GNU GDB Debugger Command Cheat Sheet

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 reachaaspecified line number. Also: function name, address, filename:function or filename:line-number.
stepi si nexti ni	step/next assembly/processor instruction.
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 number f number	Select frame number.
up number down number	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.
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 type	GDB 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::string	pstring stl_variable
std::widestring	pwstring 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

01 #include <iostream> 02 #include <vector> 03 #include <string> 04 05 using namespace std; 06 07 main() 08 { 09 vector<int> II; 10 11 II.push_back(10); 12 II.push_back(20); 13 II.push_back(30); 14 15 cout << II.size() << endl; 16 17 }

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

01 #include <iostream> 02 #include <vector> 03 04 using namespace std; 05 06 main() 07 { 08 vector< vector<int> > vI2Matrix(3, vector<int>(2,0)); 09 10 vI2Matrix[0][0] = 0; 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++) 21 { 22 for(jj=0; jj < 2; jj++) 23 { 24 cout << vI2Matrix[ii][jj] << endl; 25 } 26 } 27 }

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.