diff --git a/docs/examples/htmltitle.cpp b/docs/examples/htmltitle.cpp
index 7986b94640..cce6d16e11 100644
--- a/docs/examples/htmltitle.cpp
+++ b/docs/examples/htmltitle.cpp
@@ -73,7 +73,7 @@ static std::string buffer;
 static size_t writer(char *data, size_t size, size_t nmemb,
                      std::string *writerData)
 {
-  if(writerData == NULL)
+  if(!writerData)
     return 0;
 
   writerData->append(data, size * nmemb);
diff --git a/lib/cfilters.h b/lib/cfilters.h
index 17cc634b37..13bb428b55 100644
--- a/lib/cfilters.h
+++ b/lib/cfilters.h
@@ -657,7 +657,7 @@ struct cf_call_data {
 #define CF_DATA_SAVE(save, cf, data) \
   do { \
     (save) = CF_CTX_CALL_DATA(cf); \
-    DEBUGASSERT((save).data == NULL || (save).depth > 0); \
+    DEBUGASSERT(!(save).data || (save).depth > 0); \
     CF_CTX_CALL_DATA(cf).depth++;  \
     CF_CTX_CALL_DATA(cf).data = (struct Curl_easy *)CURL_UNCONST(data); \
   } while(0)
@@ -665,7 +665,7 @@ struct cf_call_data {
 #define CF_DATA_RESTORE(cf, save) \
   do { \
     DEBUGASSERT(CF_CTX_CALL_DATA(cf).depth == (save).depth + 1); \
-    DEBUGASSERT((save).data == NULL || (save).depth > 0); \
+    DEBUGASSERT(!(save).data || (save).depth > 0); \
     CF_CTX_CALL_DATA(cf) = (save); \
   } while(0)
 
diff --git a/lib/easy.c b/lib/easy.c
index ce3d00200a..8d61241952 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -942,7 +942,7 @@ static void dupeasy_meta_freeentry(void *p)
   /* Always FALSE. Cannot use a 0 assert here since compilers
    * are not in agreement if they then want a NORETURN attribute or
    * not. *sigh* */
-  DEBUGASSERT(p == NULL);
+  DEBUGASSERT(!p);
 }
 
 /*
diff --git a/lib/ftplistparser.c b/lib/ftplistparser.c
index a4fffc86dc..f205848b6d 100644
--- a/lib/ftplistparser.c
+++ b/lib/ftplistparser.c
@@ -196,7 +196,7 @@ void Curl_wildcard_dtor(struct WildcardData **wcp)
     wc->dtor = ZERO_NULL;
     wc->ftpwc = NULL;
   }
-  DEBUGASSERT(wc->ftpwc == NULL);
+  DEBUGASSERT(!wc->ftpwc);
 
   Curl_llist_destroy(&wc->filelist, NULL);
   curlx_safefree(wc->path);
diff --git a/lib/multi.c b/lib/multi.c
index fe9bcfaeaf..dd29328a17 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -220,7 +220,7 @@ static void ph_freeentry(void *p)
   /* Always FALSE. Cannot use a 0 assert here since compilers
    * are not in agreement if they then want a NORETURN attribute or
    * not. *sigh* */
-  DEBUGASSERT(p == NULL);
+  DEBUGASSERT(!p);
 }
 
 /*
diff --git a/lib/pingpong.c b/lib/pingpong.c
index 6952d659a9..ae3f7faa30 100644
--- a/lib/pingpong.c
+++ b/lib/pingpong.c
@@ -152,7 +152,7 @@ CURLcode Curl_pp_vsendf(struct Curl_easy *data,
 
   DEBUGASSERT(pp->sendleft == 0);
   DEBUGASSERT(pp->sendsize == 0);
-  DEBUGASSERT(pp->sendthis == NULL);
+  DEBUGASSERT(!pp->sendthis);
 
   if(!conn)
     /* cannot send without a connection! */
diff --git a/lib/url.c b/lib/url.c
index 99463551e8..bc9308438a 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -442,7 +442,7 @@ static void easy_meta_freeentry(void *p)
   /* Always FALSE. Cannot use a 0 assert here since compilers
    * are not in agreement if they then want a NORETURN attribute or
    * not. *sigh* */
-  DEBUGASSERT(p == NULL);
+  DEBUGASSERT(!p);
 }
 
