GDB

GDB is free software, protected by the gnu General Public License (GPL).

help – The help provides help on commands associated with GDB

(gdb) help break –displays details about setting breakpoint

info – The command can be used for describing the state of your program

(gdb) info registers– displays value of registers used by the program

show- The command can be used to get the state of gdb itself

(gdb) show path –displays program path
The set environment command can be used to set the environmental variables used by your program.

(gdb) set environment HOME=/home/penchala

                (gdb) set environment PWD=/home/usr

(gdb) set environment PATH=/home/pencha 

Running programs under GDB  The debugging information can be generated with the option g

Example gcc –g –o client client.c

 Starting your program inside GDB
  file command can be used to load symbols from the executable file.

Example:  file a.out reads symbols from a.out 

run or r (creates a process) starts the execution of the program whose symbols are loaded.
 Debugging already running process
   attach pid The command halts process execution immediately. 
   detach pid The command detaches the process from the gdb.

Debugging programs with multiple threads

the gdb provides features to help debugging programs with several threads.

thread threadnumber – The control switches to thread denoted by threadnumber.

info threads - display the thread number assigned to thread by gdb and thread id given by the system.

 break linenumber threadno – stops the thread when it reaches the line number specified.

Watch points are used to watch the value of an expression

watch - The program stops execution when value of variable changes.

(gdb) watch a - The program stops execution when the value of a changes.

Removing Break , Watch points

clear break point/catch point/watch point – This command removes the specified break point

(gdb) break 10

(gdb) info break

1 breakpoint keep y 0x08048323 in main at s6.c:15

(gdb) clear 10

Deleted break point 1 

delete - The function removes all catch / break points. 

(gdb) delete

Remove all break points ? Y

(gdb) info break

No break points or watch points

Continuing and stepping

Continue (c)– Resume execution of the program from the current address where your program last stopped. 
step [count] – The program is used to execute the number of lines specified by count.
  until (u) – continue execution of your program until the specified location is reached or the current stack frame returns. 
stepi - execute one machine instruction then stop and return to debugger. 
nexti – execute one machine instruction then stop and return to debugger. But if it is a function call proceed until the function returns.

 Examining the stack

frame args- This command displays the stack frame currently executing that is innermost stack frame.

  Back traces – The back trace is a summary of how your program got where it is. 

    bt Print a back trace of the entire stack: one line per frame for all frames in the stack. 

    bt n Similar, but print only the innermost n frames

 
  Printing source lines 

      list function displays lines centered around the function named or line number given as argument.

Example show list size

                                 Number of source lines gdb will list by default is 5.

               set listsize count – The count of number of lines for list to print.

              Example set listsize 10

Examining the data

The print statement is used to print data and variables in program

print /f expr

/f – The format used can be x, d, u, o, c, f denoting radix, integer, unsigned, float etc.

expr – gdb supports any kind of constant, variable or operator supported by the programming language used.

• Example –

                 (gdb) int a=10

               (gdb) print a

              print a=10 – sets the value of a to 10

             The print command can be used set values to variables.