diff --git a/crypto/bio/bio.h b/crypto/bio/bio.h
index ce8b19ce2e3c77dfab451f02942ef5eff96da2b2..e8bb8aa4deb09c339633640e6f799ba7642b6032 100644
--- a/crypto/bio/bio.h
+++ b/crypto/bio/bio.h
@@ -612,10 +612,17 @@ void BIO_copy_next_retry(BIO *b);
 
 /*long BIO_ghbn_ctrl(int cmd,int iarg,char *parg);*/
 
-int BIO_printf(BIO *bio, const char *format, ...);
-int BIO_vprintf(BIO *bio, const char *format, va_list args);
-int BIO_snprintf(char *buf, size_t n, const char *format, ...);
-int BIO_vsnprintf(char *buf, size_t n, const char *format, va_list args);
+#ifndef __GNUC__
+#define __attribute__(x)
+#endif
+int BIO_printf(BIO *bio, const char *format, ...)
+	__attribute__((__format__(__printf__,2,3)));
+int BIO_vprintf(BIO *bio, const char *format, va_list args)
+	__attribute__((__format__(__printf__,2,0)));
+int BIO_snprintf(char *buf, size_t n, const char *format, ...)
+	__attribute__((__format__(__printf__,3,4)));
+int BIO_vsnprintf(char *buf, size_t n, const char *format, va_list args)
+	__attribute__((__format__(__printf__,3,0)));
 
 /* BEGIN ERROR CODES */
 /* The following lines are auto generated by the script mkerr.pl. Any changes