The gdb debugger

The gdb debugger
These notes are by no means comprehensive – for further details, use gdb’s built-in help function.

Compiling to be able to debug:

g++ -g fname – e.g., g++ -g try1.cpp

Running the debugger on the resulting object-file

gdb objFile – e.g., gdb a.out

Debugging functions:

Examining the source code

list – displays a listing of (the current 10 lines of) the program

list n1, n2 – displays a listing of lines n1 through n2 of the program

list routine-name – displays a listing of (the first 10 lines of) the routine

In case the source spans multiple files: list fname:n1, fname:n2 (etc.)

Running the program

run argList – e.g., run > outFile

continue – if the program halts at a breakpoint (see below), continue its execution

repeat run with no arguments specified – use the same arguments as previously

To see the arguments – show args

To set new arguments – set args newArgList

To terminate a program – CTRL-C (returns control to gdb)

To exit gdb – quit

Single-step execution:

next – executes function-calls in a single step

step – steps through function calls as well

Examining/ changing variables

backtrace – if a program terminates abnormally, shows where this happened

print varName – outputs the value of the variable varName

whatis varName – outputs the type of the variable varName

ptype varName – if varName is not a simple type (e.g., it’s a class), outputs the definition of the type

set varName = expression – sets variable varName’s value

Setting breakpoints

break lineNo – insert a breakpoint prior to this line

break functionName – insert a breakpoint prior to entering this function

In case the source spans multiple files, break fname::lineNo (etc.)

Conditional breakpoints – break lineNo if condition (etc.)

Watchpoints – like conditional breakpoints, but not attached to any point in the program (major performance hit!!)

gdb assigns each breakpoint a number – can see these with info breakpoints

To get rid of breakpoints

delete i – delete the breakpoint numbered i

clear lineNo – delete all breakpoints in the specified line

clear functionName – delete the breakpoint at the entry of the specified function

Additional run commands

call functionName – call and execute this function

finish – finish executing the current function and print its return value

return value – cancel execution of the current function, and return value to the caller

For further help:

Use gdb’s built-in help feature: type help within gdb to obtain a list of help topics. Full documentation on each gdb function is available three levels down the help hierarchy.