The C11 has added an attempt to force compilers to initialize padding of structures and unions under certain circumstances. Unfortunately the situation has become confusing now, since it still foresees that padding can be treated differently from other parts of structures that are not initialized explicitly.
This post is mainly identical to a defect report that hopefully will be discussed by the C standards committee on their next meeting. I found that problem that this raises needs to be better known before people start using this interface more widely, so I decided to also publish it here.
The thread interfaces as they are declared in the
threads.h header are largely underspecified, such that interpreting them is often just guess-work and leaves room for a wide range of interpretations. This is particularly irritating since there already is an ISO standard about threads that is quite elaborated and mature, namely ISO/IEC 9945:2009, commonly know as POSIX 2010. C11 mentions ISO/IEC 9945:2009, but completely misses to technically relate to it on the thread interface. The semantic specification of C11 threads is in parts so loose, that a stringent implementation of C11 threads on top of POSIX doesn’t seem possible.
Other platforms that are less formalized than POSIX have their own technical restrictions that should additionally be taken into account. The “other platform” for threads that clearly had been targeted by the committee are threads on Microsoft Windows platforms. Most other widely used commodity operating systems are POSIX compatible (from mainframes down to Android phones). But we should not underestimate the potential of the C threads interface. Because it has a reduced interface it might be suitable for a larger range of platforms than we can foresee today. Because C threads don’t enforce a complete share of the address space, such platforms could e.g be accelerators (providing a portable thread interface on GPU?) or networks on chips. The only memory that must be shared by C threads are objects with static storage duration and objects allocated through
malloc and friends. Thus freestanding environments without
malloc would only be required to shared statically allocated objects.
In the following I only give an incomplete list of the defects as I noticed them, I suspect that there might be a lot of others.