When creating constructor functions (at least within a shared library, not
sure about within a mainline program), that is, a function with an
attribute "constructor", the order of constructor execution is
undetermined.  This causes a problem if you link with the pthread library
and perform any significant action within another constructor.  The
pthread library's pthread_initialize() function initializes the initial
thread's struct _ptheread_descr_struct.p_errnop member to point to _errno,
but if your constructor runs before pthread_initialize, then it is still
at its default initial value of NULL.

This causes probelems if your constructor makes a system call that fails
and attempts to set errno, or calls any of the printf family, or probably
other functions that touch errno.

I think that this could be easily fixed if the initializer for
__pthread_initial_thread specified &_errno for p_errnop (and &_h_errno
for p_h_errnop) instead of NULL.  I don't see any reason that it
needs to be initialized to NULL.

A way to produce the problem is:
static void myfunc() __attribute__((constructor));
static void myfunc() {
	printf("hello");
}

Build this into a shared library.  Build another process that links
with this library and the pthread library.  If myfunc() runs before
pthread_initialize(), then the printf() will try to touch errno and
will cause a segmentation violation.