autotools, cmake: work around an Xcode 15+ issue.

There appears to be no way to build tcpdump on macOS Ventura with Xcode
15 with the system libpcap and have the resulting program run without
getting an error due to failing to find pcap_open() or
pcap_findalldevs_ex() at startup.

In particular, there appears to be no way to use __builtin_available()
to protect accesses to the routines that showed up in Sonoma, so that
the run-time linker doesn't fail if the routine in question isn't
present.  Perhaps it requires more compiler command-line arguments.

So, instead, only check for pcap_open() and pcap_findalldevs_ex() if 1)
this isn't macOS or 2) we're not building with the system libpcap.

Author: guyharris

CMakeLists.txt

@@ -713,7 +713,54 @@ check_function_exists(pcap_set_tstamp_precision HAVE_PCAP_SET_TSTAMP_PRECISION)
 #
 check_function_exists(pcap_set_immediate_mode HAVE_PCAP_SET_IMMEDIATE_MODE)
 check_function_exists(pcap_dump_ftell64 HAVE_PCAP_DUMP_FTELL64)
-check_function_exists(pcap_open HAVE_PCAP_OPEN)
+#
+# macOS Sonoma's libpcap includes stub versions of the remote-
+# capture APIs.  They are exported as "weakly linked symbols".
+#
+# Xcode 15 offers only a macOS Sonoma SDK, which has a .tbd
+# file for libpcap that claims it includes those APIs.  (Newer
+# versions of macOS don't provide the system shared libraries,
+# they only provide the dyld shared cache containing those
+# libraries, so the OS provides SDKs that include a .tbd file
+# to use when linking.)
+#
+# This means that check_function_exists() will think that
+# the remote-capture APIs are present, including pcap_open().
+#
+# However, they are *not* present in macOS Ventura and earlier,
+# which means that building on Ventura with Xcode 15 produces
+# executables that fail to start because one of those APIs
+# isn't found in the system libpcap.
+#
+# Protecting calls to those APIs with __builtin_available()
+# does not prevent this, because the libpcap header files
+# in the Sonoma SDK mark them as being first available
+# in macOS 10.13, just like all the other routines introduced
+# in libpcap 1.9, even though they're only available if libpcap
+# is built with remote capture enabled or stub routines are
+# provided.  (A fix to enable this has been checked into the
+# libpcap repository, and may end up in a later version of
+# the SDK.)
+#
+# Given all that, and given that the versions of the
+# remote-capture APIs in Sonoma are stubs that always fail,
+# there doesn't seem to be any point in checking for pcap_open()
+# if we're linking against the Apple libpcap.
+#
+# However, if we're *not* linking against the Apple libpcap,
+# we should check for it, so that we can use it if it's present.
+#
+# So we check for pcap_open if 1) this isn't macOS or 2) the
+# the libpcap we found is not a system library, meaning that
+# its path begins neither with /usr/lib (meaning it's a system
+# dylib) nor /Application/Xcode.app (meaning it's a file in
+# the Xcode SDK).
+#
+if(NOT APPLE OR NOT
+   (PCAP_LIBRARIES MATCHES "/usr/lib/.*" OR
+    PCAP_LIBRARIES MATCHES "/Application/Xcode.app/.*"))
+    check_function_exists(pcap_open HAVE_PCAP_OPEN)
+endif()
 
 #
 # On Windows, check for pcap_wsockinit(); if we don't have it, check for

aclocal.m4

@@ -454,6 +454,59 @@ AC_DEFUN(AC_LBL_LIBPCAP,
                         # ignore those values.
                         #
                         _broken_apple_pcap_config=yes
+
+                        #
+                        # Furthermore:
+                        #
+                        # macOS Sonoma's libpcap includes stub versions
+                        # of the remote-capture APIs.  They are exported
+                        # as "weakly linked symbols".
+                        #
+                        # Xcode 15 offers only a macOS Sonoma SDK, which
+                        # has a .tbd file for libpcap that claims it
+                        # includes those APIs.  (Newer versions of macOS
+                        # don't provide the system shared libraries,
+                        # they only provide the dyld shared cache
+                        # containing those libraries, so the OS provides
+                        # SDKs that include a .tbd file to use when
+                        # linking.)
+                        #
+                        # This means that AC_CHECK_FUNCS() will think
+                        # that the remote-capture APIs are present,
+                        # including pcap_open() and
+                        # pcap_findalldevs_ex().
+                        #
+                        # However, they are *not* present in macOS
+                        # Ventura and earlier, which means that building
+                        # on Ventura with Xcode 15 produces executables
+                        # that fail to start because one of those APIs
+                        # isn't found in the system libpcap.
+                        #
+                        # Protecting calls to those APIs with
+                        # __builtin_available() does not appear to
+                        # prevent this, for some unknown reason, and it
+                        # doesn't even allow the program to compile with
+                        # versions of Xcode prior to Xcode 15, as the
+                        # pcap.h file doesn't specify minimum OS
+                        # versions for those functions.
+                        #
+                        # Given all that, and given that the versions of
+                        # the remote-capture APIs in Sonoma are stubs
+                        # that always fail, there doesn't seem to be any
+                        # point in checking for pcap_open() if we're
+                        # linking against the Apple libpcap.
+                        #
+                        # However, if we're *not* linking against the
+                        # Apple libpcap, we should check for it, so that
+                        # we can use it if it's present.
+                        #
+                        # We know this is macOS and that we're using
+                        # the system-provided pcap-config to find
+                        # libpcap, so we know it'll be the system
+                        # libpcap, and note that we should not search
+                        # for remote-capture APIs.
+                        #
+                        _dont_check_for_remote_apis=yes
                         ;;
 
                     solaris*)

configure.ac

@@ -580,7 +580,14 @@ AC_CHECK_FUNCS(pcap_set_tstamp_precision)
 # if we have them.
 #
 AC_CHECK_FUNCS(pcap_set_immediate_mode pcap_dump_ftell64)
-AC_CHECK_FUNCS(pcap_open pcap_findalldevs_ex)
+#
+# See the comment in AC_LBL_LIBPCAP in aclocal.m4 for the reason
+# why we don't check for remote-capture APIs if we're building
+# with the system libpcap on macOS.
+#
+if test "$_dont_check_for_remote_apis" != "yes"; then
+	AC_CHECK_FUNCS(pcap_open pcap_findalldevs_ex)
+fi
 
 #
 # Check for special debugging functions