Add non-null and unused-result warnings for the external API.

GCC (and clang) supports extensions to annotate functions so that their
 results must be used and so that their arguments can't be statically
 provable to be null. If a caller violates these requirements they
 get a warning, so this helps them write correct code.

I deployed this in libopus a couple years ago with good success, and
 the implementation here is basically copied straight from that.

One consideration is that the non-null annotation teaches the optimizer
 and will actually compile out runtime non-nullness checks as dead-code.
 Since this is usually not whats wanted, the non-null annotations are
 disabled when compiling the library itself.

The commit also removes some dead inclusions of assert.h and introduces
 compatibility macros for restrict and inline in preparation for some
 portability improvements.

include/secp256k1.h

@@ -5,6 +5,54 @@
 extern "C" {
 # endif
 
+# if !defined(SECP256K1_GNUC_PREREQ)
+#  if defined(__GNUC__)&&defined(__GNUC_MINOR__)
+#   define SECP256K1_GNUC_PREREQ(_maj,_min) \
+ ((__GNUC__<<16)+__GNUC_MINOR__>=((_maj)<<16)+(_min))
+#  else
+#   define SECP256K1_GNUC_PREREQ(_maj,_min) 0
+#  endif
+# endif
+
+# if (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L) )
+#  if SECP256K1_GNUC_PREREQ(3,0)
+#   define SECP256K1_RESTRICT __restrict__
+#  elif (defined(_MSC_VER) && _MSC_VER >= 1400)
+#   define SECP256K1_RESTRICT __restrict
+#  else
+#   define SECP256K1_RESTRICT
+#  endif
+# else
+#  define SECP256K1_RESTRICT restrict
+# endif
+
+# if (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L) )
+#  if SECP256K1_GNUC_PREREQ(2,7)
+#   define SECP256K1_INLINE __inline__
+#  elif (defined(_MSC_VER))
+#   define SECP256K1_INLINE __inline
+#  else
+#   define SECP256K1_INLINE
+#  endif
+# else
+#  define SECP256K1_INLINE inline
+# endif
+
+/**Warning attributes
+  * NONNULL is not used if SECP256K1_BUILD is set to avoid the compiler optimizing out
+  * some paranoid null checks. */
+# if defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
+#  define SECP256K1_WARN_UNUSED_RESULT __attribute__ ((__warn_unused_result__))
+# else
+#  define SECP256K1_WARN_UNUSED_RESULT
+# endif
+# if !defined(SECP256K1_BUILD) && defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
+#  define SECP256K1_ARG_NONNULL(_x)  __attribute__ ((__nonnull__(_x)))
+# else
+#  define SECP256K1_ARG_NONNULL(_x)
+# endif
+
+
 /** Flags to pass to secp256k1_start. */
 # define SECP256K1_START_VERIFY (1 << 0)
 # define SECP256K1_START_SIGN   (1 << 1)
@@ -34,9 +82,14 @@ void secp256k1_stop(void);
  *           pubkeylen: the length of pubkey
  * Requires starting using SECP256K1_START_VERIFY.
  */
-int secp256k1_ecdsa_verify(const unsigned char *msg, int msglen,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_verify(
-                           const unsigned char *sig, int siglen,
+  const unsigned char *msg,
-                           const unsigned char *pubkey, int pubkeylen);
+  int msglen,
+  const unsigned char *sig,
+  int siglen,
+  const unsigned char *pubkey,
+  int pubkeylen
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(5);
 
 /** Create an ECDSA signature.
  *  Returns: 1: signature created
@@ -50,10 +103,14 @@ int secp256k1_ecdsa_verify(const unsigned char *msg, int msglen,
  *                   to contain the actual signature length (<=72).
  * Requires starting using SECP256K1_START_SIGN.
  */
-int secp256k1_ecdsa_sign(const unsigned char *msg, int msglen,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_sign(
-                         unsigned char *sig, int *siglen,
+  const unsigned char *msg,
+  int msglen,
+  unsigned char *sig,
+  int *siglen,
   const unsigned char *seckey,
-                         const unsigned char *nonce);
+  const unsigned char *nonce
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(6);
 
 /** Create a compact ECDSA signature (64 byte + recovery id).
  *  Returns: 1: signature created
@@ -66,11 +123,14 @@ int secp256k1_ecdsa_sign(const unsigned char *msg, int msglen,
  *           recid:  pointer to an int, which will be updated to contain the recovery id (can be NULL)
  * Requires starting using SECP256K1_START_SIGN.
  */
-int secp256k1_ecdsa_sign_compact(const unsigned char *msg, int msglen,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_sign_compact(
+  const unsigned char *msg,
+  int msglen,
   unsigned char *sig64,
   const unsigned char *seckey,
   const unsigned char *nonce,
-                                 int *recid);
+  int *recid
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
 
 /** Recover an ECDSA public key from a compact signature.
  *  Returns: 1: public key successfully recovered (which guarantees a correct signature).
@@ -84,17 +144,22 @@ int secp256k1_ecdsa_sign_compact(const unsigned char *msg, int msglen,
  *           pubkeylen:  pointer to an int that will contain the pubkey length (cannot be NULL)
  * Requires starting using SECP256K1_START_VERIFY.
  */
-int secp256k1_ecdsa_recover_compact(const unsigned char *msg, int msglen,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_recover_compact(
+  const unsigned char *msg,
+  int msglen,
   const unsigned char *sig64,
-                                    unsigned char *pubkey, int *pubkeylen,
+  unsigned char *pubkey,
-                                    int compressed, int recid);
+  int *pubkeylen,
+  int compressed,
+  int recid
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
 
