Test assumptions we make about the platform

Things that are not guaranteed by the standard but should be true of all
platforms of interest to us:
- 8-bit chars
- NULL pointers represented by all-bits-zero

include/mbedtls/check_config.h

@@ -30,6 +30,15 @@
 #ifndef MBEDTLS_CHECK_CONFIG_H
 #define MBEDTLS_CHECK_CONFIG_H
 
+/*
+ * We assume CHAR_BIT is 8 in many places. In practice, this is true on our
+ * target platforms, so not an issue, but let's just be extra sure.
+ */
+#include <limits.h>
+#if CHAR_BIT != 8
+#error "mbed TLS requires a platform with 8-bit chars"
+#endif
+
 #if defined(MBEDTLS_DEPRECATED_WARNING) && \
     !defined(__GNUC__) && !defined(__clang__)
 #error "MBEDTLS_DEPRECATED_WARNING only works with GCC and Clang"

programs/test/selftest.c

@@ -72,6 +72,19 @@ int main( int argc, char *argv[] )
 #if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C)
     unsigned char buf[1000000];
 #endif
+    void *pointer;
+
+    /*
+     * The C standard doesn't guarantee that all-bits-0 is the representation
+     * of a NULL pointer. We do however use that in our code for initializing
+     * structures, which should work on every modern platform. Let's be sure.
+     */
+    memset( &pointer, 0, sizeof( void * ) );
+    if( pointer != NULL )
+    {
+        mbedtls_printf( "all-bits-zero is not a NULL pointer\n" );
+        return( 1 );
+    }
 
     if( argc == 2 && strcmp( argv[1], "-quiet" ) == 0 )
         v = 0;

tests/suites/main_test.function

@@ -216,6 +216,7 @@ int main()
     FILE *file;
     char buf[5000];
     char *params[50];
+    void *pointer;
 
 #if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C) && \
     !defined(TEST_SUITE_MEMORY_BUFFER_ALLOC)
@@ -223,6 +224,18 @@ int main()
     mbedtls_memory_buffer_alloc_init( alloc_buf, sizeof(alloc_buf) );
 #endif
 
+    /*
+     * The C standard doesn't guarantee that all-bits-0 is the representation
+     * of a NULL pointer. We do however use that in our code for initializing
+     * structures, which should work on every modern platform. Let's be sure.
+     */
+    memset( &pointer, 0, sizeof( void * ) );
+    if( pointer != NULL )
+    {
+        mbedtls_fprintf( stderr, "all-bits-zero is not a NULL pointer\n" );
+        return( 1 );
+    }
+
     file = fopen( filename, "r" );
     if( file == NULL )
     {