fix(posix): tighten dirent shim and use nullptr

The dirent() shim's scandir() growth path used `sizeof(*next)` where the
inner type is `struct dirent **`, which clang-tidy's
bugprone-sizeof-expression flags as a pointer-to-aggregate sizeof. Spell
the element type out (`struct dirent *`) so the realloc/qsort sizes are
unambiguous on both 32- and 64-bit. Convert the remaining NULL tokens in
the shim and the lockstep test to nullptr to satisfy hicpp-use-nullptr.

Signed-off-by: Nuno Marques <n.marques21@hotmail.com>

platforms/posix/include/windows_shim/dirent.h

@@ -108,10 +108,10 @@ typedef struct px4_dir_shim {
  */
 static inline DIR *opendir(const char *name)
 {
-	if (!name) { errno = EINVAL; return NULL; }
+	if (!name) { errno = EINVAL; return nullptr; }
 
 	DIR *d = (DIR *)calloc(1, sizeof(DIR));
-	if (!d) { errno = ENOMEM; return NULL; }
+	if (!d) { errno = ENOMEM; return nullptr; }
 
 	/* Translate "foo" -> "foo\*" so FindFirstFile enumerates the
 	 * directory's contents rather than matching the directory itself. */
@@ -119,14 +119,14 @@ static inline DIR *opendir(const char *name)
 	if (n <= 0 || (size_t)n >= sizeof(d->pattern)) {
 		free(d);
 		errno = ENAMETOOLONG;
-		return NULL;
+		return nullptr;
 	}
 
 	d->handle = FindFirstFileA(d->pattern, &d->find_data);
 	if (d->handle == INVALID_HANDLE_VALUE) {
 		free(d);
 		errno = ENOENT;
-		return NULL;
+		return nullptr;
 	}
 	d->first = 1;
 	d->offset = 0;
@@ -141,12 +141,12 @@ static inline DIR *opendir(const char *name)
  */
 static inline struct dirent *readdir(DIR *d)
 {
-	if (!d) { errno = EBADF; return NULL; }
+	if (!d) { errno = EBADF; return nullptr; }
 
 	if (d->first) {
 		d->first = 0;
 	} else if (!FindNextFileA(d->handle, &d->find_data)) {
-		return NULL; /* end of directory */
+		return nullptr; /* end of directory */
 	}
 
 	strncpy(d->entry.d_name, d->find_data.cFileName, NAME_MAX);
@@ -245,7 +245,7 @@ static inline int scandir(const char *dirp, struct dirent ***namelist,
 		return -1;
 	}
 
-	*namelist = NULL;
+	*namelist = nullptr;
 
 	DIR *dir = opendir(dirp);
 
@@ -255,17 +255,17 @@ static inline int scandir(const char *dirp, struct dirent ***namelist,
 
 	size_t count = 0;
 	size_t capacity = 0;
-	struct dirent **list = NULL;
-	struct dirent *entry = NULL;
+	struct dirent **list = nullptr;
+	struct dirent *entry = nullptr;
 
-	while ((entry = readdir(dir)) != NULL) {
+	while ((entry = readdir(dir)) != nullptr) {
 		if (select && !select(entry)) {
 			continue;
 		}
 
 		if (count == capacity) {
 			size_t next_capacity = capacity ? capacity * 2 : 16;
-			struct dirent **next = (struct dirent **)realloc(list, next_capacity * sizeof(*next));
+			struct dirent **next = (struct dirent **)realloc(list, next_capacity * sizeof(struct dirent *));
 
 			if (!next) {
 				for (size_t i = 0; i < count; ++i) {
@@ -302,7 +302,7 @@ static inline int scandir(const char *dirp, struct dirent ***namelist,
 	closedir(dir);
 
 	if (compar && count > 1) {
-		qsort(list, count, sizeof(*list), (int (*)(const void *, const void *))compar);
+		qsort(list, count, sizeof(struct dirent *), (int (*)(const void *, const void *))compar);
 	}
 
 	*namelist = list;

platforms/posix/src/px4/common/lockstep_scheduler/test/src/lockstep_scheduler_test.cpp

@@ -127,10 +127,10 @@ void test_locked_semaphore_getting_unlocked()
 void test_condition_signal_wakes_without_time_advance()
 {
 	pthread_cond_t cond;
-	pthread_cond_init(&cond, NULL);
+	pthread_cond_init(&cond, nullptr);
 
 	pthread_mutex_t lock;
-	pthread_mutex_init(&lock, NULL);
+	pthread_mutex_init(&lock, nullptr);
 
 	LockstepScheduler ls;
 	ls.set_absolute_time(some_time_us);