libc: io: implement file io buffering and internal locking for concurrency

lib/libc/io/stdio/fputs.c

@@ -0,0 +1,31 @@
+#include "file.h"
+
+#include <stdio.h>
+
+extern int __fputc(int c, struct __opaque_file *stream);
+
+int fputs(const char *restrict str, struct __opaque_file *restrict stream)
+{
+	__libc_file_lock(stream);
+	if (stream->f_flags & (FILE_EOF | FILE_ERR)) {
+		__libc_file_unlock(stream);
+		return EOF;
+	}
+
+	size_t i = 0;
+	int err = 0;
+	for (i = 0; str[i]; i++) {
+		err = __fputc(str[i], stream);
+		if (err < 0) {
+			break;
+		}
+	}
+
+	__libc_file_unlock(stream);
+
+	if (i > 0) {
+		return i;
+	}
+
+	return err;
+}