From 09c9e82a5ca2c8d83b36b36aacb7aa389403fad8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:28:44 +0100
Subject: [PATCH 01/25] ocamlmerlin.c: consistent definition of PATHSZ

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 6aafd6483..d4f00e20b 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -21,7 +21,6 @@
 #endif
 #ifdef _MSC_VER
 typedef SSIZE_T ssize_t;
-#define PATH_MAX MAX_PATH
 #ifndef _UCRT
 #define snprintf _snprintf
 #endif
@@ -92,7 +91,11 @@ static void failwith(const char *msg)
   exit(EXIT_FAILURE);
 }
 
+#ifdef _WIN32
+#define PATHSZ (MAX_PATH+1)
+#else
 #define PATHSZ (PATH_MAX+1)
+#endif
 
 /* On Linux, sun_path size is 108 bytes.
    On macOS it's 104.

From d3bcd918f3308bccca178371470a0c308c1289c0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:17 +0100
Subject: [PATCH 02/25] ocamlmerlin.c: improve location of socket path

On Windows, use [GetTempPath2][] if available or [GetTempPath][] to
retrieve the temporary directory path.

On other systems, follow the [XDG Base Directory Specification][XDG]
and put the socket file in the XDG runtime directory. It is indicated
by the `XDG_RUNTIME_DIR` environment variable, that is always set on
platforms implementing the spec. Otherwise, fallback to `TMPDIR` or
`/tmp`.

[GetTempPath2]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-gettemppath2a
[GetTempPath2]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-gettemppatha
[XDG]: https://specifications.freedesktop.org/basedir/latest/
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 29 ++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index d4f00e20b..d439e87d0 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -113,12 +113,29 @@ static void failwith(const char *msg)
 
 static const char *path_socketdir(void)
 {
-  static const char *tmpdir = NULL;
-  if (tmpdir == NULL)
-    tmpdir = getenv("TMPDIR");
-  if (tmpdir == NULL)
-    tmpdir = "/tmp";
-  return tmpdir;
+#ifdef _WIN32
+  static char dir[MAX_PATH+1] = { 0 };
+  if (dir[0] == 0) {
+    DWORD (WINAPI *pGetTempPath2A)(DWORD, LPSTR) =
+      (DWORD (WINAPI *)(DWORD, LPSTR))
+      (void (WINAPI *)(void))
+      GetProcAddress(GetModuleHandle("KERNEL32.DLL"), "GetTempPath2A");
+    DWORD rc;
+    if (pGetTempPath2A != NULL)
+      rc = pGetTempPath2A(_countof(dir), dir);
+    else
+      rc = GetTempPathA(_countof(dir), dir);
+    if (rc == 0)
+      failwith("could not get temporary path");
+  }
+#else
+  static const char *dir = NULL;
+  if (dir == NULL &&
+      (dir = getenv("XDG_RUNTIME_DIR")) == NULL &&
+      (dir = getenv("TMPDIR")) == NULL)
+    dir = "/tmp";
+#endif
+  return dir;
 }
 
 #ifdef _WIN32

From 61549c19af22d1991f8bcc9713b3219dc29f5dcc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:17 +0100
Subject: [PATCH 03/25] Improve passing arrays as parameters

In C, arrays passed as arguments to functions decay into pointers, and
the size information is lost. This means all of these are equivalent:

    void f(char *s);
    void f(char s[]);
    void f(char s[3615]);

One way to partially retain this information is to use `static n` with
`n` being the array size: the compiler now knows that the first `n`
elements after the pointed address can be accessed. This C99 feature
is unfortunately not supported by MSVC (but supported by clang-cl).
The compiler uses this info to check that the caller supplies a long
enough buffer to the callee. The size information is not used inside
the callee.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 16 +++++++++++-----
 src/platform/os_ipc_stub.c             |  8 +++++++-
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index d439e87d0..16519daa4 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -47,6 +47,12 @@ typedef SSIZE_T ssize_t;
 #include <sys/param.h>
 #endif
 
+#if !(defined(_MSC_VER) && !defined(__clang__))
+#define ATLEAST static
+#else
+#define ATLEAST
+#endif
+
 /** Portability information **/
 
 /* Determine OS, http://stackoverflow.com/questions/6649936
@@ -141,7 +147,7 @@ static const char *path_socketdir(void)
 #ifdef _WIN32
 /** Deal with Windows IPC **/
 
-static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds[3])
+static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds[ATLEAST 3])
 {
   DWORD dwNumberOfBytesWritten;
   if (!WriteFile(hPipe, fds, 3 * sizeof(HANDLE), &dwNumberOfBytesWritten, NULL) || dwNumberOfBytesWritten != 3 * sizeof(HANDLE))
@@ -153,7 +159,7 @@ static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds
 #else
 /** Deal with UNIX IPC **/
 
