extmod/os_dupterm: Let mp_os_dupterm_tx_strn() return num bytes written.

In case of multiple outputs, the minimum successful write length is
returned.  In line with this, in case any output has a write error, zero is
returned.

In case of no outputs, -1 is returned.

The return value can be used to assess whether writes were attempted, and
if so, whether they succeeded.

Signed-off-by: Maarten van der Schrieck <maarten@thingsconnected.nl>

1 files changed, 4 insertions, 2 deletions

diff --git a/extmod/misc.h b/extmod/misc.h
index 80e5b59a0..8ea51a981 100644
--- a/extmod/misc.h
+++ b/extmod/misc.h
@@ -39,10 +39,12 @@ bool mp_os_dupterm_is_builtin_stream(mp_const_obj_t stream);
 void mp_os_dupterm_stream_detached_attached(mp_obj_t stream_detached, mp_obj_t stream_attached);
 uintptr_t mp_os_dupterm_poll(uintptr_t poll_flags);
 int mp_os_dupterm_rx_chr(void);
-void mp_os_dupterm_tx_strn(const char *str, size_t len);
+int mp_os_dupterm_tx_strn(const char *str, size_t len);
 void mp_os_deactivate(size_t dupterm_idx, const char *msg, mp_obj_t exc);
 #else
-#define mp_os_dupterm_tx_strn(s, l)
+static inline int mp_os_dupterm_tx_strn(const char *s, size_t l) {
+    return -1;
+}
 #endif
 
 #endif // MICROPY_INCLUDED_EXTMOD_MISC_H