app: Use NRF_MODEM_MAX_SOCKET_COUNT

File descriptor counting has been refactored in Zephyr.
CONFIG_POSIX_OPEN_MAX should not be used anymore.
SM should enable 8 sockets allowed by the modem and
NRF_MODEM_MAX_SOCKET_COUNT is for this purpose.
This was brought into NCS by the 2026.01.12 upmerge:
nrfconnect/sdk-nrf#26468

Signed-off-by: Tommi Rantanen <tommi.rantanen@nordicsemi.no>

Author: trantanen

app/prj.conf

@@ -39,8 +39,6 @@ CONFIG_NET_IPV6=y
 # Modem library
 CONFIG_NRF_MODEM_LIB=y
 CONFIG_AT_CMD_CUSTOM=y
-# Align the max FD entry to NRF_MODEM_MAX_SOCKET_COUNT(8)
-CONFIG_ZVFS_OPEN_MAX=8
 
 # Handle modem fault
 CONFIG_NRF_MODEM_LIB_ON_FAULT_APPLICATION_SPECIFIC=y

app/src/sm_at_socket.c

@@ -20,8 +20,10 @@
 
 LOG_MODULE_REGISTER(sm_sock, CONFIG_SM_LOG_LEVEL);
 
-#define SM_FDS_COUNT CONFIG_POSIX_OPEN_MAX
-#define SM_MAX_SOCKET_COUNT (SM_FDS_COUNT - 1)
+/* Ensure the socket count so we do a change consciously when it is changed. */
+BUILD_ASSERT(NRF_MODEM_MAX_SOCKET_COUNT == 8, "NRF_MODEM_MAX_SOCKET_COUNT must be 8");
+
+#define SM_MAX_SOCKET_COUNT NRF_MODEM_MAX_SOCKET_COUNT
 
 /**@brief Socketopt operations. */
 enum sm_socketopt_operation {

app/tests/at_socket/src/test_at_socket.c

@@ -392,15 +392,15 @@ void test_xsocket_invalid_type(void)
 
 /*
  * Test: Create maximum number of sockets
- * - Tests that the application can create up to CONFIG_POSIX_OPEN_MAX-1 sockets
+ * - Tests that the application can create up to NRF_MODEM_MAX_SOCKET_COUNT sockets
  * - Verifies that attempting to create one more socket fails
  */
 void test_xsocket_max_sockets(void)
 {
 	const char *response;
-	int max_sockets = CONFIG_POSIX_OPEN_MAX - 1;
+	int max_sockets = NRF_MODEM_MAX_SOCKET_COUNT;
 
-	/* Create 3 sockets (determined by CONFIG_POSIX_OPEN_MAX - 1) */
+	/* Create 3 sockets (determined by NRF_MODEM_MAX_SOCKET_COUNT) */
 	for (int i = 0; i < max_sockets; i++) {
 
 		/* Mock nrf_socket to return fd */
@@ -432,11 +432,11 @@ void test_xsocket_max_sockets(void)
 
 	/* Clean up: close all 3 sockets */
 	for (int i = 0; i < max_sockets; i++) {
+		char send_buf[20];
 		__cmock_nrf_close_ExpectAndReturn(i, 0);
+		sprintf(send_buf, "AT#XCLOSE=%d\r\n", i);
+		send_at_command(send_buf);
 	}
-	send_at_command("AT#XCLOSE=0\r\n");
-	send_at_command("AT#XCLOSE=1\r\n");
-	send_at_command("AT#XCLOSE=2\r\n");
 }
 
 /*
@@ -2051,7 +2051,7 @@ void test_xapoll_stop_socket(void)
 void test_xapoll_start_all_sockets(void)
 {
 	const char *response;
-	int max_sockets = CONFIG_POSIX_OPEN_MAX - 1;
+	int max_sockets = NRF_MODEM_MAX_SOCKET_COUNT;
 
 	/* Create multiple sockets */
 	for (int i = 0; i < max_sockets; i++) {
@@ -2075,11 +2075,11 @@ void test_xapoll_start_all_sockets(void)
 
 	/* Close all sockets */
 	for (int i = 0; i < max_sockets; i++) {
+		char send_buf[20];
 		__cmock_nrf_close_ExpectAndReturn(i, 0);
+		sprintf(send_buf, "AT#XCLOSE=%d\r\n", i);
+		send_at_command(send_buf);
 	}
-	send_at_command("AT#XCLOSE=0\r\n");
-	send_at_command("AT#XCLOSE=1\r\n");
-	send_at_command("AT#XCLOSE=2\r\n");
 }
 
 /*
@@ -2090,7 +2090,7 @@ void test_xapoll_start_all_sockets(void)
 void test_xapoll_stop_all_sockets(void)
 {
 	const char *response;
-	int max_sockets = CONFIG_POSIX_OPEN_MAX - 1;
+	int max_sockets = NRF_MODEM_MAX_SOCKET_COUNT;
 
 	/* Create multiple sockets */
 	for (int i = 0; i < max_sockets; i++) {
@@ -2118,11 +2118,11 @@ void test_xapoll_stop_all_sockets(void)
 
 	/* Close all sockets */
 	for (int i = 0; i < max_sockets; i++) {
+		char send_buf[20];
 		__cmock_nrf_close_ExpectAndReturn(i, 0);
+		sprintf(send_buf, "AT#XCLOSE=%d\r\n", i);
+		send_at_command(send_buf);
 	}
-	send_at_command("AT#XCLOSE=0\r\n");
-	send_at_command("AT#XCLOSE=1\r\n");
-	send_at_command("AT#XCLOSE=2\r\n");
 }
 
 /*
@@ -2234,7 +2234,7 @@ void test_xapoll_invalid_socket(void)
 void test_xclose_operation(void)
 {
 	const char *response;
-	int max_sockets = CONFIG_POSIX_OPEN_MAX - 1;
+	int max_sockets = NRF_MODEM_MAX_SOCKET_COUNT;
 
 	/* Test 1: Close single socket using handle */
 	/* Create one socket */