-static void ipc_send(int fd, unsigned char *buffer, size_t len, int fds[3])
+static void ipc_send(int fd, unsigned char *buffer, size_t len, int fds[ATLEAST 3])
 {
   char msg_control[CMSG_SPACE(3 * sizeof(int))];
   struct iovec iov = { .iov_base = buffer, .iov_len = len };
@@ -506,7 +512,7 @@ static char ocamlmerlin_server[] = "ocamlmerlin-server.exe";
 static char ocamlmerlin_server[] = "ocamlmerlin-server";
 #endif
 
-static void compute_merlinpath(char merlin_path[PATHSZ], const char *argv0, struct stat *st)
+static void compute_merlinpath(char merlin_path[ATLEAST PATHSZ], const char *argv0, struct stat *st)
 {
   char argv0_dirname[PATHSZ];
   size_t strsz;
@@ -593,9 +599,9 @@ LPSTR retrieve_user_sid_string()
   return usidstr;
 }
 
-static void compute_socketname(char socketname[PATHSZ], char eventname[PATHSZ], const char merlin_path[PATHSZ])
+static void compute_socketname(char socketname[ATLEAST PATHSZ], char eventname[ATLEAST PATHSZ], const char merlin_path[ATLEAST PATHSZ])
 #else
-static void compute_socketname(char socketname[SOCKSZ], struct stat *st)
+static void compute_socketname(char socketname[ATLEAST SOCKSZ], struct stat *st)
 #endif
 {
 #ifdef _WIN32
diff --git a/src/platform/os_ipc_stub.c b/src/platform/os_ipc_stub.c
index a95fe4754..465cdb7ec 100644
--- a/src/platform/os_ipc_stub.c
+++ b/src/platform/os_ipc_stub.c
@@ -30,6 +30,12 @@ typedef SSIZE_T ssize_t;
 #include <caml/alloc.h>
 #include <caml/threads.h>
 
+#if !(defined(_MSC_VER) && !defined(__clang__))
+#define ATLEAST static
+#else
+#define ATLEAST
+#endif
+
 #ifdef _MSC_VER
 extern __declspec(dllimport) char **environ;
 #else
@@ -75,7 +81,7 @@ static unsigned char buffer[BUFFER_SIZE];
 
 #define unbyte(x,n) (((unsigned char)x) << (n * 8))
 
-static ssize_t recv_buffer(int fd, int fds[3])
+static ssize_t recv_buffer(int fd, int fds[ATLEAST 3])
 {
   char msg_control[CMSG_SPACE(3 * sizeof(int))];
   struct iovec iov = { .iov_base = buffer, .iov_len = sizeof(buffer) };

From f8debc1d7f3b06aa1624031174cae36b190394dc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:17 +0100
Subject: [PATCH 04/25] ocamlmerlin.c: improve declaration of
 retrieve_user_sid_string

- make the function static;
- don't use K&R-style declaration, compilers now reject it before
  C23. After C23 `f(void)` is similar to `f()`.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 16519daa4..aa96ef5c4 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -560,7 +560,7 @@ static void compute_merlinpath(char merlin_path[ATLEAST PATHSZ], const char *arg
 #ifdef _WIN32
 
 /* May return NULL */
-LPSTR retrieve_user_sid_string()
+static LPSTR retrieve_user_sid_string(void)
 {
   LPSTR usidstr;
   HANDLE process_token;

From 2c8acdfdaba630be124e5e5c6e9ee2d2fc71e491 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:17 +0100
Subject: [PATCH 05/25] ocamlmerlin.c: use centralized exit of function with
 gotos pattern

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 31 +++++++++-----------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index aa96ef5c4..ffa4d4d7b 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -562,7 +562,7 @@ static void compute_merlinpath(char merlin_path[ATLEAST PATHSZ], const char *arg
 /* May return NULL */
 static LPSTR retrieve_user_sid_string(void)
 {
-  LPSTR usidstr;
+  LPSTR usidstr = NULL;
   HANDLE process_token;
   if ( ! OpenProcessToken( GetCurrentProcess(), TOKEN_QUERY, &process_token ) )
     return NULL;
@@ -570,32 +570,23 @@ static LPSTR retrieve_user_sid_string(void)
   DWORD sid_buffer_size;
   if ( ! GetTokenInformation(process_token, TokenUser, NULL, 0, &sid_buffer_size ) &&
         ( GetLastError() != ERROR_INSUFFICIENT_BUFFER ) )
-  {
-    CloseHandle(process_token);
-    return NULL;
-  }
+    goto close_process_token;
 
-  TOKEN_USER * token_user_ptr = (PTOKEN_USER) malloc(sid_buffer_size);
-  if ( ! token_user_ptr )
-  {
-    CloseHandle( process_token);
-    return NULL;
-  }
+  TOKEN_USER * token_user = (PTOKEN_USER) malloc(sid_buffer_size);
+  if ( ! token_user )
+    goto close_process_token;
 
