diff --git a/configure.ac b/configure.ac
index a390d0c..dbad6f4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -132,6 +132,7 @@ AC_CONFIG_FILES([
 	man/Makefile
 	src/Makefile
 	src/libbsd.pc
+	src/libbsd-ctor.pc
 	src/libbsd-overlay.pc
 	test/Makefile
 ])
diff --git a/include/bsd/unistd.h b/include/bsd/unistd.h
index 193dd35..5dfa605 100644
--- a/include/bsd/unistd.h
+++ b/include/bsd/unistd.h
@@ -56,6 +56,10 @@ void *setmode(const char *mode_str);
 
 void closefrom(int lowfd);
 
+/* Compatibility with sendmail implementations. */
+#define initsetproctitle(c, a, e) setproctitle_init((c), (a), (e))
+
+void setproctitle_init(int argc, char *argv[], char *envp[]);
 void setproctitle(const char *fmt, ...);
 
 int getpeereid(int s, uid_t *euid, gid_t *egid);
diff --git a/man/setproctitle.3 b/man/setproctitle.3
index d42498e..dd5df59 100644
--- a/man/setproctitle.3
+++ b/man/setproctitle.3
@@ -33,6 +33,8 @@
 .In sys/types.h
 .In bsd/unistd.h
 .Ft void
+.Fn setproctitle_init "int argc" "char *argv[]" "char *envp[]"
+.Ft void
 .Fn setproctitle "const char *fmt" "..."
 .Sh DESCRIPTION
 The
@@ -41,6 +43,15 @@ library routine sets the process title that appears on the
 .Xr ps 1
 command.
 .Pp
+The
+.Fn setproctitle_init
+library routine only needs to be called (before any call to
+.Fn setproctitle
+and with
+.Fn main
+arguments), if the automatic constructor support has not
+been linked in through the libbsd-ctor pkg-config file.
+.Pp
 The title is set from the executable's name, followed by the
 result of a
 .Xr printf 3
@@ -101,6 +112,11 @@ first appeared in
 .Fx 2.2 .
 Other operating systems have
 similar functions.
+.Pp
+The
+.Fn setproctitle_init
+function is a libbsd extension not present on the BSDs, avoid using it
+in portable code.
 .Sh AUTHORS
 .An -nosplit
 .An Peter Wemm Aq peter@FreeBSD.org
diff --git a/src/Makefile.am b/src/Makefile.am
index 96fee3e..46f8634 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -10,6 +10,7 @@ AM_CPPFLAGS = \
 EXTRA_DIST = \
 	libbsd.map \
 	libbsd.pc.in \
+	libbsd-ctor.pc.in \
 	libbsd-overlay.pc.in \
 	hash/helper.c \
 	$(nil)
@@ -21,10 +22,12 @@ CLEANFILES = \
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = \
 	libbsd.pc \
+	libbsd-ctor.pc \
 	libbsd-overlay.pc \
 	$(nil)
 
 lib_LTLIBRARIES = libbsd.la
+lib_LIBRARIES = libbsd-ctor.a
 
 hash/md5hl.c: $(srcdir)/hash/helper.c
 	$(AM_V_at) $(MKDIR_P) hash
@@ -75,6 +78,10 @@ libbsd_la_SOURCES = \
 	wcslcpy.c \
 	$(nil)
 
+libbsd_ctor_a_SOURCES = \
+	setproctitle_ctor.c \
+	$(nil)
+
 runtimelibdir = $(libdir)
 
