Greetings,

WCSLIB 8.9 is now available from

  https://www.atnf.csiro.au/computing/software/wcs/wcslib-releases

This release is the product of Thomas Robitaille's investigations into
thread-related problems experienced by Astropy, as detailed below.  It
is a recommended upgrade if you use WCSLIB in threaded execution.

Thanks Tom!

Mark Calabretta



WCSLIB version 8.9 (2026/06/18)
-------------------------------

WCSLIB policy over the years has been to use the oldest version of C
that is sufficient for the job, currently C99.  However, as of this
release, C11 is essentially now a prerequisite for thread-safe
execution.  This arises from the need to use functions from stdatomic.h
to deal with race conditions that may arise in the wcserr routines (see
below).  For now, C99 is still sufficient for thread-safe execution if
the C99 compiler supports thread-local storage as required by wcsprintf
(most do), and if wcserr is not used.  However, that situation may not
persist.

It does not seem unreasonable, nor particularly onerous, to require a
more modern, thread-aware compiler, namely C11, for threaded execution.
(It's still pretty old!)

This release is the product of Thomas Robitaille's investigations into
thread-related problems experienced by Astropy.

* C library

  - In wcsprintf.c, added the thread-local storage (TLS) specifier to
    static variables.  Without it the module-static output buffer is
    shared across all threads.  Consequently, if two threads call
    wcsprintf() concurrently after either has called wcsprintf_set(NULL)
    they will interleave their writes into the same wcsprintf_buff.
    While not a problem in itself, if the buffer needs to grow they will
    race on the realloc() of wcsprintf_buff, thereby corrupting the
    heap and typically crashing inside realloc().  Reported by Thomas
    Robitaille with patch.

  - In wcserr.c, conflicts may arise among threads that share a struct
    containing a wcserr struct (namely celprm, disprm, linprm, prjprm,
    spcprm, spxprm, tabprm, and wcsprm).  Two threads may attempt
    simultaneously to operate on the shared wcserr struct, i.e. to set,
    clear, or copy it, leading to race conditions and potentially
    segfaults if one thread attempts to read a wcserr struct that is in
    the process of being cleared by another.  The solution consists of
    a simple locking mechanism - a spinlock native to C11, provided by
    stdatomic.h - implemented as a pre-processor macro.  For C99, the
    macro defaults to a no-op, which means that the problem persists.
    Consequently, C11 is now required for thread-safe execution if
    wcserr is used.  Reported by Thomas Robitaille with patch. 

  - In wcs.c, amended error return mapping for the lin routines to
    account for the addition of LINERR_BAD_PARAM in WCSLIB 8.7.
    Reported by Thomas Robitaille with patch. 

  - In wcshdo(), fixed a bug introduced in the last release in
    converting from sprintf() to snprintf() when writing the keycomment
    for AXIS.jhat.  Reported by Thomas Robitaille with patch.

  - New test program, twcsprintf_pthread, exercises wcsprintf() in
    threaded execution.  Contributed by Thomas Robitaille.

* Installation

  - Bumped the value of _POSIX_C_SOURCE to 200112L in configure.ac.  The
    old value, 1, restricts stdio.h to POSIX.1-1990 interfaces, which
    does not include snprintf().  WCSLIB was failing to compile on macOS
    with recent versions of clang (16 and later), which promote implicit
    function declarations from a warning to a hard error.  Reported by
    Thomas Robitaille.

  - In configure.ac, added tests for an appropriate thread-local storage
    (TLS) qualifier for C99 (it's native in C11).  Propagated by pre-
    processor macro WCSLIB_TLS via CPPFLAGS, specifically to the static
    variables in wcsprintf.c (see above).

* User manual

  - Updated the section on thread-safety in light of the above changes
    to wcsprintf and wcserr.