-  if ( ! GetTokenInformation(process_token, TokenUser, token_user_ptr,
+  if ( ! GetTokenInformation(process_token, TokenUser, token_user,
                              sid_buffer_size, &sid_buffer_size))
-  {
-    free(token_user_ptr);
-    CloseHandle(process_token);
-    return NULL;
-  }
+    goto free_token_user;
 
-  if (! ConvertSidToStringSid(token_user_ptr->User.Sid, &usidstr))
+  if (! ConvertSidToStringSid(token_user->User.Sid, &usidstr))
     usidstr = NULL;
 
-  free(token_user_ptr);
+ free_token_user:
+  free(token_user);
+ close_process_token:
   CloseHandle(process_token);
-
   return usidstr;
 }
 

From 99883e8be76d1cb50efaf5c48dfee7920dcbf9e3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:17 +0100
Subject: [PATCH 06/25] ocamlmerlin.c: don't hard-code size of unix domain
 socket path

Quoting POSIX 2024:

> The size of sun_path is required to be constant, but intentionally
> does not have a specified name for that constant. Historically,
> different implementations used different sizes. For example, 4.3 BSD
> used a size of 108, and 4.4 BSD used a size of 104. Since most
> implementations originate from BSD versions, the size is typically
> in the range 92 to 108. An application can deduce the size by using
> `sizeof(((struct sockaddr_un *)0)->sun_path)`.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index ffa4d4d7b..5b0e15891 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -99,16 +99,12 @@ static void failwith(const char *msg)
 
 #ifdef _WIN32
 #define PATHSZ (MAX_PATH+1)
+#define SOCKSZ (MAX_PATH+1)
 #else
 #define PATHSZ (PATH_MAX+1)
+#define SOCKSZ (sizeof(((struct sockaddr_un *)0)->sun_path) - sizeof("./"))
 #endif
 
-/* On Linux, sun_path size is 108 bytes.
-   On macOS it's 104.
-   We use 102 buffer, as we later append './'
-*/
-#define SOCKSZ (102)
-
 #define BEGIN_PROTECTCWD \
   { char previous_cwd[PATHSZ]; \
     if (!getcwd(previous_cwd, PATHSZ)) previous_cwd[0] = '\0';
@@ -291,7 +287,7 @@ static int connect_socket(const char *socketname, int fail)
     /* Return from chdir is ignored */
     err = chdir(path_socketdir());
     address.sun_family = AF_UNIX;
-    snprintf(address.sun_path, 104, "./%s", socketname);
+    snprintf(address.sun_path, sizeof(address.sun_path), "./%s", socketname);
     address_len = strlen(address.sun_path) + sizeof(address.sun_family) + 1;
 
     NO_EINTR(err, connect(sock, (struct sockaddr*)&address, address_len));
@@ -385,7 +381,7 @@ static void start_server(const char *socketname, const char* ignored, const char
     /* Return from chdir is ignored */
     err = chdir(path_socketdir());
     address.sun_family = AF_UNIX;
-    snprintf(address.sun_path, 104, "./%s", socketname);
+    snprintf(address.sun_path, sizeof(address.sun_path), "./%s", socketname);
     address_len = strlen(address.sun_path) + sizeof(address.sun_family) + 1;
     unlink(address.sun_path);
 

From 860a67c1c8d6e142ce1ddc75c8c17e70fca60919 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 07/25] ocamlmerlin.c: cleanly separate Windows and non-Windows
 code

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 5b0e15891..bd64d3d5e 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -587,11 +587,7 @@ static LPSTR retrieve_user_sid_string(void)
 }
 
 static void compute_socketname(char socketname[ATLEAST PATHSZ], char eventname[ATLEAST PATHSZ], const char merlin_path[ATLEAST PATHSZ])
-#else
-static void compute_socketname(char socketname[ATLEAST SOCKSZ], struct stat *st)
-#endif
 {
-#ifdef _WIN32
   BY_HANDLE_FILE_INFORMATION info;
   LPSTR user_sid_string;
   HANDLE hFile = CreateFile(merlin_path, FILE_READ_ATTRIBUTES, FILE_SHARE_DELETE | FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
@@ -613,14 +609,18 @@ static void compute_socketname(char socketname[ATLEAST SOCKSZ], struct stat *st)
       "\\\\.\\pipe\\%s", eventname);
 
   LocalFree(user_sid_string);
+}
+
 #else
+static void compute_socketname(char socketname[ATLEAST SOCKSZ], struct stat *st)
+{
   snprintf(socketname, SOCKSZ,
       "ocamlmerlin_%llu_%llu_%llu.socket",
       (unsigned long long)getuid(),
       (unsigned long long)st->st_dev,
       (unsigned long long)st->st_ino);
-#endif
 }
+#endif
 
 /* Main */
 

From 31b3426a09e544172771fe3fb881295e5c7d6e74 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 08/25] ocamlmerlin.c: improve initialization of struct
 sockaddr_un

See also [`sys/un.h`][1] and [unix(7)][2].

