Fix for --gc-sections on NetBSD

This patch fixes http://bugs.ntp.org/1844, works around
http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=40401, by
improving the test for support of --gc-sections to run a program which
reads a file using stdio built with --gc-sections, instead of simply
link the binary.  This catches the buggy linker as the garbage
collection removes a tag NetBSD uses to distinguish its own elf
binaries from Linux ones, causing it to treat conftest as a Linux
binary and run it with the wrong syscall table.

diff --git a/configure.in b/configure.in
index 3b7d1a5814f1f9ce625e6228d75b985be28b73dc..340d9d2310587edefd41e3e270bd8b0b00cb7670 100644 (file)
--- a/configure.in
+++ b/configure.in
@@ -662,26 +662,64 @@ LIBEVENT_GC_SECTIONS=
 if test "$GCC" = yes && test "$enable_function_sections" = yes ; then
     AC_CACHE_CHECK(
        [if linker supports omitting unused code and data],
-       [libevent_cv_gc_sections_works],
+       [libevent_cv_gc_sections_runs],
        [
+           dnl  NetBSD will link but likely not run with --gc-sections
+           dnl  http://bugs.ntp.org/1844
+           dnl  http://gnats.netbsd.org/40401
+           dnl  --gc-sections causes attempt to load as linux elf, with
+           dnl  wrong syscalls in place.  Test a little gauntlet of
+           dnl  simple stdio read code checking for errors, expecting
+           dnl  enough syscall differences that the NetBSD code will
+           dnl  fail even with Linux emulation working as designed.
+           dnl  A shorter test could be refined by someone with access
+           dnl  to a NetBSD host with Linux emulation working.
            origCFLAGS="$CFLAGS"
            CFLAGS="$CFLAGS -Wl,--gc-sections"
            AC_LINK_IFELSE(
-               [AC_LANG_PROGRAM([[]], [[]])],
+               [AC_LANG_PROGRAM(
+                   [[
+                       #include <stdlib.h>
+                       #include <stdio.h>
+                   ]],
+                   [[
+                       FILE *  fpC;
+                       char    buf[32];
+                       size_t  cch;
+                       int     read_success_once;
+
+                       fpC = fopen("conftest.c", "r");
+                       if (NULL == fpC)
+                               exit(1);
+                       do {
+                               cch = fread(buf, sizeof(buf), 1, fpC);
+                               read_success_once |= (0 != cch);
+                       } while (0 != cch);
+                       if (!read_success_once)
+                               exit(2);
+                       if (!feof(fpC))
+                               exit(3);
+                       if (0 != fclose(fpC))
+                               exit(4);
+
+                       exit(EXIT_SUCCESS);
+                   ]]
+               )],
                [
-                   if grep gc-sections conftest.err ; then
-                       libevent_cv_gc_sections_works=no
+                   if test "X$cross_compiling" = "Xyes" || grep gc-sections conftest.err ; then
+                       libevent_cv_gc_sections_runs=no
                    else
-                       libevent_cv_gc_sections_works=yes
+                       libevent_cv_gc_sections_runs=no
+                       ./conftest >/dev/null 2>&1 && libevent_cv_gc_sections_runs=yes
                    fi
                ],
-               [libevent_cv_gc_sections_works=no]
+               [libevent_cv_gc_sections_runs=no]
            )
            CFLAGS="$origCFLAGS"
            AS_UNSET([origCFLAGS])
        ]
     )
-    case "$libevent_cv_gc_sections_works" in
+    case "$libevent_cv_gc_sections_runs" in
      yes)
        CFLAGS="-ffunction-sections -fdata-sections $CFLAGS"
        LIBEVENT_GC_SECTIONS="-Wl,--gc-sections"