Add basic test for openat

tests/Makefile.tests.mk

@@ -62,6 +62,7 @@ EXPECT_NAMES=\
 	fcntl/create \
 	fcntl/fcntl \
 	fcntl/open \
+	fcntl/openat \
 	fcntl/posix_fallocate \
 	features \
 	fnmatch \

tests/fcntl/openat.c

@@ -0,0 +1,43 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <dirent.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include "test_helpers.h"
+
+int main(void) {
+    int m = mkdir("test_dir", 0777);
+    ERROR_IF(mkdir, m, == -1);
+
+    int fd_file = open("test_dir/test_file.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
+    ERROR_IF(open, fd_file, == -1);
+    UNEXP_IF(open, fd_file, < 0);
+
+    int c1 = close(fd_file);
+    ERROR_IF(close, c1, == -1);
+
+    DIR *dir_stream = opendir("test_dir");
+    ERROR_IF(opendir, dir_stream, == NULL);
+
+    int dfd = dirfd(dir_stream);
+    ERROR_IF(dirfd, dfd, == -1);
+    UNEXP_IF(dirfd, dfd, < 0);
+
+    int fd_at = openat(dfd, "../test_dir/test_file.txt", O_RDONLY);
+    ERROR_IF(openat, fd_at, == -1);
+    UNEXP_IF(openat, fd_at, < 0);
+
+    int c2 = close(fd_at);
+    ERROR_IF(close, c2, == -1);
+
+    int c3 = closedir(dir_stream);
+    ERROR_IF(closedir, c3, == -1);
+
+    int u = unlink("test_dir/test_file.txt");
+    ERROR_IF(unlink, u, == -1);
+
+    int r = rmdir("test_dir");
+    ERROR_IF(rmdir, r, == -1);
+}