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_un.h.html
[2]: https://man7.org/linux/man-pages/man7/unix.7.html
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index bd64d3d5e..7c14a34eb 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -1,3 +1,4 @@
+#include <stddef.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -282,13 +283,14 @@ static int connect_socket(const char *socketname, int fail)
 
   BEGIN_PROTECTCWD
     struct sockaddr_un address;
-    int address_len;
+    socklen_t address_len;
+    memset(&address, 0, sizeof(address));
 
     /* Return from chdir is ignored */
     err = chdir(path_socketdir());
     address.sun_family = AF_UNIX;
     snprintf(address.sun_path, sizeof(address.sun_path), "./%s", socketname);
-    address_len = strlen(address.sun_path) + sizeof(address.sun_family) + 1;
+    address_len = offsetof(struct sockaddr_un, sun_path) + strlen(address.sun_path) + 1;
 
     NO_EINTR(err, connect(sock, (struct sockaddr*)&address, address_len));
   END_PROTECTCWD
@@ -376,13 +378,14 @@ static void start_server(const char *socketname, const char* ignored, const char
 
   BEGIN_PROTECTCWD
     struct sockaddr_un address;
-    int address_len;
+    socklen_t address_len;
+    memset(&address, 0, sizeof(address));
 
     /* Return from chdir is ignored */
     err = chdir(path_socketdir());
     address.sun_family = AF_UNIX;
     snprintf(address.sun_path, sizeof(address.sun_path), "./%s", socketname);
-    address_len = strlen(address.sun_path) + sizeof(address.sun_family) + 1;
+    address_len = offsetof(struct sockaddr_un, sun_path) + strlen(address.sun_path) + 1;
     unlink(address.sun_path);
 
     NO_EINTR(err, bind(sock, (struct sockaddr*)&address, address_len));

From c5829724838acf235f359250d5b05743a7aacbbe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 09/25] Always use __declspec(import) on environ on Windows

It works with mingw-w64 too.

    warning: '__p__environ' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]

https://gcc.gnu.org/onlinedocs/gcc/Microsoft-Windows-Variable-Attributes.html#index-dllimport-variable-attribute
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 2 +-
 src/platform/os_ipc_stub.c             | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 7c14a34eb..bc702b900 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -218,7 +218,7 @@ static void append_argument(unsigned char *buffer, size_t len, ssize_t *pos, con
   *pos = j;
 }
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 extern __declspec(dllimport) char **environ;
 #else
 extern char **environ;
diff --git a/src/platform/os_ipc_stub.c b/src/platform/os_ipc_stub.c
index 465cdb7ec..6f7384129 100644
--- a/src/platform/os_ipc_stub.c
+++ b/src/platform/os_ipc_stub.c
@@ -36,7 +36,7 @@ typedef SSIZE_T ssize_t;
 #define ATLEAST
 #endif
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 extern __declspec(dllimport) char **environ;
 #else
 extern char **environ;

From cac70c8b3f75c38bdbed00ff25712653d834b545 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 10/25] ocamlmerlin.c: fix unused variable warnings

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index bc702b900..5d8b3ab60 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -313,7 +313,7 @@ static void start_server(const char *socketname, const char* eventname, const ch
   PROCESS_INFORMATION pi;
   STARTUPINFO si;
   HANDLE hEvent = CreateEvent(NULL, FALSE, FALSE, eventname);
-  DWORD dwResult;
+
   sprintf(buf, "%s server %s %s", exec_path, socketname, eventname);
   ZeroMemory(&si, sizeof(si));
   si.cb = sizeof(si);
@@ -331,7 +331,7 @@ static void start_server(const char *socketname, const char* eventname, const ch
     failwith_perror("execlp");
 }
 #else
-static void make_daemon(int sock)
+static void make_daemon(void)
 {
   /* On success: The child process becomes session leader */
   if (setsid() < 0)
@@ -349,13 +349,6 @@ static void make_daemon(int sock)
   if (chdir("/") != 0)
     failwith_perror("chdir");
 
-  //int x;
-  //for (x = sysconf(_SC_OPEN_MAX); x>2; x--)
-  //{
-  //  if (x != sock)
-  //    close(x);
-  //}
-
   pid_t child = fork();
   signal(SIGHUP, SIG_IGN);
 
