cmsc212-gdb-tutorial.md

GDB Tutorial: A Walkthrough with Examples

source: cs.umd.edu

• additional step when compiling, like -g switch for gcc to enable built-in debugging support needed by gdb

• starting up gdb and load a program to debug using command file as (gdb) file myprog.x

can directly start with gdb myprog.x

• auto-complete words with TAB key, can check help with help <command>

• run command runs the loaded program, in case of issues it shall show some facts about it

• can break program in middle with break file1.c:15, if program ever reaches mentioned file:line pair it will pause and prompt

• to make gdb break at a function call use break my_func

• can use run again to continue after pause, to reach to next breakpoint use continue

• can use step with fine-grained control to proceed, similarly next single-steps as well the next sub-routine instead just next instruction

• print <var> command prints value of var, print/x prints value in hexadecimal

• set watchpoint on state like value of variables changing, example (gdb) watch my_var, it will interrup run and print old/new values

• other useful commands

• backtrace produces a stack trace related to seg fault

• where shows the next statement to be executed, where seg fault occured

• finish completes fun till current function's end

• delete to remove a breakpoint

• info breakpoints shows info on all declared breakpoints

• using conditional breakpoints allow more fluidity, like (gdb) break file1.c:6 if i >= SOMESIZE

Fun with pointers

• assume following struct

struct book {
  int uuid;
  char *name;
  float prince;
  int pages;
}

• assume we are in gdb at some point in execution after a line that looks like struct book * b1 = <somebook>;

• to see value of pointer (gdb) print b1

• checking particular field of struct by referencing

(gdb) print b1->uuid
(gdb) print b1->name
(gdb) print b1->price
(gdb) print b1->pages

• can also use

(gdb) print (*b1).uuid
(gdb) print (*b1).name
(gdb) print (*b1).price
(gdb) print (*b1).pages

• to see entire content of struct (gdb) print *b1

• can follow pointers iteratively as (gdb) print list_ptr->next->next->data, for something like linked list

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