Add unbuffered print for fiber

A simple solution to avoid unexpected output by the buffered I/O 'printf' is
using low-level I/O interface 'write'. It works because 'printf' will wait to
write to STDOUT until the buffer is full, or on some other conditions. Using
write, however, can write to STDOUT immediately.

Here is a naive implementation for the idea with some limitation and weakness
that need improvement:
1. It will fail if the formatted string with length >64
2. The function 'write' can write less than n bytes. It will need further
   handling if happens.

Author: RinHizakura

fiber/fiber.c

@@ -19,6 +19,26 @@
 #include <sys/wait.h>  /* For wait */
 #include <unistd.h>    /* For getpid */
 
+/* A simple solution to avoid unexpected output by the buffered I/O 'printf' is
+ * using low-level I/O interface 'write'. It works because 'printf' will wait to
+ * write to STDOUT until the buffer is full, or on some other conditions. Using
+ * write, however, can write to STDOUT immediately.
+ *
+ * Here is a naive implementation for the idea with some limitation and weakness
+ * that need improvement:
+ * 1. It will fail if the formatted string with length >64
+ * 2. The function 'write' can write less than n bytes. It will need further
+ *    handling if happens.
+ */
+#define BUF_LEN 64
+#define printf_unbuffered(fmt, ...)                                         \
+    do {                                                                    \
+        char str[BUF_LEN + 1];                                              \
+        int n = snprintf(str, BUF_LEN + 1, fmt __VA_OPT__(, ) __VA_ARGS__); \
+        write(1, str, n);                                                   \
+    } while (0)
+#define printf printf_unbuffered
+
 typedef struct {
     pid_t pid;   /* The pid of the child thread as returned by clone */
     void *stack; /* The stack pointer */