@@ -370,6 +363,7 @@ static void make_daemon(int sock)
 
 static void start_server(const char *socketname, const char* ignored, const char *exec_path)
 {
+  (void) ignored;
   int sock = socket(PF_UNIX, SOCK_STREAM, 0);
   if (sock == -1)
     failwith_perror("socket");
@@ -405,7 +399,7 @@ static void start_server(const char *socketname, const char* ignored, const char
 
   if (child == 0)
   {
-    make_daemon(sock);
+    make_daemon();
 
     char socket_fd[50], socket_path[PATHSZ];
     sprintf(socket_fd, "%d", sock);

From ac648d9a71dee569b0ebb4618891ae6f424c0f27 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 11/25] ocamlmerlin.c: use the cool _Countof op from C2y

to count the number of elements in an array.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 5d8b3ab60..278d0598b 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -54,6 +54,19 @@ typedef SSIZE_T ssize_t;
 #define ATLEAST
 #endif
 
+#ifndef __has_feature
+#define __has_feature(x) 0
+#endif
+#ifndef __has_extension
+#define __has_extension __has_feature
+#endif
+
+#if __has_feature(c_countof)
+#define countof _Countof
+#else
+#define countof(a) (sizeof(a)/sizeof(*(a)))
+#endif
+
 /** Portability information **/
 
 /* Determine OS, http://stackoverflow.com/questions/6649936
@@ -125,9 +138,9 @@ static const char *path_socketdir(void)
       GetProcAddress(GetModuleHandle("KERNEL32.DLL"), "GetTempPath2A");
     DWORD rc;
     if (pGetTempPath2A != NULL)
-      rc = pGetTempPath2A(_countof(dir), dir);
+      rc = pGetTempPath2A(countof(dir), dir);
     else
-      rc = GetTempPathA(_countof(dir), dir);
+      rc = GetTempPathA(countof(dir), dir);
     if (rc == 0)
       failwith("could not get temporary path");
   }
@@ -534,7 +547,7 @@ static void compute_merlinpath(char merlin_path[ATLEAST PATHSZ], const char *arg
   strsz = strlen(merlin_path);
 
   // Append ocamlmerlin-server
-  if (strsz + sizeof(ocamlmerlin_server) + 8 > PATHSZ)
+  if (strsz + countof(ocamlmerlin_server) + 8 > PATHSZ)
     failwith("path is too long");
 
   strcpy(merlin_path + strsz, ocamlmerlin_server);

From 19fc132effa241f9b097ce6f56cd1a273eddbca8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 12/25] ocamlmerlin.c: fearless booleans

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 278d0598b..b2a8e7481 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <signal.h>
+#include <stdbool.h>
 #ifdef _WIN32
 /* GetNamedPipeServerProcessId requires Windows Vista+ */
 #undef _WIN32_WINNT
@@ -276,7 +277,7 @@ static ssize_t prepare_args(unsigned char *buffer, size_t len, int argc, char **
 
 #ifdef _WIN32
 #define IPC_SOCKET_TYPE HANDLE
-static HANDLE connect_socket(const char *socketname, int fail)
+static HANDLE connect_socket(const char *socketname, bool fail)
 {
   HANDLE hPipe;
   hPipe = CreateFile(socketname, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, 0);
@@ -287,7 +288,7 @@ static HANDLE connect_socket(const char *socketname, int fail)
 #else
 #define IPC_SOCKET_TYPE int
 #define INVALID_HANDLE_VALUE -1
-static int connect_socket(const char *socketname, int fail)
+static int connect_socket(const char *socketname, bool fail)
 {
   int sock = socket(PF_UNIX, SOCK_STREAM, 0);
   if (sock == -1) failwith_perror("socket");
@@ -429,12 +430,12 @@ static void start_server(const char *socketname, const char* ignored, const char
 
 static IPC_SOCKET_TYPE connect_and_serve(const char *socket_path, const char* event_path, const char *exec_path)
 {
-  IPC_SOCKET_TYPE sock = connect_socket(socket_path, 0);
+  IPC_SOCKET_TYPE sock = connect_socket(socket_path, false);
 
   if (sock == INVALID_HANDLE_VALUE)
   {
     start_server(socket_path, event_path, exec_path);
-    sock = connect_socket(socket_path, 1);
+    sock = connect_socket(socket_path, true);
   }
 
   if (sock == INVALID_HANDLE_VALUE)
@@ -648,14 +649,14 @@ static void dumpinfo(void)
 
 static void unexpected_termination(int argc, char **argv)
 {
-  int sexp = 0;
+  bool sexp = false;
   int i;
 
   for (i = 1; i < argc - 1; ++i)
   {
     if (strcmp(argv[i], "-protocol") == 0 &&
         strcmp(argv[i+1], "sexp") == 0)
-      sexp = 1;
+      sexp = true;
   }
 
   puts(sexp

From 7475d0aa4e21c945113e65510758943cf6788959 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 19 Nov 2025 08:35:18 +0100
Subject: [PATCH 13/25] ocamlmerlin.c: correctly report WinAPI errors

