diff --git a/compat/msvc-posix.h b/compat/msvc-posix.h
index c500b8b4aa..7ce39b8d3f 100644
--- a/compat/msvc-posix.h
+++ b/compat/msvc-posix.h
@@ -16,7 +16,6 @@
 #define __attribute__(x)
 #define strcasecmp   _stricmp
 #define strncasecmp  _strnicmp
-#define ftruncate    _chsize
 #define strtoull     _strtoui64
 #define strtoll      _strtoi64
 
@@ -30,4 +29,27 @@ typedef int sigset_t;
 
 #include "mingw-posix.h"
 
+/*
+ * MSVC's `_chsize()` takes a 32-bit `long` and silently truncates files
+ * to 2 GiB. `_chsize_s()` accepts a 64-bit length but returns 0 on
+ * success or an errno value on failure, rather than the -1/errno
+ * convention POSIX `ftruncate()` callers expect. Wrap it so callers
+ * that test the return value as `< 0` or against `-1` keep working.
+ *
+ * Note: this declaration must follow `#include "mingw-posix.h"` so
+ * `off_t` resolves to `off64_t` and the parameter type matches the
+ * underlying `_chsize_s()` width.
+ */
+static inline int msvc_ftruncate(int fd, off_t length)
+{
+	int err = _chsize_s(fd, length);
+
+	if (err) {
+		errno = err;
+		return -1;
+	}
+	return 0;
+}
+#define ftruncate msvc_ftruncate
+
 #endif /* COMPAT_MSVC_POSIX_H */