sys/stdio_base: add optional function stdio_available

A couple of `stdio` backend implementations allow to check for the number of available bytes for reading before the blocking `stdio_read` is called. This helps to implement non-blocking functionalities while waiting for `stdin`.

makefiles/pseudomodules.inc.mk

@@ -152,6 +152,7 @@ PSEUDOMODULES += sock_udp
 PSEUDOMODULES += socket_zep_hello
 PSEUDOMODULES += soft_uart_modecfg
 PSEUDOMODULES += stdin
+PSEUDOMODULES += stdio_available
 PSEUDOMODULES += stdio_cdc_acm
 PSEUDOMODULES += stdio_ethos
 PSEUDOMODULES += stdio_uart_rx

sys/Kconfig.stdio

@@ -53,6 +53,11 @@ config MODULE_STDIO_UART_RX
     help
         Reception when using UART-based STDIO needs to be enabled.
 
+config MODULE_STDIO_AVAILABLE
+    bool
+    help
+        Indicates that the implementation supports function stdio_available
+
 config MODULE_PRINTF_FLOAT
     bool "Float support in printf"
 

sys/include/stdio_base.h

@@ -25,6 +25,8 @@
 
 #include <unistd.h>
 
+#include "kernel_defines.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -34,6 +36,18 @@ extern "C" {
  */
 void stdio_init(void);
 
+#if IS_USED(MODULE_STDIO_AVAILABLE) || DOXYGEN
+/**
+ * @brief   Get the number of bytes available for reading from stdio.
+ *
+ * @warning This function is only available if the implementation supports
+ *          it and the @c stdio_available module is enabled.
+ *
+ * @return  number of available bytes
+ */
+int stdio_available(void);
+#endif
+
 /**
  * @brief read @p len bytes from stdio uart into @p buffer
  *