`perror` can be used with functions from the CRT that set `errno`,
otherwise `GetLastError` and `FormatMessage` have to be used instead.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 41 ++++++++++++++++++--------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index b2a8e7481..d89f51bdb 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -98,6 +98,23 @@ typedef SSIZE_T ssize_t;
 
 static void dumpinfo(void);
 
+#ifdef _WIN32
+static void failwith_formatmessage(const char *msg)
+{
+    char err[512];
+    FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
+                  NULL,                 /* message source */
+                  GetLastError(),       /* error number */
+                  0,                    /* default language */
+                  err,                  /* destination */
+                  countof(err),         /* size of destination */
+                  NULL);                /* no inserts */
+    fprintf(stderr, "%s: %s\n", err, msg);
+    dumpinfo();
+    exit(EXIT_FAILURE);
+}
+#endif
+
 static void failwith_perror(const char *msg)
 {
   perror(msg);
@@ -143,7 +160,7 @@ static const char *path_socketdir(void)
     else
       rc = GetTempPathA(countof(dir), dir);
     if (rc == 0)
-      failwith("could not get temporary path");
+      failwith_formatmessage("GetTempPath");
   }
 #else
   static const char *dir = NULL;
@@ -162,9 +179,9 @@ static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds
 {
   DWORD dwNumberOfBytesWritten;
   if (!WriteFile(hPipe, fds, 3 * sizeof(HANDLE), &dwNumberOfBytesWritten, NULL) || dwNumberOfBytesWritten != 3 * sizeof(HANDLE))
-    failwith_perror("sendmsg");
+    failwith_formatmessage("WriteFile/sendmsg");
   if (!WriteFile(hPipe, buffer, len, &dwNumberOfBytesWritten, NULL) || dwNumberOfBytesWritten != len)
-    failwith_perror("send");
+    failwith_formatmessage("WriteFile/send");
 }
 
 #else
@@ -282,7 +299,7 @@ static HANDLE connect_socket(const char *socketname, bool fail)
   HANDLE hPipe;
   hPipe = CreateFile(socketname, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, 0);
   if (hPipe == INVALID_HANDLE_VALUE)
-    if (fail) failwith_perror("connect");
+    if (fail) failwith_formatmessage("CreateFile/connect");
   return hPipe;
 }
 #else
@@ -338,11 +355,11 @@ static void start_server(const char *socketname, const char* eventname, const ch
   /* Note that DETACHED_PROCESS means that the process does not appear in Task Manager
      but the server can still be stopped with ocamlmerlin server stop-server */
   if (!CreateProcess(exec_path, buf, NULL, NULL, FALSE, DETACHED_PROCESS, NULL, lpSystemDir, &si, &pi))
-    failwith_perror("fork");
+    failwith_formatmessage("CreateProcess/fork");
   CloseHandle(pi.hProcess);
   CloseHandle(pi.hThread);
   if (WaitForSingleObject(hEvent, 5000) != WAIT_OBJECT_0)
-    failwith_perror("execlp");
+    failwith_formatmessage("WaitForSingleObject/execlp");
 }
 #else
 static void make_daemon(void)
@@ -603,7 +620,7 @@ static void compute_socketname(char socketname[ATLEAST PATHSZ], char eventname[A
   LPSTR user_sid_string;
   HANDLE hFile = CreateFile(merlin_path, FILE_READ_ATTRIBUTES, FILE_SHARE_DELETE | FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
   if (hFile == INVALID_HANDLE_VALUE || !GetFileInformationByHandle(hFile, &info))
-    failwith_perror("stat (cannot find ocamlmerlin binary)");
+    failwith_formatmessage("CreateFile/stat (cannot find ocamlmerlin binary)");
   CloseHandle(hFile);
 
   user_sid_string = retrieve_user_sid_string() ;
@@ -697,17 +714,17 @@ int main(int argc, char **argv)
 #ifdef _WIN32
     hProcess = GetCurrentProcess();
     if (!GetNamedPipeServerProcessId(sock, &pid))
-      failwith_perror("GetNamedPipeServerProcessId");
+      failwith_formatmessage("GetNamedPipeServerProcessId");
     hServerProcess = OpenProcess(PROCESS_DUP_HANDLE, FALSE, pid);
     if (hServerProcess == INVALID_HANDLE_VALUE)
-      failwith_perror("OpenProcess");
+      failwith_formatmessage("OpenProcess");
     if (!DuplicateHandle(hProcess, GetStdHandle(STD_INPUT_HANDLE), hServerProcess, &fds[0], 0, FALSE, DUPLICATE_SAME_ACCESS))
-      failwith_perror("DuplicateHandle(stdin)");
+      failwith_formatmessage("DuplicateHandle(stdin)");
     if (!DuplicateHandle(hProcess, GetStdHandle(STD_OUTPUT_HANDLE), hServerProcess, &fds[1], 0, FALSE, DUPLICATE_SAME_ACCESS))
-      failwith_perror("DuplicateHandle(stdout)");
+      failwith_formatmessage("DuplicateHandle(stdout)");
     CloseHandle(GetStdHandle(STD_OUTPUT_HANDLE));
     if (!DuplicateHandle(hProcess, GetStdHandle(STD_ERROR_HANDLE), hServerProcess, &fds[2], 0, FALSE, DUPLICATE_SAME_ACCESS))
-      failwith_perror("DuplicateHandle(stderr)");
+      failwith_formatmessage("DuplicateHandle(stderr)");
 #else
     int fds[3] = { STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO };
 #endif

