Question

I was asked from where do we know that when passing NULL as a second argument in pthread_create() function the thread is made joinable.

I mean, I know that man pages state so, but a justification in code was demanded.
I know that when NULL is passed in, default attributes are used:

const struct pthread_attr *iattr = (struct pthread_attr *) attr;
if (iattr == NULL)
/* Is this the best idea? On NUMA machines this could mean accessing far-away memory. */
iattr = &default_attr;

I know that it should be somewhere in the code of pthread library, but I don’t know where exactly.

I know that the definition of default_attr is in pthread_create.c:

static const struct pthread_attr default_attr = { /* Just some value > 0 which gets rounded to the nearest page size. */ .guardsize = 1, };

http://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_create.c;h=4fe0755079e5491ad360c3b4f26c182543a0bd6e;hb=HEAD#l457

but I do not know where is exactly stated in the code that this result in a joinable thread.

Thanks in advance.

Answer 1

First off, from the code you pasted you can see that default_attr contains zeroes in almost all fields (there’s no such thing as a half-initialized variable in C: if you only initialize some fields, the others are set to 0).

Second, pthread_create contains this code:

/* Initialize the field for the ID of the thread which is waiting
   for us.  This is a self-reference in case the thread is created
   detached.  */
pd->joinid = iattr->flags & ATTR_FLAG_DETACHSTATE ? pd : NULL;

This line checks whether iattr->flags has the ATTR_FLAG_DETACHSTATE bit set, which (for default_attr) it doesn’t because default_attr.flags is 0. Thus it sets pd->joinid to NULL and not to pd as for detached threads.

(Note that this answer only applies to GNU glibc and not to POSIX pthreads in general.)