Cleanup stdout write redirection.

- comments, naming, missing include

core/include/stdout_redirect.h

@@ -7,6 +7,8 @@
 #ifndef _STDOUT_REDIRECT_H_
 #define _STDOUT_REDIRECT_H_
 
+#include <sys/reent.h>
+
 #ifdef __cplusplus
 extern "C"
 {
@@ -14,7 +16,26 @@ extern "C"
 
 typedef long _WriteFunction(struct _reent *r, int fd, const char *ptr, int len );
 
+/** Set implementation of write syscall for stdout.
+ *
+ *  Use this function to redirect stdout for further processing (save to file, send over network).
+ *  It may be good idea to save result of `get_write_stdout()` beforehand and call
+ *  it at the end of the supplied function.
+ *
+ *  NOTE: use NULL to reset to default implementation.
+ *
+ *  @param[in] f  New code to handle stdout output
+ *
+ */
 void set_write_stdout(_WriteFunction *f);
+
+/** Get current implementation of write syscall for stdout.
+ *
+ *  Save returned value before calling `set_write_stdout`, it allows for chaining
+ *  multiple independent handlers.
+ *
+ *  @returns current stdout handler
+ */
 _WriteFunction *get_write_stdout();
 
 #ifdef __cplusplus

core/newlib_syscalls.c

@@ -55,20 +55,20 @@ __attribute__((weak)) long _write_stdout_r(struct _reent *r, int fd, const char
     return len;
 }
 
-_WriteFunction *_current_stdout_write_r = &_write_stdout_r;
+static _WriteFunction *current_stdout_write_r = &_write_stdout_r;
 
 void set_write_stdout(_WriteFunction *f)
 {
     if  (f != NULL) {
-        _current_stdout_write_r = f;
+        current_stdout_write_r = f;
     } else {
-        _current_stdout_write_r = &_write_stdout_r;
+        current_stdout_write_r = &_write_stdout_r;
     }
 }
 
 _WriteFunction *get_write_stdout()
 {
-    return _current_stdout_write_r;
+    return current_stdout_write_r;
 }
 
 /* default implementation, replace in a filesystem */
@@ -83,7 +83,7 @@ __attribute__((weak)) long _write_r(struct _reent *r, int fd, const char *ptr, i
     if(fd != r->_stdout->_file) {
         return _write_filesystem_r(r, fd, ptr, len);
     }
-    return _current_stdout_write_r(r, fd, ptr, len);
+    return current_stdout_write_r(r, fd, ptr, len);
 }
 
 /* syscall implementation for stdio read from UART */