From dee4d3a98935a72743b13e29c1c70c15ec074fe0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 09:50:32 +0100
Subject: [PATCH 14/25] ocamlmerlin.c: use C99-conforming snprintf with
 mingw+msvcrt

_snprintf doesn't null-terminate strings.
MSVCRT doens't provide snprintf, but if __USE_MINGW_ANSI_STDIO is
defined, then mingw-w64 will provide this function.
There are environments where mingw-w64 is configured with MSVCRT, and
others where it is configured with UCRT. It seems safer to use the
compatibility shims.
UCRT provides snprintf, and MSVC defaults to UCRT.
---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index d89f51bdb..a76b89ecc 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -1,3 +1,4 @@
+#define __USE_MINGW_ANSI_STDIO 1
 #include <stddef.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -23,9 +24,6 @@
 #endif
 #ifdef _MSC_VER
 typedef SSIZE_T ssize_t;
-#ifndef _UCRT
-#define snprintf _snprintf
-#endif
 #endif
 #else
 #include <unistd.h>

From a29431fbc51f16118958da7fa4111c4b8d47713a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 10:04:07 +0100
Subject: [PATCH 15/25] ocamlmerlin.c: mark some objects as const

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index a76b89ecc..d5481ef00 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -173,7 +173,7 @@ static const char *path_socketdir(void)
 #ifdef _WIN32
 /** Deal with Windows IPC **/
 
-static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds[ATLEAST 3])
+static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, const HANDLE fds[ATLEAST 3])
 {
   DWORD dwNumberOfBytesWritten;
   if (!WriteFile(hPipe, fds, 3 * sizeof(HANDLE), &dwNumberOfBytesWritten, NULL) || dwNumberOfBytesWritten != 3 * sizeof(HANDLE))
@@ -185,7 +185,7 @@ static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, HANDLE fds
 #else
 /** Deal with UNIX IPC **/
 
-static void ipc_send(int fd, unsigned char *buffer, size_t len, int fds[ATLEAST 3])
+static void ipc_send(int fd, unsigned char *buffer, size_t len, const int fds[ATLEAST 3])
 {
   char msg_control[CMSG_SPACE(3 * sizeof(int))];
   struct iovec iov = { .iov_base = buffer, .iov_len = len };
@@ -253,7 +253,7 @@ extern __declspec(dllimport) char **environ;
 extern char **environ;
 #endif
 
-static ssize_t prepare_args(unsigned char *buffer, size_t len, int argc, char **argv)
+static ssize_t prepare_args(unsigned char *buffer, size_t len, int argc, char * const *argv)
 {
   int i = 0;
   ssize_t j = 4;
@@ -638,7 +638,7 @@ static void compute_socketname(char socketname[ATLEAST PATHSZ], char eventname[A
 }
 
 #else
-static void compute_socketname(char socketname[ATLEAST SOCKSZ], struct stat *st)
+static void compute_socketname(char socketname[ATLEAST SOCKSZ], const struct stat *st)
 {
   snprintf(socketname, SOCKSZ,
       "ocamlmerlin_%llu_%llu_%llu.socket",
@@ -662,7 +662,7 @@ static void dumpinfo(void)
       "merlin path: %s\nsocket path: %s/%s\n", merlin_path, path_socketdir(), socketname);
 }
 
-static void unexpected_termination(int argc, char **argv)
+static void unexpected_termination(int argc, char * const *argv)
 {
   bool sexp = false;
   int i;

From 1e493ebf30fd1a30ad2061937588c6ce8d56e1bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 11:09:21 +0100
Subject: [PATCH 16/25] ocamlmerlin.c: fix sign-compare warnings

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index d5481ef00..40318bf28 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -173,7 +173,7 @@ static const char *path_socketdir(void)
 #ifdef _WIN32
 /** Deal with Windows IPC **/
 
