diff --git a/build-aux/build-mes.sh b/build-aux/build-mes.sh
index 4699eec0..397fdc09 100755
--- a/build-aux/build-mes.sh
+++ b/build-aux/build-mes.sh
@@ -32,7 +32,7 @@ fi
 [ "$mes_p" -a ! "$gcc_p" ] && cp -f lib/linux/$mes_arch/crt1.S lib/$mes_arch/crt1.S
 [ "$mes_p" -a ! "$gcc_p" ] && cp -f lib/linux/$mes_arch/crt1.o lib/$mes_arch/crt1.o
 
-[ ! "$mesc_p" -a ! "$posix_p" ] && (program_prefix= compile lib/linux/$mes_arch/crt0)
+ [ ! "$mesc_p" -a ! "$with_glibc_p" ] && (program_prefix= compile lib/linux/$mes_arch/crt0)
 [ "$mes_p" -a "$gcc_p" ] && (program_prefix= compile lib/linux/$mes_arch/crti)
 [ "$mes_p" -a "$gcc_p" ] && (program_prefix= compile lib/linux/$mes_arch/crtn)
 
diff --git a/build-aux/build.sh.in b/build-aux/build.sh.in
index 68727d30..d9704677 100644
--- a/build-aux/build.sh.in
+++ b/build-aux/build.sh.in
@@ -34,7 +34,7 @@ elif [ -d "$MES_SEED" ]; then
 fi
 
 ## FIXME: remove this and have user configure/build/install for each compiler?
-unset CFLAGS CPPFLAGS LDFLAGS gcc_p tcc_p posix_p
+unset CFLAGS CPPFLAGS LDFLAGS gcc_p tcc_p with_glibc_p
 MES=guile
 mesc_p=1
 mes_p=1
diff --git a/build-aux/cc.sh b/build-aux/cc.sh
index 0d03235b..20d1ee07 100755
--- a/build-aux/cc.sh
+++ b/build-aux/cc.sh
@@ -41,7 +41,7 @@ archive () {
 
 link () {
     lib=$libc
-    [ "$posix_p" ] && lib='-l mes'
+    [ "$with_glibc_p" ] && lib='-l mes'
     out=$(dirname "$1")/${program_prefix}$(basename "$1")
     trace "CCLD       $1" $CC $CFLAGS $LDFLAGS -o" $out" $crt1 "$1".${program_prefix}o $2 $lib
 }
diff --git a/build-aux/config.make.in b/build-aux/config.make.in
index caffe8d4..19ad218d 100644
--- a/build-aux/config.make.in
+++ b/build-aux/config.make.in
@@ -64,7 +64,7 @@ infodir:=@infodir@
 libdir:=@libdir@
 mandir:=@mandir@
 moduledir:=@moduledir@
-posix_p:=@posix_p@
+with_glibc_p:=@with_glibc_p@
 program_prefix:=@program_prefix@
 srcdest:=@srcdest@
 srcdir:=@srcdir@
diff --git a/build-aux/config.sh b/build-aux/config.sh
index 75d1d42b..de59582b 100644
--- a/build-aux/config.sh
+++ b/build-aux/config.sh
@@ -81,7 +81,7 @@ export mes_p
 export mesc_p
 export tcc_p
 export mes_arch
-export posix_p
+export with_glibc_p
 
 CPPFLAGS=${CPPFLAGS-"
 -D 'VERSION=\"$VERSION\"'
@@ -92,7 +92,7 @@ CPPFLAGS=${CPPFLAGS-"
 -I ${srcdest}include
 "}
 
-[ "$posix_p" ] && CPPFLAGS="$CPPFLAGS -D POSIX=1 -D WITH_GLIBC=1"
+[ "$with_glibc_p" ] && CPPFLAGS="$CPPFLAGS -D WITH_GLIBC=1"
 
 LDFLAGS=${LDFLAGS-"
 -v
diff --git a/build-aux/config.status.in b/build-aux/config.status.in
index b7705007..cbe88d40 100644
--- a/build-aux/config.status.in
+++ b/build-aux/config.status.in
@@ -64,7 +64,7 @@ infodir="@infodir@"
 libdir="@libdir@"
 mandir="@mandir@"
 moduledir="@moduledir@"
-posix_p="@posix_p@"
+with_glibc_p="@with_glibc_p@"
 program_prefix="@program_prefix@"
 srcdest="@srcdest@"
 srcdir="@srcdir@"
diff --git a/build-aux/test.sh b/build-aux/test.sh
index eb9bba14..55a4fed6 100755
--- a/build-aux/test.sh
+++ b/build-aux/test.sh
@@ -26,7 +26,7 @@ sh ${srcdest}build-aux/test-cc.sh $1
 
