Quartz 4

allocate_stack_thread

2 min read

🧵 Summary:

you could find the full code here

Base init

  struct pthread *pd;
  size_t size;
  size_t pagesize_m1 = __getpagesize () - 1; // 4096
  size_t tls_static_size_for_stack = __nptl_tls_static_size_for_stack ();
  size_t tls_static_align_m1 = GLRO (dl_tls_static_align) - 1;
 
  assert (powerof2 (pagesize_m1 + 1));
  assert (TCB_ALIGNMENT >= STACK_ALIGN);

the __getpagesize on x86_64 just return an hard coded value of 4096 i would of thought this was base on the cpuid

the __nptl_tls_static_size_for_stack() is set at the end of the libc in the __libc_setup_tls() with the function init_static_tls() this will init the tls

stack size

/* Get the stack size from the attribute if it is set.  Otherwise we
   use the default we determined at start time.  */
if (attr->stacksize != 0)
  size = attr->stacksize;
else
  {
    lll_lock (__default_pthread_attr_lock, LLL_PRIVATE);
    size = __default_pthread_attr.internal.stacksize;
    lll_unlock (__default_pthread_attr_lock, LLL_PRIVATE);
  }

Explanation:

This snippet retrieves the desired stack size for a new thread:

  • If the caller has set a custom stack size via pthread_attr_setstacksize, it uses that.
  • Otherwise, it safely falls back to a global default.

This fallback is mostly a safeguard: the default stack size is typically set by init_pthread_attr() or through the default pthread_attr_t object used by the main thread.

See part 2 – Thread Attribute Handling for where the default is initialized.

Get memory for the stack

In this section, I’ll only cover the standard path—the else branch of the allocation logic. I’m skipping the if (attr->flags & ATTR_FLAG_STACKADDR) case for now, which handles user-provided stacks; I might revisit it later.

Note: We’ll assume TLS_TCB_AT_TP, which is the default for modern platforms like x86_64 and AArch64. and other 64 byte system

memory plage

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