Your understanding about str reside on heap is not correct. Its global variable which gets stored into the data segment. Regarding your print global variable, you can do as follows on my GNU/Linux terminal.
$ gcc -g -Wall hello.c
$ gdb -q ./a.out
Reading symbols from /home/mantosh/practice/a.out...done.
(gdb) break main
Breakpoint 1 at 0x400524: file hello.c, line 6.
(gdb) run
Starting program: /home/mantosh/practice/a.out
Breakpoint 1, main () at bakwas.c:6
6 printf("%s",str);
(gdb) disassemble main
Dump of assembler code for function main:
0x0000000000400520 <+0>: push %rbp
0x0000000000400521 <+1>: mov %rsp,%rbp
=> 0x0000000000400524 <+4>: mov $0x601020,%esi
0x0000000000400529 <+9>: mov $0x4005e4,%edi
0x000000000040052e <+14>: mov $0x0,%eax
0x0000000000400533 <+19>: callq 0x4003f0 <printf@plt>
0x0000000000400538 <+24>: mov $0x0,%eax
0x000000000040053d <+29>: pop %rbp
0x000000000040053e <+30>: retq
End of assembler dump.
(gdb) p str
$1 = "justatest\000\000\000\000\000"
(gdb) p &str
$2 = (char (*)[15]) 0x601020
// These are addresses of two arguments which would be passed in printf.
// From assembly instruction we can verify that before calling the printf
// these are getting stored into the registers.
(gdb) x/s 0x4005e4
0x4005e4: "%s"
(gdb) x/s 0x601020
0x601020 <str>: "justatest