 install-exec-hook:
diff --git a/src/libbsd-ctor.pc.in b/src/libbsd-ctor.pc.in
new file mode 100644
index 0000000..117b823
--- /dev/null
+++ b/src/libbsd-ctor.pc.in
@@ -0,0 +1,12 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: libbsd-ctor
+Description: Automatic constructor functions for libbsd
+Version: @VERSION@
+URL: http://libbsd.freedesktop.org/
+Cflags: -I${includedir}
+Libs: -L${libdir} -Wl,-z,nodlopen -Wl,-u,libbsd_init_func -lbsd-ctor
+Requires: libbsd
diff --git a/src/libbsd.map b/src/libbsd.map
index 4067e2c..52df212 100644
--- a/src/libbsd.map
+++ b/src/libbsd.map
@@ -105,3 +105,8 @@ LIBBSD_0.5 {
     wcslcat;
     wcslcpy;
 } LIBBSD_0.4;
+
+LIBBSD_0.6 {
+    /* Exported to cope with the constructor+dlopen+threads mess. */
+    setproctitle_init;
+} LIBBSD_0.5;
diff --git a/src/setproctitle.c b/src/setproctitle.c
index 7e4b165..305b563 100644
--- a/src/setproctitle.c
+++ b/src/setproctitle.c
@@ -153,8 +153,8 @@ spt_copyargs(int argc, char *argv[])
 	return 0;
 }
 
-static void
-spt_init(int argc, char *argv[], char *envp[])
+void
+setproctitle_init(int argc, char *argv[], char *envp[])
 {
 	char *base, *end, *nul, *tmp;
 	int i, envc, error;
@@ -215,14 +215,6 @@ spt_init(int argc, char *argv[], char *envp[])
 	SPT.end  = end;
 }
 
-/*
- * Force spt_init() function into the .init_array section instead of expecting
- * either the compiler to place constructors there or the linker to move them
- * from .ctors to .init_array.
- */
-void (*spt_init_func)(int argc, char *argv[], char *envp[])
-	__attribute__((section(".init_array"))) = spt_init;
-
 #ifndef SPT_MAXTITLE
 #define SPT_MAXTITLE 255
 #endif
diff --git a/src/setproctitle_ctor.c b/src/setproctitle_ctor.c
new file mode 100644
index 0000000..9360774
--- /dev/null
+++ b/src/setproctitle_ctor.c
@@ -0,0 +1,52 @@
+/*
+ * Copyright © 2013 Guillem Jover <guillem@hadrons.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
+ * THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+ * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+ * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <unistd.h>
+
+/*
+ * The automatic initialization cannot be part of the main shared library,
+ * because there is no thread-safe way to change the environ global
+ * variable. This is not a problem if the initializaion happens just at
+ * program load time, but becomes one if the shared library is directly or
+ * indirectly dlopen()ed during the execution of the program, which could
+ * have either kept references to the old environ or could change it in
+ * some other thread. This has been observed for example on systems using
+ * Samba NSS modules.
+ *
+ * To avoid any other possible fallout, the constructor is split into a
+ * new static library that needs to be linked explicitly into programs
+ * using setproctitle(). As an additional safety measure the pkg-config
+ * linker flags will mark the program as not allowing to be dlopen()ed
+ * so that we make sure to avoid the problem described above.
+ */
+
+/*
+ * Force setproctitle_init() function into the .init_array section instead of
+ * expecting either the compiler to place constructors there or the linker to
+ * move them from .ctors to .init_array.
+ */
+void (*libbsd_init_func)(int argc, char *argv[], char *envp[])
+	__attribute__((section(".init_array"))) = setproctitle_init;
diff --git a/test/Makefile.am b/test/Makefile.am
index 73a1e91..d0e0f0b 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -18,6 +18,10 @@ check_PROGRAMS = \
 
 humanize_LDFLAGS = $(top_builddir)/src/libbsd.la
 fgetln_LDFLAGS = $(top_builddir)/src/libbsd.la
-proctitle_LDFLAGS = $(top_builddir)/src/libbsd.la
+proctitle_LDFLAGS = \
+	-Wl,-u,libbsd_init_func \
+	$(top_builddir)/src/libbsd-ctor.a \
+	$(top_builddir)/src/libbsd.la \
+	$(nil)
 
 TESTS = $(check_PROGRAMS)