data fields>},
      members of std::_Vector_base<int,std::allocator<int> >::_Vector_impl:
      _M_start = 0x804b028,
      _M_finish = 0x804b034,
      _M_end_of_storage = 0x804b038
    }
  }, <No data fields>}
(gdb) pvector II
elem[0]: $2 = 10
elem[1]: $3 = 20
elem[2]: $4 = 30
Vector size = 3
Vector capacity = 4
Element type = int *
(gdb) c
Continuing.
3

Program exited normally.
(gdb) quit
Notice the native GDB print "p" results in an cryptic display while the "pvector" routine from the GDB script provided a human decipherable display of your data.


De-Referencing a 2-D vector of vectors:

Example: STL_vector_int_2.cpp

#include <iostream>
#include <vector>

using namespace std;

main()
{
   vector< vector<int> > vI2Matrix(3, vector<int>(2,0));

   vI2Matrix[0][0] = 0;
   vI2Matrix[0][1] = 1;
   vI2Matrix[1][0] = 10;
   vI2Matrix[1][1] = 11;
   vI2Matrix[2][0] = 20;
   vI2Matrix[2][1] = 21;

   cout << "Loop by index:" << endl;

   int ii, jj;
   for(ii=0; ii < 3; ii++)
   {
      for(jj=0; jj < 2; jj++)
      {
         cout << vI2Matrix[ii][jj] << endl;
      }
   }
}
Compile: g++ -g STL_vector_int_2.cpp

Debug in GDB: gdb a.out

(gdb) l
1       #include <iostream>
2       #include <vector>
3
4       using namespace std;
5
6       main()
7       {
8          vector< vector<int> > vI2Matrix(3, vector<int>(2,0));
9
10         vI2Matrix[0][0] = 0;
(gdb) l
11         vI2Matrix[0][1] = 1;
12         vI2Matrix[1][0] = 10;
13         vI2Matrix[1][1] = 11;
14         vI2Matrix[2][0] = 20;
15         vI2Matrix[2][1] = 21;
16
17         cout << "Loop by index:" << endl;
18
19         int ii, jj;
20         for(ii=0; ii < 3; ii++)
(gdb) break 17
Breakpoint 1 at 0x8048a19: file STL_vector_2.cpp, line 17.
(gdb) r
Starting program: /home/userx/a.out

Breakpoint 1, main () at STL_vector_2.cpp:17
17         cout << "Loop by index:" << endl;
(gdb) pvector vI2Matrix
elem[0]: $1 = {
  <std::_Vector_base<int,std::allocator<int> >> = {
    _M_impl = {
      <std::allocator<int>> = {
        <__gnu_cxx::new_allocator<int>> = {<No data fields>}, <No data fields>},
      members of std::_Vector_base<int,std::allocator<int> >::_Vector_impl:
      _M_start = 0x804b040,
      _M_finish = 0x804b048,
      _M_end_of_storage = 0x804b048
    }
  }, <No data fields>}
elem[1]: $2 = {
  <std::_Vector_base<int,std::allocator<int> >> = {
    _M_impl = {
      <std::allocator<int>> = {
        <__gnu_cxx::new_allocator<int>> = {<No data fields>}, <No data fields>},
      members of std::_Vector_base<int,std::allocator<int> >::_Vector_impl:
      _M_start = 0x804b050,
      _M_finish = 0x804b058,
      _M_end_of_storage = 0x804b058
    }
  }, <No data fields>}
elem[2]: $3 = {
  <std::_Vector_base<int,std::allocator<int> >> = {
    _M_impl = {
      <std::allocator<int>> = {
        <__gnu_cxx::new_allocator<int>> = {<No data fields>}, <No data fields>},
      members of std::_Vector_base<int,std::allocator<int> >::_Vector_impl:
      _M_start = 0x804b060,
      _M_finish = 0x804b068,
      _M_end_of_storage = 0x804b068
---Type <return> to continue, or q <return> to quit---
    }
  }, <No data fields>}
Vector size = 3
Vector capacity = 3
Element type = class std::vector<int,std::allocator<int> > *
(gdb) pvector $1
elem[0]: $4 = 0
elem[1]: $5 = 1
Vector size = 2
Vector capacity = 2
Element type = int *
(gdb) pvector $2
elem[0]: $6 = 10
elem[1]: $7 = 11
Vector size = 2
Vector capacity = 2
Element type = int *
(gdb) pvector $3
elem[0]: $8 = 20
elem[1]: $9 = 21
Vector size = 2
Vector capacity = 2
Element type = int *
(gdb) p vI2Matrix
$10 = {
  <std::_Vector_base<std::vector<int, std::allocator<int> >,std::allocator<std::vector<int, std::allocator<int> > > >> = {
    _M_impl = {
      <std::allocator<std::vector<int, std::allocator<int> > >> = {
        <__gnu_cxx::new_allocator<std::vector<int, std::allocator<int> > >> = {<No data fields>}, <No data fields>},
      members of std::_Vector_base<std::vector<int, std::allocator<int> >,std::allocator<std::vector<int, std::allocator<int> > > >::_Vector_impl:
      _M_start = 0x804b018,
      _M_finish = 0x804b03c,
      _M_end_of_storage = 0x804b03c
    }
  }, <No data fields>}
(gdb) quit
Note "pvector" does not de-reference the entire vector of vectors all at once but returns vectors $1, $2 and $3. The "pvector" command then helps us traverse the information by examining the contents of each element in the individual "terminal" vectors. Note that the native gdb "p vI2Matrix" (last command) was much less informative.


Man Pages:

  • gdb - GNU debugger
  • ld - Linker
  • gcc/g++ - GNU project C and C++ compiler


Links:


Books:

"Debugging with GDB: The GNU Source-Level Debugger"
by Richard Stallman, Roland H. Pesch, Stan Shebs
ISBN # 1882114884, Free Software Foundation; 9th edition (January 1, 2002)

Amazon.com
"GDB Pocket Reference"
by Arnold Robbins
ISBN # 0596100272, O'Reilly

Amazon.com
"Advanced Linux Programming"
by Mark Mitchell, Jeffrey Oldham, Alex Samuel, Jeffery Oldham
ISBN # 0735710430, New Riders

Good book for programmers who already know how to program and just need to know the Linux specifics. Covers a variety of Linux tools, libraries, API's and techniques. If you don't know how to program, start with a book on C.

Amazon.com

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