gdb basics tutorial

      Note these commands are universal except the first two are specific for llvm build and debug. For other application you want to debug, simply build a debug version using your own way (either make or cmake), then just 'gdb yourApplication' . For some complex applications you may need --args to pass some flags.

(1)    Build a debug version executable.  

cmake -DGCC_INSTALL_PREFIX=/home/llvm/gcc/install/gcc-6.2.0 -DCMAKE_BUILD_TYPE="Debug" -DBUILD_SHARED_LIBS=ON -DLLVM_ENABLE_ASSERTIONS=ON  ~/src/llvm ; make -j20

(2)    Start gdb:

gdb --args /home/jtony/debug/bin/llc  test/CodeGen/PowerPC/expand-isel.ll

(3)    Query function:

  info func expandMergeableISELs  (info func MergeableISEL also works, fuzzy matching?)

(4)     Set break points:

Function breakpoint:

b[reak] function_name

e.g: b(reak) PPCExpandISEL::expandMergeableISELs

set breakpoint at a specified file-line pair:

b[reak] file.cpp:7

break current line:

b[reak]

conditional break is also allowed:  b[reak] file.cpp:7 if a > 5

(5)    Start the program and run to breakpoint

r[un]

Run to next breakpoint:

c[ontinue]

typing r(un) again would restart the program from beginning

(6)    Switch to TUI mode

lay src          (ctrl + x  + a)

(7)    Debug

Step into:   ‘s[tep]’             -- will go into a function call, and walk through every line of code

Step over:  ‘n[ext]’            -- treat function call as one instruction, won’t go into it

Step out:    ‘fin[ish]’          --  finish the rest of the function call and return to the caller

Note: press ENTER gdb will repeat the same command you just gave to it

(8)    Save/restore/show breakpoints

Save breakpoints:  save breakpoints filename

Reload breakpints:  source filename

Show breakpoints:   ‘info break’

(9)    Show commands history: 

‘show commands’

(10) Print/ptype

Print value of a variable:      p[rint] varName

Print in hexadecimal:       p[rint]/x  varName

print the type of var:             ptype ‘varName’

(11) Calling stack

Print calling stack:   ‘where’ or ‘backtrace’

Navigate between calling stack:

Go up one level:       ‘up’

Go down one level:  ‘down’

Go up N level:           ‘up  N’

Go down N level;     ‘down N’

refresh the screen:   ‘refresh’

(12) Call function

call a function: call function_name(parameters)

(13) jump to a line (not safe, unpredictable, unless you know what you are doing)   

                 First set a break point on the line you want to jump to:   b[reak]   line_number    e.g  b 321

                 Then jump line number:  jump  line_number                       e.g  jump 321

New skills learned during the tutorial:

n(ext) N       will step over N steps, if N = 1, is equivalent to just n(ext)