 if [ ! "$mesc_p" ]; then
     #FIXME: c&p
-    unset CFLAGS CPPFLAGS LDFLAGS gcc_p tcc_p posix_p
+    unset CFLAGS CPPFLAGS LDFLAGS gcc_p tcc_p with_glibc_p
     MES=${MES-guile}
     mesc_p=1
     mes_p=1
diff --git a/configure b/configure
index 6615e5cc..520007d1 100755
--- a/configure
+++ b/configure
@@ -533,7 +533,7 @@ Some influential environment variables:
                      ("@mes_arch@" . ,mes-arch)
                      ("@mes_p@" . ,(if mes? "1" ""))
                      ("@mesc_p@" . ,(if mesc? "1" ""))
-                     ("@posix_p@" . ,(if posix? "1" ""))
+                     ("@with_glibc_p@" . ,(if posix? "1" ""))
                      ("@tcc_p@" . ,(if tcc? "1" ""))
 
                      ("@abs_top_srcdir@" . ,abs-top-srcdir)
diff --git a/configure.sh b/configure.sh
index 701eac05..a2798cef 100755
--- a/configure.sh
+++ b/configure.sh
@@ -111,7 +111,7 @@ subst () {
     -e s,"@mesc_p@,$mesc_p,"\
     -e s,"@tcc_p@,$tcc_p,"\
     -e s,"@mes_arch@,$mes_arch,"\
-    -e s,"@posix_p@,$posix_p,"\
+    -e s,"@with_glibc_p@,$with_glibc_p,"\
     -e s,"@abs_top_srcdir@,$abs_top_srcdir,"\
     -e s,"@abs_top_builddir@,$abs_top_builddir,"\
     -e s,"@top_builddir@,$top_builddir,"\
@@ -195,7 +195,7 @@ if [ ! "$mesc_p" ]; then
     mes_arch=$mes_arch-$compiler
 fi
 if [ ! "$mesc_p" -a ! "$mes_p" ]; then
-    posix_p=1
+    with_glibc_p=1
 fi
 
 subst ${srcdest}mes/module/mes/boot-0.scm.in mes/module/mes/boot-0.scm
diff --git a/include/libmes.h b/include/libmes.h
index 5377b1db..e15a0e40 100644
--- a/include/libmes.h
+++ b/include/libmes.h
@@ -23,6 +23,11 @@
 
 #include <libmes-mini.h>
 
+#if WITH_GLIBC
+int mes_open (char const *file_name, int flags, ...);
+#define open mes_open
+#endif
+
 int __mes_debug ();
 void __ungetc_init ();
 void __ungetc_clear (int filedes);
diff --git a/include/linux/x86_64/syscall.h b/include/linux/x86_64/syscall.h
index 269546e2..5ad16fb8 100644
--- a/include/linux/x86_64/syscall.h
+++ b/include/linux/x86_64/syscall.h
@@ -77,7 +77,7 @@
 #define SYS_getegid   0x6c
 #define SYS_getppid   0x6e
 
-// make+POSIX
+// make+WITH_GLIBC
 #define SYS_rt_sigprocmask 0x0e
 
 #endif // __MES_LINUX_X86_64_SYSCALL_H
diff --git a/lib/libmes.c b/lib/libmes.c
index 0c32f3da..49974b67 100644
--- a/lib/libmes.c
+++ b/lib/libmes.c
@@ -41,7 +41,10 @@
 #include <mes/fdputs.c>
 #include <mes/fdungetc.c>
 
-#if POSIX
+#if WITH_GLIBC
+#undef open
+#include <fcntl.h>
+#include <stdarg.h>
 // The Mes C Library defines and initializes these in crt1
 int __stdin = STDIN;
 int __stdout = STDOUT;
@@ -63,7 +66,7 @@ mes_open (char const *file_name, int flags, ...)
 
 #include <mes/eputs.c>
 #include <mes/oputs.c>
-#endif // POSIX
+#endif // WITH_GLIBC
 
