[hjmjohnson]

GetCurrentProcessFileName() reads /proc/curproc/file on FreeBSD, DragonFly, and OpenBSD. procfs is not mounted by default on modern FreeBSD, and OpenBSD has no procfs at all, so on a default install the readlink fails and the function returns nullptr. This asks the kernel through sysctl instead, keeping the procfs read as a fallback.

The BSD branch has been compiled against each BSD's real headers but not executed on a BSD host. A BSD reviewer confirming the returned path would close that gap.

The change

FreeBSD and DragonFly expose the executable path as CTL_KERN.KERN_PROC.KERN_PROC_PATHNAME; NetBSD spells it CTL_KERN.KERN_PROC_ARGS.KERN_PROC_PATHNAME:

#if defined(KERN_PROC_PATHNAME)
#if defined(__NetBSD__)
  int mib[4] = { CTL_KERN, KERN_PROC_ARGS, -1, KERN_PROC_PATHNAME };
#else
  int mib[4] = { CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1 };
#endif
  size_t pathlen = sizeof(path);
  if ( sysctl(mib, sizeof(mib) / sizeof(mib[0]), path, &pathlen, nullptr, 0) == 0 && pathlen > 1 )
    {
    return path;
    }
#endif

• The procfs readlink stays as a fallback, so a system that mounts procfs behaves as it does today.
• The inner guard is KERN_PROC_PATHNAME itself, a macro in <sys/sysctl.h>. The outer platform list remains, because <sys/sysctl.h> is only included on the BSDs.
• OpenBSD has no KERN_PROC_PATHNAME (confirmed in its 7.9 headers), so it keeps the fallback and is no worse off than before.

The commented-out NetBSD block is dropped now that NetBSD is handled.

readlink termination: hardening, not a live bug

readlink does not NUL-terminate and returns the buffer size when it truncates. Both call sites now pass sizeof(path) - 1 and terminate explicitly. The buffers are static, hence zero-initialized, so today this only matters for a link target at or beyond PATH_MAX.

Verification

Platform	Coverage
FreeBSD 15.1 headers and libc++ (clang 21 cross)	full gdcmSystem.cxx compiles with 0 warnings at -Wall -Wextra; the sysctl path is selected (KERN_PROC_PATHNAME = 12)
NetBSD 11.0 headers	the BSD block compiles clean at -Wall -Wextra -Werror; KERN_PROC_ARGS = 48, KERN_PROC_PATHNAME = 5
OpenBSD 7.9 headers	compiles clean at -Werror; no KERN_PROC_PATHNAME, fallback only
glibc x86_64 (Ubuntu 24.04), musl (Alpine), i386 Debian	built and tested; failure set identical to master with gdcmData and gdcmDataExtra
macOS arm64	built and tested; failure set identical to master
FreeBSD / DragonFly / NetBSD / OpenBSD runtime	not executed

The NetBSD and OpenBSD sets carry no C++ standard library headers, so for those two the BSD block was extracted verbatim from gdcmSystem.cxx into a translation unit that needs only C headers.

Independent of #228 and #230. Touches the same file as #229 but a different function; the two merge cleanly.