From 77022abafdc60fc965de0b9667f7272e9aaffb0a Mon Sep 17 00:00:00 2001
From: apio <blobs.trading@gmail.com>
Date: Sat, 22 Jul 2023 11:25:20 +0200
Subject: [PATCH] libc: Implement ungetc

---
 libc/src/stdio.cpp | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/libc/src/stdio.cpp b/libc/src/stdio.cpp
index 4e3440ff..6e84e267 100644
--- a/libc/src/stdio.cpp
+++ b/libc/src/stdio.cpp
@@ -688,9 +688,18 @@ extern "C"
         return f;
     }
 
-    int ungetc(int, FILE*)
+    int ungetc(int c, FILE* stream)
     {
-        fail("FIXME: ungetc: not implemented");
+        if (stream->_buf.index == 0)
+            return EOF; // No data currently in the read buffer, or no data has been read from it.
+
+        if (stream->_buf.mode == _IONBF)
+            return EOF; // FIXME: C doesn't state that ungetc() should only work on buffered streams.
+
+        stream->_buf.index--;
+        stream->_buf.buffer[stream->_buf.index] = (char)c;
+
+        return 0;
     }
 
     int setvbuf(FILE* stream, char* buf, int mode, size_t size)