Hook vfork() and replace it with fork().

tests/Makefile.am

@@ -1,9 +1,11 @@
 TESTS = run.sh
-check_PROGRAMS = example
+check_PROGRAMS = example example_vfork
 example_SOURCES = example.c
+example_vfork_SOURCES = example_vfork.c
 
 dist_check_SCRIPTS = run.sh lib.sh
 dist_check_DATA = example.expected \
+                  example_vfork.expected \
                   example-noforce.sh \
                   example-noforce.sh.expected \
                   example-redirects.sh \

tests/example_vfork.c

@@ -0,0 +1,43 @@
+/*
+ * Test issues with vfork().
+ *
+ * Copyright (C) 2013  Simon Ruderich
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+    pid_t pid;
+
+    fprintf(stderr, "Before vfork().\n");
+
+    pid = vfork();
+    if (pid == 0) {
+        /* This violates the POSIX standard! The "child" is only allowed to
+         * modify the result of vfork(), e.g. the pid variable. Some programs
+         * (e.g. gdb) do it anyway so we have to workaround it. */
+        dup2(STDOUT_FILENO, STDERR_FILENO);
+
+        _exit(2);
+    }
+
+    fprintf(stderr, "After vfork().\n");
+    puts("");
+
+    return EXIT_SUCCESS;
+}

tests/example_vfork.expected

@@ -0,0 +1,3 @@
+>stderr>Before vfork().
+<stderr<>stderr>After vfork().
+<stderr<

tests/run.sh

@@ -33,3 +33,4 @@ force_write=1
 test_script example-simple.sh
 test_script example-redirects.sh
 test_program example
+test_program example_vfork