samples: net: echo_async_select: Use read()/write() if possible

If build with full POSIX API, use read()/write() instead of
recv()/send() calls for sockets.

We have read()/write() support for a while, but no samples/tests
actually performed at least a build test for it (so it will be
done now).

Fixes: #25407
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

samples/net/sockets/echo_async_select/prj.conf

@@ -1,6 +1,7 @@
 # General config
 CONFIG_NEWLIB_LIBC=y
 CONFIG_MAIN_STACK_SIZE=1200
+CONFIG_POSIX_API=y
 
 # Networking config
 CONFIG_NETWORKING=y

samples/net/sockets/echo_async_select/src/socket_echo_select.c

@@ -18,12 +18,20 @@
 #include <unistd.h>
 #include <fcntl.h>
 
+/* Generic read()/write() is available in POSIX config, so use it. */
+#define READ(fd, buf, sz) read(fd, buf, sz)
+#define WRITE(fd, buf, sz) write(fd, buf, sz)
+
 #else
 
 #include <fcntl.h>
 #include <net/socket.h>
 #include <kernel.h>
 
+/* Generic read()/write() are not defined, so use socket-specific recv(). */
+#define READ(fd, buf, sz) recv(fd, buf, sz, 0)
+#define WRITE(fd, buf, sz) send(fd, buf, sz, 0)
+
 #endif
 
 /* For Zephyr, keep max number of fd's in sync with max poll() capacity */
@@ -178,17 +186,18 @@ void main(void)
 				       addr_str, client);
 				if (pollfds_add(client) < 0) {
 					static char msg[] = "Too many connections\n";
-					send(client, msg, sizeof(msg) - 1, 0);
+					WRITE(client, msg, sizeof(msg) - 1);
 					close(client);
 				} else {
 					setblocking(client, false);
 				}
 			} else {
 				char buf[128];
-				int len = recv(fd, buf, sizeof(buf), 0);
+				int len = READ(fd, buf, sizeof(buf));
 				if (len <= 0) {
 					if (len < 0) {
-						printf("error: recv: %d\n", errno);
+						printf("error: RECV: %d\n",
+						       errno);
 					}
 error:
 					pollfds_del(fd);
@@ -207,10 +216,10 @@ void main(void)
 					setblocking(fd, true);
 
 					for (p = buf; len; len -= out_len) {
-						out_len = send(fd, p, len, 0);
+						out_len = WRITE(fd, p, len);
 						if (out_len < 0) {
 							printf("error: "
-							       "send: %d\n",
+							       "WRITE: %d\n",
 							       errno);
 							goto error;
 						}