diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index 0c8bfe4..3447f5b 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -308,10 +308,10 @@
 OPENSSL_EXPORT void BIO_copy_next_retry(BIO *bio);
 
 
-/* Printf functions.
- *
- * These functions are versions of printf functions that output to a BIO rather
- * than a FILE. */
+/* Printf functions. */
+
+/* BIO_printf behaves like |printf| but outputs to |bio| rather than a |FILE|.
+ * It returns the number of bytes written or a negative number on error. */
 OPENSSL_EXPORT int BIO_printf(BIO *bio, const char *format, ...)
     OPENSSL_PRINTF_FORMAT_FUNC(2, 3);
 
diff --git a/include/openssl/mem.h b/include/openssl/mem.h
index 46d75d1..ba48f07 100644
--- a/include/openssl/mem.h
+++ b/include/openssl/mem.h
@@ -114,14 +114,11 @@
  * representation of the given type. */
 #define DECIMAL_SIZE(type)	((sizeof(type)*8+2)/3+1)
 
-/* Printf functions.
- *
- * These functions are either OpenSSL wrappers for standard functions (i.e.
- * |BIO_snprintf| and |BIO_vsnprintf|) which don't exist in C89, or are
- * versions of printf functions that output to a BIO rather than a FILE. */
+/* BIO_snprintf has the same behavior as snprintf(3). */
 OPENSSL_EXPORT int BIO_snprintf(char *buf, size_t n, const char *format, ...)
     OPENSSL_PRINTF_FORMAT_FUNC(3, 4);
 
+/* BIO_vsnprintf has the same behavior as vsnprintf(3). */
 OPENSSL_EXPORT int BIO_vsnprintf(char *buf, size_t n, const char *format,
                                  va_list args)
     OPENSSL_PRINTF_FORMAT_FUNC(3, 0);