 /**
@@ -2591,7 +2591,7 @@ static void conn_meta_freeentry(void *p)
   /* Always FALSE. Cannot use a 0 assert here since compilers
    * are not in agreement if they then want a NORETURN attribute or
    * not. *sigh* */
-  DEBUGASSERT(p == NULL);
+  DEBUGASSERT(!p);
 }
 
 static CURLcode url_create_needle(struct Curl_easy *data,
diff --git a/lib/vssh/libssh.c b/lib/vssh/libssh.c
index 4842ff3a91..8bad2ec4ae 100644
--- a/lib/vssh/libssh.c
+++ b/lib/vssh/libssh.c
@@ -1888,8 +1888,8 @@ static void sshc_cleanup(struct ssh_conn *sshc)
     }
 
     /* worst-case scenario cleanup */
-    DEBUGASSERT(sshc->ssh_session == NULL);
-    DEBUGASSERT(sshc->scp_session == NULL);
+    DEBUGASSERT(!sshc->ssh_session);
+    DEBUGASSERT(!sshc->scp_session);
 
     if(sshc->readdir_tmp) {
       ssh_string_free_char(sshc->readdir_tmp);
diff --git a/lib/vssh/libssh2.c b/lib/vssh/libssh2.c
index 027ef71b99..e0dccc7dde 100644
--- a/lib/vssh/libssh2.c
+++ b/lib/vssh/libssh2.c
@@ -152,8 +152,7 @@ static void kbd_callback(const char *name, int name_len,
     /* this function must allocate memory that can be freed by libssh2, which
        uses the LIBSSH2_FREE_FUNC callback */
     responses[0].text = Curl_cstrdup(passwd);
-    responses[0].length =
-      responses[0].text == NULL ? 0 : curlx_uztoui(strlen(passwd));
+    responses[0].length = responses[0].text ? curlx_uztoui(strlen(passwd)) : 0;
   }
   (void)prompts;
 } /* kbd_callback */
@@ -2607,12 +2606,12 @@ static CURLcode sshc_cleanup(struct ssh_conn *sshc, struct Curl_easy *data,
   }
 
   /* worst-case scenario cleanup */
-  DEBUGASSERT(sshc->ssh_session == NULL);
-  DEBUGASSERT(sshc->ssh_channel == NULL);
-  DEBUGASSERT(sshc->sftp_session == NULL);
-  DEBUGASSERT(sshc->sftp_handle == NULL);
-  DEBUGASSERT(sshc->kh == NULL);
-  DEBUGASSERT(sshc->ssh_agent == NULL);
+  DEBUGASSERT(!sshc->ssh_session);
+  DEBUGASSERT(!sshc->ssh_channel);
+  DEBUGASSERT(!sshc->sftp_session);
+  DEBUGASSERT(!sshc->sftp_handle);
+  DEBUGASSERT(!sshc->kh);
+  DEBUGASSERT(!sshc->ssh_agent);
 
   curlx_safefree(sshc->rsa_pub);
   curlx_safefree(sshc->rsa);
diff --git a/lib/vtls/rustls.c b/lib/vtls/rustls.c
index 90a37cbeda..d69ec03f8f 100644
--- a/lib/vtls/rustls.c
+++ b/lib/vtls/rustls.c
@@ -1100,7 +1100,7 @@ static CURLcode cr_init_backend(struct Curl_cfilter *cf,
     return CURLE_SSL_CONNECT_ERROR;
   }
 
