$ gcc -g -Wall -Werror -o prog prog.c
$ gdb prog
Copyright (C) 2014 Free Software Foundation, Inc...
(gdb) break 9
Breakpoint 1 at 0x100f03: file prog.c, line 9.
(gdb) run
/Users/comp1921 Starting program: ..../prog 

Breakpoint 1, main (argc=1, argv=0x7ffbc8) at prog.c:9
9               for (i = 1; i <= 3; i++ { 
(gdb) next
10              sum += a[i];
(gdb) print sum
$1 = 0
(gdb) print a[i]
$2 = 4
(gdb) print i
$3 = 1
(gdb) print a@1
$4 = {{7, 4, 3}}
(gdb) cont
...