How can I get a thread's stack bounds?

I've had to work out how to get the current thread's stack bounds a couple of times lately, so it's time I wrote it down.

If you ask the internets, the usual answer you'll get back is to look at the pthread_attr_t you supplied to pthread_create(3). One problem with this is that the values you get will be the values you supplied, not the potentially rounded and/or aligned values that were actually used. More importantly, it doesn't work at all for threads you didn't create yourself; this can be a problem if you're a library, for example, called by arbitrary code on an arbitrary thread.

The real answer is pthread_getattr_np(3). Here's the synopsis:
#include <pthread.h>

int pthread_getattr_np(pthread_t thread, pthread_attr_t* attr);

You call pthread_getattr_np(3) with an uninitialized pthread_attr_t, make your queries, and destroy the pthread_attr_t as normal.
pthread_attr_t attributes;
errno = pthread_getattr_np(thread, &attributes);
if (errno != 0) {
  PLOG(FATAL) << "pthread_getattr_np failed";

void* stack_address;
size_t stack_size;
errno = pthread_attr_getstack(&attributes, &stack_address, &stack_size);
if (errno != 0) {
  PLOG(FATAL) << "pthread_attr_getstack failed";

errno = pthread_attr_destroy(&attributes);
if (errno != 0) {
  PLOG(FATAL) << "pthread_attr_destroy failed";

Note that you don't want the obsolete pthread_attr_getstackaddr(3) and pthread_attr_getstacksize(3) functions; they were sufficiently poorly defined that POSIX replaced them with pthread_attr_getstack(3).

Note also that the returned address is the lowest valid address, so your current stack pointer is (hopefully) a lot closer to stack_address + stack_size than stack_address. (I actually check that my current sp is in bounds as a sanity check that I got the right values. Better to find out sooner than later.)

Why should you call pthread_getattr_np(3) even if you created the thread yourself? Because what you ask for and what you get aren't necessarily exactly the same, and because you might want to know what you got without wanting to specify anything other than the defaults.

Oh, yeah... The "_np" suffix means "non-portable". The pthread_getattr_np(3) function is available on glibc and bionic, but not (afaik) on Mac OS. But let's face it; any app that genuinely needs to query stack addresses and sizes is probably going to contain an #ifdef or two anyway...