How do you decide which constraints to use for variables when working with inline assembly?

Question

I'm trying to write a string into a file but haven't been able to figure out how to use more than one constraint at a time. For example:

// write(3, "This is a test.\n", 16);
asm volatile (
               "movq $4,%%rax\n\t"
               "movq $3,%%rdi\n\t"
               "movq %0,%%rsi\n\t"
               "movq $0x10,%%rdx\n\t"
               "int $0x80\n\t"
               :"=g"(ret)
               :"0"(msg)
   );

The above code works for my string, although I am not sure how to do the same for the length. My first thought was to use the '1' constraint but that did not work. I'm pretty certain that using the numbered constraints here isn't the best idea, but thus far it is the only way I have managed to successfully write.

I've looked at various online sources and guides covering inline assembly but I still can't figure out how to do it properly.

** Edit **: I haven't had much luck with naming the constraints either:

asm volatile (
                   "movq $4,%%rax\n\t"
                   "movq $3,%%rdi\n\t"
                   "movq %[msg],%%rsi\n\t"
                   "movq %[len],%%rdx\n\t"
                   "int $0x80\n\t"
                   ::[msg]"g"(msg), [len]"g"(len)
                   :"cc"
       );

This is the 'truss' output when run: write(3,0x4,16) ERR#22 'Invalid argument'. It is really strange since now the length is working, but the string isn't getting worked into the call.

If someone could tell me what I am doing wrong I would really appreciate it.

Thank you.