 #include <mes/eputc.c>
 #include <mes/oputc.c>
diff --git a/lib/tests/stdio/70-printf-hello.c b/lib/tests/stdio/70-printf-hello.c
index 7169a334..f61246af 100644
--- a/lib/tests/stdio/70-printf-hello.c
+++ b/lib/tests/stdio/70-printf-hello.c
@@ -27,7 +27,7 @@
 int
 main ()
 {
-#if __GNUC__ && __x86_64__ && !POSIX
+#if __GNUC__ && __x86_64__ && !WITH_GLIBC
   return 0;
 #endif
   int i = 42;
diff --git a/lib/tests/stdio/70-printf-simple.c b/lib/tests/stdio/70-printf-simple.c
index b8a3a40e..02814bc0 100644
--- a/lib/tests/stdio/70-printf-simple.c
+++ b/lib/tests/stdio/70-printf-simple.c
@@ -27,7 +27,7 @@
 int
 main ()
 {
-#if __GNUC__ && __x86_64__ && !POSIX
+#if __GNUC__ && __x86_64__ && !WITH_GLIBC
   return 0;
 #endif
   char *s = "mes";
diff --git a/lib/tests/stdio/80-sscanf.c b/lib/tests/stdio/80-sscanf.c
index 34313ecc..7691eba2 100644
--- a/lib/tests/stdio/80-sscanf.c
+++ b/lib/tests/stdio/80-sscanf.c
@@ -26,7 +26,7 @@
 int
 main ()
 {
-#if __GNUC__ && __x86_64__ && !POSIX
+#if __GNUC__ && __x86_64__ && !WITH_GLIBC
   return 0;
 #endif
   int i;
diff --git a/lib/tests/stdlib/50-malloc.c b/lib/tests/stdlib/50-malloc.c
index 4d022d1e..fd3cb8e3 100644
--- a/lib/tests/stdlib/50-malloc.c
+++ b/lib/tests/stdlib/50-malloc.c
@@ -18,8 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#if POSIX
-#error "POSIX not supported"
+#if WITH_GLIBC
+#error "WITH_GLIBC not supported"
 #endif
 
 #include <libmes.h>
diff --git a/module/mescc/preprocess.scm b/module/mescc/preprocess.scm
index 0f6607e6..cb046013 100644
--- a/module/mescc/preprocess.scm
+++ b/module/mescc/preprocess.scm
@@ -84,8 +84,8 @@
      #:cpp-defs `(
                   "NULL=0"
                   "__linux__=1"
-                  "POSIX=0"
                   "_POSIX_SOURCE=0"
+                  "WITH_GLIBC=0"
                   "__STDC__=1"
                   "__MESC__=1"
                   ,(if mes? "__MESC_MES__=1" "__MESC_MES__=0")
diff --git a/scaffold/cons-mes.c b/scaffold/cons-mes.c
index 09a06691..41b3506d 100644
--- a/scaffold/cons-mes.c
+++ b/scaffold/cons-mes.c
@@ -18,8 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#if POSIX
-#error "POSIX not supported"
+#if WITH_GLIBC
+#error "WITH_GLIBC not supported"
 #endif
 
 #include <stdio.h>
diff --git a/scaffold/lib/stdlib/malloc.c b/scaffold/lib/stdlib/malloc.c
index 6491fd95..b3f26128 100644
--- a/scaffold/lib/stdlib/malloc.c
+++ b/scaffold/lib/stdlib/malloc.c
@@ -18,8 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#if POSIX
-#error "POSIX not supported"
+#if WITH_GLIBC
+#error "WITH_GLIBC not supported"
 #endif
 
 #include <libmes.h>
diff --git a/scaffold/micro-mes.c b/scaffold/micro-mes.c
index 5b539bf6..4dda7a7c 100644
--- a/scaffold/micro-mes.c
+++ b/scaffold/micro-mes.c
@@ -18,8 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#if POSIX
-#error "POSIX not supported"
+#if WITH_GLIBC
+#error "WITH_GLIBC not supported"
 #endif
 
 #include <stdio.h>
diff --git a/scaffold/tests/61-array.c b/scaffold/tests/61-array.c
index 18b62399..6f024ab6 100644
--- a/scaffold/tests/61-array.c
+++ b/scaffold/tests/61-array.c
@@ -26,7 +26,7 @@
 
 char *env[] = {"foo", "bar", "baz", 0};
 
-#if 0//!POSIX
+#if 0//!WITH_GLIBC
 #define getenv xgetenv
 
 char *
diff --git a/scaffold/tiny-mes.c b/scaffold/tiny-mes.c
index a7f6566e..32c92bd4 100644
--- a/scaffold/tiny-mes.c
+++ b/scaffold/tiny-mes.c
@@ -18,8 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#if POSIX
-#error "POSIX not supported"
+#if WITH_GLIBC
+#error "WITH_GLIBC not supported"
 #endif
 
 #include <libmes.h>
diff --git a/src/mes.c b/src/mes.c
index be9133a0..fe31b968 100644
--- a/src/mes.c
+++ b/src/mes.c
@@ -27,7 +27,7 @@
 #include <libmes.h>
 
 //#define MES_MINI 1
-#if POSIX
+#if WITH_GLIBC
 long ARENA_SIZE = 100000000; // 2.3GiB
 #else
 long ARENA_SIZE = 300000; // 32b: 3MiB, 64b: 6 MiB