diff --git a/configure.ac b/configure.ac
index 0c4f20f..6947d0b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -113,9 +113,11 @@ AS_IF([test "$with_libc"           = yes], [with_libc=yes],           [with_libc
 #############
 
 AS_IF([test "$enable_tests"        = yes -a "$host_cpu" != "$build_cpu" ], AC_MSG_ERROR([can not build cross-platform tests]))
-AS_IF([test "$enable_tests"        = yes -a "$enable_freestanding" = yes], AC_MSG_ERROR([can not build freestanding tests]))
 AS_IF([test "$enable_tests_python" = yes -a "$host_cpu" != "$build_cpu" ], AC_MSG_ERROR([can not build cross-platform tests]))
+AS_IF([test "$enable_tests"        = yes -a "$enable_freestanding" = yes], AC_MSG_ERROR([can not build freestanding tests]))
 AS_IF([test "$enable_tests_python" = yes -a "$enable_freestanding" = yes], AC_MSG_ERROR([can not build freestanding tests]))
+AS_IF([test "$enable_tests"        = yes -a "$with_libc"           = yes], AC_MSG_ERROR([can not use package `libc' with tests]))
+AS_IF([test "$enable_tests_python" = yes -a "$with_libc"           = yes], AC_MSG_ERROR([can not use package `libc' with tests]))
 
 AS_IF([test "$with_printf" = yes -a "$with_ntoa"       = no], AC_MSG_ERROR([package `printf' requires package `ntoa']))
 AS_IF([test "$with_printf" = yes -a "$with_printf_fmt" = no], AC_MSG_ERROR([package `printf' requires package `printf-fmt']))
diff --git a/libc/string.h b/libc/string.h
index 5be69eb..2808391 100644
--- a/libc/string.h
+++ b/libc/string.h
@@ -7,6 +7,7 @@ extern "C" {
 
 #include <stddef.h>
 
+int memcmp(const void *s1, const void *s2, size_t n);
 void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);
diff --git a/src/libc.c b/src/libc.c
index 7394aa1..2ecb1a1 100644
--- a/src/libc.c
+++ b/src/libc.c
@@ -29,10 +29,18 @@ int isspace(const int c)
     return c == ' ' || (unsigned)c - '\t' < 5;
 }
 
+int memcmp(const void *s1, const void *s2, size_t n)
+{
+    for (const unsigned char *p1 = s1, *p2 = s2; n--; ++p1, ++p2) {
+        if (*p1 != *p2) return *p1 - *p2;
+    }
+    return 0;
+}
+
 void *memcpy(void *dest, const void *src, size_t n)
 {
     char *dest_cell = dest;
-    char *src_cell = src;
+    char *src_cell = (char*)src;
     while (n--) *dest_cell++ = *src_cell++;
     return dest;
 }
@@ -40,7 +48,7 @@ void *memcpy(void *dest, const void *src, size_t n)
 void *memmove(void *dest, const void *src, size_t n)
 {
     char *dest_cell = dest;
-    char *src_cell = src;
+    char *src_cell = (char*)src;
     if (dest_cell <= src_cell) {
         while (n--) *dest_cell++ = *src_cell++;
     } else {