why is the Operation not permitted?

[Guy]

 Hello!

 This might be a trivial question:
 I am debbuging my_program with gdb such as:
 gdb my_program
 GNU gdb (GDB) 7.4
 ...
 Reading symbols from my_program
 (gdb) run
 ...

 And now I want to see what is happening:
 strace -p <pid of my_program>

 and get an error:
 attach: ptrace(PTRACE_ATTACH, ...): Operation not permitted

 Why this?
 Is there any "read_only" options?

 Thanks
-- 
 Guy Maurel
 Sebastian-Fischer-Weg 13
 89077 Ulm