Added setStdIoUnbuffered for unbuffered stdio (fixes #2146).

lib/system.nim

@@ -2708,6 +2708,9 @@ when not defined(JS): #and not defined(nimscript):
       ##
       ## Default mode is readonly. Returns true iff the file could be reopened.
 
+    proc setStdIoUnbuffered() {.tags: [], benign.}
+      ## Configures `stdin`, `stdout` and `stderr` to be unbuffered.
+
     proc close*(f: File) {.importc: "fclose", header: "<stdio.h>", tags: [].}
       ## Closes the file.
 

lib/system/sysio.nim

@@ -354,4 +354,9 @@ proc writeFile(filename, content: string) =
   else:
     sysFatal(IOError, "cannot open: ", filename)
 
+proc setStdIoUnbuffered() =
+  discard setvbuf(stdout, nil, IONBF, 0)
+  discard setvbuf(stderr, nil, IONBF, 0)
+  discard setvbuf(stdin, nil, IONBF, 0)
+
 {.pop.}

web/news/version_0_15_released.rst

@@ -14,6 +14,7 @@ Changes affecting backwards compatibility
 Library Additions
 -----------------
 
+- Added ``setStdIoUnbuffered`` proc to ``system.nim`` to enable unbuffered I/O.
 
 Compiler Additions
 ------------------