printf stops printing after first newline

[jmattsson]

It appears that for some strange reason the regular C printf() function stops printing when it encounters the first newline (\n) in a string. This can be trivially reproduced by attempting to print two lines in the one call; see below for a patch to the official 01_hello_world example.

diff --git i/examples/01_hello_world/main/hello_world_main.c w/examples/01_hello_world/main/hello_world_main.c
index ad2c0ac..ca907ea 100644
--- i/examples/01_hello_world/main/hello_world_main.c
+++ w/examples/01_hello_world/main/hello_world_main.c
@@ -14,7 +14,7 @@

 void hello_task(void *pvParameter)
 {
-    printf("Hello world!\n");
+    printf("Hello world!\nHiding in plain sight...\n");
     for (int i = 10; i >= 0; i--) {
         printf("Restarting in %d seconds...\n", i);
         vTaskDelay(1000 / portTICK_RATE_MS);

Despite applying the above patch, the output stays at:

Hello world!

If using the %s modifier, a newline in the argument string also causes printf() to stop outputting characters, effectively cutting short the output. E.g.

printf("Hello%s world!\n", "Hiding\n in plain sight...");

yields the following output:

HelloHiding

[projectgus]

That @jmattsson . I committed a patch for this into our review queue yesterday, it's a bug in the write syscall.

Because everyone's on holidays this week I'm not sure if we'll get the fix merged to github until next week (review policy), but I might be able to pop up a temporary branch.

[jmattsson]

Really? I instrumented _write_r() but by the time it got there the string had already been truncated at the \n.

[projectgus]

Yeah it's not an obvious bug though., _write_r should return number of bytes written, but doesn't. Stdout is line buffered by default: so the first line gets sent to _write_r & TXed,, _write_r returns zero, newlib discards the rest of the buffer thinking there was an error.

Fix is incoming now, should be auto-pushed github shortly...

[jmattsson]

Ah, sneaky bug! Glad it was an easy fix though.