-  DEBUGASSERT(rconn == NULL);
+  DEBUGASSERT(!rconn);
   rr = rustls_client_connection_new(backend->config,
                                     connssl->peer.dest->hostname,
                                     &rconn);
diff --git a/src/tool_doswin.c b/src/tool_doswin.c
index 2864c47dea..f836af9506 100644
--- a/src/tool_doswin.c
+++ b/src/tool_doswin.c
@@ -773,7 +773,7 @@ curl_socket_t win32_stdin_read_thread(void)
     assert(stdin_thread);
     return socket_r;
   }
-  assert(stdin_thread == NULL);
+  assert(!stdin_thread);
 
   do {
     curl_socklen_t socksize = 0;
diff --git a/tests/libtest/lib1536.c b/tests/libtest/lib1536.c
index 9debd78807..cd31116194 100644
--- a/tests/libtest/lib1536.c
+++ b/tests/libtest/lib1536.c
@@ -76,8 +76,7 @@ static CURLcode test_lib1536(const char *URL)
   if(!scheme || memcmp(scheme, "http", 5) != 0) {
     curl_mfprintf(stderr, "%s:%d scheme of http resource is incorrect; "
                   "expected 'http' but is %s\n",
-                  __FILE__, __LINE__,
-                  (scheme == NULL ? "NULL" : "invalid"));
+                  __FILE__, __LINE__, scheme ? "invalid" : "NULL");
     result = CURLE_HTTP_RETURNED_ERROR;
     goto test_cleanup;
   }
diff --git a/tests/unit/README.md b/tests/unit/README.md
index 292e3bfa2f..28908a7a15 100644
--- a/tests/unit/README.md
+++ b/tests/unit/README.md
@@ -55,8 +55,8 @@ For the actual C file, here's a simple example:
 
       /* here you start doing things and checking that the results are good */
 
-      fail_unless( size == 0 , "initial size should be zero" );
-      fail_if( head == NULL , "head should not be initiated to NULL" );
+      fail_unless(size == 0, "initial size should be zero");
+      fail_if(!head, "head should not be initiated to NULL");
 
       /* you end the test code like this: */
 
@@ -87,8 +87,8 @@ Here's an example using optional initialization and cleanup:
 
       /* here you start doing things and checking that the results are good */
 
-      fail_unless( size == 0 , "initial size should be zero" );
-      fail_if( head == NULL , "head should not be initiated to NULL" );
+      fail_unless(size == 0, "initial size should be zero");
+      fail_if(!head, "head should not be initiated to NULL");
 
       /* you end the test code like this: */
 
diff --git a/tests/unit/unit1300.c b/tests/unit/unit1300.c
index 86112ee088..5db667cdf2 100644
--- a/tests/unit/unit1300.c
+++ b/tests/unit/unit1300.c
@@ -67,9 +67,9 @@ static CURLcode test_unit1300(const char *arg)
 
   fail_unless(Curl_llist_count(&llist) == 0,
               "list initial size should be zero");
-  fail_unless(Curl_llist_head(&llist) == NULL,
+  fail_unless(!Curl_llist_head(&llist),
               "list head should initiate to NULL");
-  fail_unless(llist_tail(&llist) == NULL,
+  fail_unless(!llist_tail(&llist),
               "list tail should initiate to NULL");
 
   /**
@@ -152,7 +152,7 @@ static CURLcode test_unit1300(const char *arg)
   fail_unless(Curl_llist_head(&llist) == element_next,
               "llist new head not modified properly");
   abort_unless(Curl_llist_head(&llist), "llist.head is NULL");
-  fail_unless(llist_node_prev(Curl_llist_head(&llist)) == NULL,
+  fail_unless(!llist_node_prev(Curl_llist_head(&llist)),
               "new head previous not set to null");
 
   /**
@@ -208,9 +208,9 @@ static CURLcode test_unit1300(const char *arg)
 
   to_remove = Curl_llist_head(&llist);
   Curl_node_remove(to_remove);
-  fail_unless(Curl_llist_head(&llist) == NULL,
+  fail_unless(!Curl_llist_head(&llist),
               "llist head is not NULL while the llist is empty");
-  fail_unless(llist_tail(&llist) == NULL,
+  fail_unless(!llist_tail(&llist),
               "llist tail is not NULL while the llist is empty");
 