 /** Verify an ECDSA secret key.
  *  Returns: 1: secret key is valid
  *           0: secret key is invalid
  *  In:      seckey: pointer to a 32-byte secret key (cannot be NULL)
  */
-int secp256k1_ec_seckey_verify(const unsigned char *seckey);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_seckey_verify(const unsigned char *seckey) SECP256K1_ARG_NONNULL(1);
 
 /** Just validate a public key.
  *  Returns: 1: valid public key
@@ -102,7 +167,7 @@ int secp256k1_ec_seckey_verify(const unsigned char *seckey);
  *  In:      pubkey:    pointer to a 33-byte or 65-byte public key (cannot be NULL).
  *           pubkeylen: length of pubkey
  */
-int secp256k1_ec_pubkey_verify(const unsigned char *pubkey, int pubkeylen);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_verify(const unsigned char *pubkey, int pubkeylen) SECP256K1_ARG_NONNULL(1);
 
 /** Compute the public key for a secret key.
  *  In:     compressed: whether the computed public key should be compressed
@@ -115,7 +180,12 @@ int secp256k1_ec_pubkey_verify(const unsigned char *pubkey, int pubkeylen);
  *           0: secret was invalid, try again.
  * Requires starting using SECP256K1_START_SIGN.
  */
-int secp256k1_ec_pubkey_create(unsigned char *pubkey, int *pubkeylen, const unsigned char *seckey, int compressed);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_create(
+  unsigned char *pubkey,
+  int *pubkeylen,
+  const unsigned char *seckey,
+  int compressed
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
 /** Decompress a public key.
  * In/Out: pubkey:    pointer to a 65-byte array to put the decompressed public key.
@@ -125,32 +195,55 @@ int secp256k1_ec_pubkey_create(unsigned char *pubkey, int *pubkeylen, const unsi
  * Returns: 0 if the passed public key was invalid, 1 otherwise. If 1 is returned, the
             pubkey is replaced with its decompressed version.
  */
-int secp256k1_ec_pubkey_decompress(unsigned char *pubkey, int *pubkeylen);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_decompress(
+  unsigned char *pubkey,
+  int *pubkeylen
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
 
 /** Export a private key in DER format. */
-int secp256k1_ec_privkey_export(const unsigned char *seckey,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_export(
-                                unsigned char *privkey, int *privkeylen,
+  const unsigned char *seckey,
-                                int compressed);
+  unsigned char *privkey,
+  int *privkeylen,
+  int compressed
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
 /** Import a private key in DER format. */
-int secp256k1_ec_privkey_import(unsigned char *seckey,
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_import(
-                                const unsigned char *privkey, int privkeylen);
+  unsigned char *seckey,
+  const unsigned char *privkey,
+  int privkeylen
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
 
 /** Tweak a private key by adding tweak to it. */
-int secp256k1_ec_privkey_tweak_add(unsigned char *seckey, const unsigned char *tweak);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_add(
+  unsigned char *seckey,
+  const unsigned char *tweak
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
 
 /** Tweak a public key by adding tweak times the generator to it.
  * Requires starting with SECP256K1_START_VERIFY.
  */
-int secp256k1_ec_pubkey_tweak_add(unsigned char *pubkey, int pubkeylen, const unsigned char *tweak);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_add(
+  unsigned char *pubkey,
+  int pubkeylen,
+  const unsigned char *tweak
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3);
 
 /** Tweak a private key by multiplying it with tweak. */
-int secp256k1_ec_privkey_tweak_mul(unsigned char *seckey, const unsigned char *tweak);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_mul(
+  unsigned char *seckey,
+  const unsigned char *tweak
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
 
 /** Tweak a public key by multiplying it with tweak.
  * Requires starting with SECP256K1_START_VERIFY.
  */
-int secp256k1_ec_pubkey_tweak_mul(unsigned char *pubkey, int pubkeylen, const unsigned char *tweak);
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_mul(
+  unsigned char *pubkey,
+  int pubkeylen,
+  const unsigned char *tweak
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3);
 
 # ifdef __cplusplus
 }

src/ecmult_gen_impl.h

@@ -5,7 +5,6 @@
 #ifndef _SECP256K1_ECMULT_GEN_IMPL_H_
 #define _SECP256K1_ECMULT_GEN_IMPL_H_
 
-#include <assert.h>
 #include "scalar.h"
 #include "group.h"
 #include "ecmult_gen.h"

src/ecmult_impl.h

@@ -5,7 +5,6 @@
 #ifndef _SECP256K1_ECMULT_IMPL_H_
 #define _SECP256K1_ECMULT_IMPL_H_
 
-#include <assert.h>
 #include "num.h"
 #include "group.h"
 #include "ecmult.h"

src/field_10x26_impl.h

@@ -6,7 +6,6 @@
 #define _SECP256K1_FIELD_REPR_IMPL_H_
 
 #include <stdio.h>
-#include <assert.h>
 #include <string.h>
 #include "util.h"
 #include "num.h"

src/field_5x52_impl.h

@@ -9,7 +9,6 @@
 #include "libsecp256k1-config.h"
 #endif
 
-#include <assert.h>
 #include <string.h>
 #include "util.h"
 #include "num.h"

src/num_gmp_impl.h

@@ -5,7 +5,6 @@
 #ifndef _SECP256K1_NUM_REPR_IMPL_H_
 #define _SECP256K1_NUM_REPR_IMPL_H_
 
-#include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 #include <gmp.h>

src/secp256k1.c

@@ -2,9 +2,10 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#define SECP256K1_BUILD (1)
+
 #include "include/secp256k1.h"
 
-#include <assert.h>
 #include "util.h"
 #include "num_impl.h"
 #include "field_impl.h"