How do you use gdb to debug your code?

gdb> p show_my_struct(struct)

My custom display of Foo:
   ...

gdb> watch foo
Watchpoint4: foo
gdb>

Program terminated with signal 11, Segmentation fault.
#0  0x08048364 in foo () at foo.c:4
4         *x = 100;

(gdb) backtrace
#0  0x08048364 in foo () at foo.c:4
#1  0x0804837f in main () at foo.c:9