   /**
diff --git a/tests/unit/unit1304.c b/tests/unit/unit1304.c
index a65a3021b8..3c51cceaa7 100644
--- a/tests/unit/unit1304.c
+++ b/tests/unit/unit1304.c
@@ -71,7 +71,7 @@ static CURLcode test_unit1304(const char *arg)
   Curl_netrc_init(&store);
   res = Curl_netrc_scan(data, &store, "test.example.com", NULL, arg, &cr_out);
   fail_unless(res == NETRC_NO_MATCH, "expected no match");
-  fail_unless(cr_out == NULL, "creds did not return NULL!");
+  fail_unless(!cr_out, "creds did not return NULL!");
   Curl_netrc_cleanup(&store);
 
   /*
diff --git a/tests/unit/unit1309.c b/tests/unit/unit1309.c
index b49a5de994..607e49aa88 100644
--- a/tests/unit/unit1309.c
+++ b/tests/unit/unit1309.c
@@ -97,7 +97,7 @@ static CURLcode test_unit1309(const char *arg)
     }
   }
 
-  fail_unless(root == NULL, "tree not empty after removing all nodes");
+  fail_unless(!root, "tree not empty after removing all nodes");
 
   /* rebuild tree */
   for(i = 0; i < NUM_NODES; i++) {
@@ -127,7 +127,7 @@ static CURLcode test_unit1309(const char *arg)
     }
   }
 
-  fail_unless(root == NULL, "tree not empty when it should be");
+  fail_unless(!root, "tree not empty when it should be");
 
   UNITTEST_END_SIMPLE
 }
diff --git a/tests/unit/unit1605.c b/tests/unit/unit1605.c
index 8c63dc3b19..baeb121085 100644
--- a/tests/unit/unit1605.c
+++ b/tests/unit/unit1605.c
@@ -53,10 +53,10 @@ static CURLcode test_unit1605(const char *arg)
   char *esc;
 
   esc = curl_easy_escape(easy, "", -1);
-  fail_unless(esc == NULL, "negative string length cannot work");
+  fail_unless(!esc, "negative string length cannot work");
 
   esc = curl_easy_unescape(easy, "%41%41%41%41", -1, &len);
-  fail_unless(esc == NULL, "negative string length cannot work");
+  fail_unless(!esc, "negative string length cannot work");
 
   UNITTEST_END(t1605_stop(easy))
 }
diff --git a/tests/unit/unit1620.c b/tests/unit/unit1620.c
index 2ce26b1ed4..57a90e85cd 100644
--- a/tests/unit/unit1620.c
+++ b/tests/unit/unit1620.c
@@ -127,7 +127,7 @@ static CURLcode test_unit1620(const char *arg)
 
   Curl_freeset(empty);
   for(i = (enum dupstring)0; i < STRING_LAST; i++) {
-    fail_unless(empty->set.str[i] == NULL, "Curl_free() did not set to NULL");
+    fail_unless(!empty->set.str[i], "Curl_free() did not set to NULL");
   }
 
   result = Curl_close(&dupe);
diff --git a/tests/unit/unit2601.c b/tests/unit/unit2601.c
index 01ea112ce6..21cdb13836 100644
--- a/tests/unit/unit2601.c
+++ b/tests/unit/unit2601.c
@@ -99,9 +99,9 @@ static void check_bufq(size_t pool_spares,
 
   fail_unless(q.chunk_size == chunk_size, "chunk_size init wrong");
   fail_unless(q.max_chunks == max_chunks, "max_chunks init wrong");
-  fail_unless(q.head == NULL, "init: head not NULL");
-  fail_unless(q.tail == NULL, "init: tail not NULL");
-  fail_unless(q.spare == NULL, "init: spare not NULL");
+  fail_unless(!q.head, "init: head not NULL");
+  fail_unless(!q.tail, "init: tail not NULL");
+  fail_unless(!q.spare, "init: spare not NULL");
   fail_unless(Curl_bufq_len(&q) == 0, "init: bufq length != 0");
 
   result = Curl_bufq_write(&q, test_data, wsize, &n2);