--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2010-12-19  Bruno Haible  <bruno@clisp.org>
+
+	sys_socket: Use POSIX compatible declarations on OSF/1 5.1.
+	* m4/sys_socket_h.m4 (gl_HEADER_SYS_SOCKET): On OSF/1, define
+	_POSIX_PII_SOCKET.
+	* doc/posix-functions/recv.texi: Document the OSF/1 problem.
+	* doc/posix-functions/recvfrom.texi: Likewise.
+	* doc/posix-functions/send.texi: Likewise.
+	* doc/posix-functions/sendto.texi: Likewise.
+
 2010-12-19  Bruno Haible  <bruno@clisp.org>
 
 	tcgetsid: Add missing declaration on OSF/1 5.1.

--- a/doc/posix-functions/recv.texi
+++ b/doc/posix-functions/recv.texi
@@ -9,6 +9,10 @@
 Portability problems fixed by Gnulib:
 @itemize
 @item
+This function has the return type @code{int} instead of @code{ssize_t}
+on some platforms:
+OSF/1 5.1.
+@item
 On Windows platforms (excluding Cygwin), error codes for @code{recv}
 are not placed in @code{errno}, and @code{WSAGetLastError} must be
 used instead.

--- a/doc/posix-functions/recvfrom.texi
+++ b/doc/posix-functions/recvfrom.texi
@@ -9,6 +9,10 @@
 Portability problems fixed by Gnulib:
 @itemize
 @item
+This function has the return type @code{int} instead of @code{ssize_t}
+on some platforms:
+OSF/1 5.1.
+@item
 On Windows platforms (excluding Cygwin), error codes for
 @code{recvfrom} are not placed in @code{errno}, and
 @code{WSAGetLastError} must be used instead.

--- a/doc/posix-functions/send.texi
+++ b/doc/posix-functions/send.texi
@@ -9,6 +9,10 @@
 Portability problems fixed by Gnulib:
 @itemize
 @item
+This function has the return type @code{int} instead of @code{ssize_t}
+on some platforms:
+OSF/1 5.1.
+@item
 On Windows platforms (excluding Cygwin), error codes for @code{send}
 are not placed in @code{errno}, and @code{WSAGetLastError} must be
 used instead.

--- a/doc/posix-functions/sendto.texi
+++ b/doc/posix-functions/sendto.texi
@@ -9,6 +9,10 @@
 Portability problems fixed by Gnulib:
 @itemize
 @item
+This function has the return type @code{int} instead of @code{ssize_t}
+on some platforms:
+OSF/1 5.1.
+@item
 On Windows platforms (excluding Cygwin), error codes for @code{sendto}
 are not placed in @code{errno}, and @code{WSAGetLastError} must be
 used instead.

--- a/m4/sys_socket_h.m4
+++ b/m4/sys_socket_h.m4
@@ -1,4 +1,4 @@
-# sys_socket_h.m4 serial 20
+# sys_socket_h.m4 serial 21
 dnl Copyright (C) 2005-2010 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -9,8 +9,20 @@
 AC_DEFUN([gl_HEADER_SYS_SOCKET],
 [
   AC_REQUIRE([gl_SYS_SOCKET_H_DEFAULTS])
+  AC_REQUIRE([AC_CANONICAL_HOST])
   AC_REQUIRE([AC_C_INLINE])
 
+  dnl On OSF/1, the functions recv(), send(), recvfrom(), sendto() have
+  dnl old-style declarations (with return type 'int' instead of 'ssize_t')
+  dnl unless _POSIX_PII_SOCKET is defined.
+  case "$host_os" in
+    osf*)
+      AC_DEFINE([_POSIX_PII_SOCKET], [1],
+        [Define to 1 in order to get the POSIX compatible declarations
+         of socket functions.])
+      ;;
+  esac
+
   AC_CACHE_CHECK([whether <sys/socket.h> is self-contained],
     [gl_cv_header_sys_socket_h_selfcontained],
     [