examples: use EXIT_... constants for main return values

Fixes issue #1609.

examples/ellswift.c

@@ -13,6 +13,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <assert.h>
 #include <string.h>
 
@@ -38,7 +39,7 @@ int main(void) {
     ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
     if (!fill_random(randomize, sizeof(randomize))) {
         printf("Failed to generate randomness\n");
-        return 1;
+        return EXIT_FAILURE;
     }
     /* Randomizing the context is recommended to protect against side-channel
      * leakage. See `secp256k1_context_randomize` in secp256k1.h for more
@@ -49,14 +50,14 @@ int main(void) {
     /*** Generate secret keys ***/
     if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) {
         printf("Failed to generate randomness\n");
-        return 1;
+        return EXIT_FAILURE;
     }
     /* If the secret key is zero or out of range (greater than secp256k1's
     * order), we fail. Note that the probability of this occurring is negligible
     * with a properly functioning random number generator. */
     if (!secp256k1_ec_seckey_verify(ctx, seckey1) || !secp256k1_ec_seckey_verify(ctx, seckey2)) {
         printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
-        return 1;
+        return EXIT_FAILURE;
     }
 
     /* Generate ElligatorSwift public keys. This should never fail with valid context and
@@ -64,7 +65,7 @@ int main(void) {
        optional, but recommended. */
     if (!fill_random(auxrand1, sizeof(auxrand1)) || !fill_random(auxrand2, sizeof(auxrand2))) {
         printf("Failed to generate randomness\n");
-        return 1;
+        return EXIT_FAILURE;
     }
     return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey1, seckey1, auxrand1);
     assert(return_val);
@@ -117,5 +118,5 @@ int main(void) {
     secure_erase(shared_secret1, sizeof(shared_secret1));
     secure_erase(shared_secret2, sizeof(shared_secret2));
 
-    return 0;
+    return EXIT_SUCCESS;
 }