Subject: [PATCH 1/7] compat/msvc: use _chsize_s for ftruncate

From: "Johannes Schindelin via GitGitGadget" <[email-masked]>
Date: Thu, 04 Jun 2026 10:51:06 +0000

From: Johannes Schindelin <[email-masked]>

On Windows, `unsigned long` and `long` are 32 bits even on 64-bit
builds. The MSVC compatibility header has shimmed `ftruncate()` with

	#define ftruncate _chsize

ever since `compat/msvc-posix.h` was introduced. `_chsize()` takes a
32-bit `long` for the new length, which silently truncates files (and
the requested size) to 2 GiB. That is enough to make t7508 test 126
"git add fails gracefully with 4 GiB and 8 GiB files" fail under
MSVC: `test-tool truncate` creates a sparse 4 GiB or 8 GiB file via
the shimmed `ftruncate()`, and the test never gets off the ground.

`_chsize_s()` is the modern replacement, accepts a 64-bit `__int64`
length, and is the only sensible target on Windows. The catch is that
it does not follow the POSIX `-1` + `errno` convention: it returns
`0` on success and an errno value (a small positive integer) on
failure. A plain `#define ftruncate _chsize_s` would therefore
silently break callers that test the return value as `< 0` or against
`-1`, of which there are several: `http.c`, `parallel-checkout.c`,
and `t/helper/test-truncate.c` among them.

Introduce a `static inline` wrapper that calls `_chsize_s()`, copies
its errno return into `errno`, and translates the result to the
familiar `-1` / `0` convention, then point `ftruncate` at the
wrapper. Place the wrapper after `#include "mingw-posix.h"` so the
`off_t` parameter resolves to the already-widened `off64_t` rather
than the 32-bit `_off_t` from `compat/vcbuild/include/unistd.h`.

MinGW is unaffected: its `ftruncate()` already takes `off_t` and
routes through `ftruncate64()` when `_FILE_OFFSET_BITS=64`, which is
the default in our build.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <[email-masked]>
---
 compat/msvc-posix.h | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

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 */
-- 
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