-static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, const HANDLE fds[ATLEAST 3])
+static void ipc_send(HANDLE hPipe, unsigned char *buffer, ssize_t len, const HANDLE fds[ATLEAST 3])
 {
   DWORD dwNumberOfBytesWritten;
   if (!WriteFile(hPipe, fds, 3 * sizeof(HANDLE), &dwNumberOfBytesWritten, NULL) || dwNumberOfBytesWritten != 3 * sizeof(HANDLE))
@@ -185,7 +185,7 @@ static void ipc_send(HANDLE hPipe, unsigned char *buffer, size_t len, const HAND
 #else
 /** Deal with UNIX IPC **/
 
-static void ipc_send(int fd, unsigned char *buffer, size_t len, const int fds[ATLEAST 3])
+static void ipc_send(int fd, unsigned char *buffer, ssize_t len, const int fds[ATLEAST 3])
 {
   char msg_control[CMSG_SPACE(3 * sizeof(int))];
   struct iovec iov = { .iov_base = buffer, .iov_len = len };
@@ -229,7 +229,7 @@ static void ipc_send(int fd, unsigned char *buffer, size_t len, const int fds[AT
 
 #define byte(x,n) ((unsigned)((x) >> (n * 8)) & 0xFF)
 
-static void append_argument(unsigned char *buffer, size_t len, ssize_t *pos, const char *p)
+static void append_argument(unsigned char *buffer, ssize_t len, ssize_t *pos, const char *p)
 {
   ssize_t j = *pos;
   while (*p && j < len)
@@ -253,7 +253,7 @@ extern __declspec(dllimport) char **environ;
 extern char **environ;
 #endif
 
-static ssize_t prepare_args(unsigned char *buffer, size_t len, int argc, char * const *argv)
+static ssize_t prepare_args(unsigned char *buffer, ssize_t len, int argc, char * const *argv)
 {
   int i = 0;
   ssize_t j = 4;

From 309b738793bee2da1a97fa5999a5bf0b361bd7a5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 11:10:57 +0100
Subject: [PATCH 17/25] ocamlmerlin.c: avoid shadowing and mixing variables

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 40318bf28..55345aa99 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -684,7 +684,6 @@ static void unexpected_termination(int argc, char * const *argv)
 int main(int argc, char **argv)
 {
   char result = 0;
-  int err = 0;
   struct stat st;
 #ifdef _WIN32
   HANDLE fds[3];
@@ -730,11 +729,11 @@ int main(int argc, char **argv)
 
 #ifdef _WIN32
     if (ReadFile(sock, &result, 1, &dwNumberOfBytesRead, NULL) && dwNumberOfBytesRead == 1)
-      err = 1;
 #else
-    NO_EINTR(err, read(sock, &result, 1));
+    ssize_t read_;
+    NO_EINTR(read_, read(sock, &result, 1));
+    if (read_ == 1)
 #endif
-    if (err == 1)
       exit(result);
 
     unexpected_termination(argc, argv);

From eee30d7b1c713b4c1cc644960c3a8faff9d9b589 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Wed, 26 Nov 2025 00:36:27 +0100
Subject: [PATCH 18/25] os_ipc_stub.c: use C99 for loop

---
 src/platform/os_ipc_stub.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/src/platform/os_ipc_stub.c b/src/platform/os_ipc_stub.c
index 6f7384129..be61f6990 100644
--- a/src/platform/os_ipc_stub.c
+++ b/src/platform/os_ipc_stub.c
@@ -57,9 +57,7 @@ ml_merlin_set_environ(value venviron)
   if (environ)
     *environ = NULL;
 
-  size_t i, j;
-
-  for (i = 0, j = 0; i < length; ++i)
+  for (size_t i = 0, j = 0; i < length; ++i)
   {
     if (buffer[i] == '\0')
     {
@@ -142,20 +140,16 @@ static ssize_t recv_buffer(int fd, int fds[ATLEAST 3])
   /* Check malformed packet */
   if (nfds != 3 || recvd != target || buffer[recvd-1] != '\0')
   {
-    int i;
-    for (i = 0; i < nfds; ++i)
+    for (int i = 0; i < nfds; ++i)
       close(fds0[i]);
     return -1;
   }
 
+  for (int i = 0; i < 3; ++i)
   {
-    int i;
-    for (i = 0; i < 3; ++i)
-    {
-      fds[i] = fds0[i];
-      if (fcntl(fds[i], F_SETFD, FD_CLOEXEC) == -1)
-        perror("fcntl");
-    }
+    fds[i] = fds0[i];
+    if (fcntl(fds[i], F_SETFD, FD_CLOEXEC) == -1)
+      perror("fcntl");
   }
 
   return recvd;

From 992016b5d801c71ed4133a2526658ea9f12eb533 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 11:21:01 +0100
Subject: [PATCH 19/25] Ensure CMSG accesses are aligned

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 22 ++++++++++------------
 src/platform/os_ipc_stub.c             | 21 +++++++++++----------
 2 files changed, 21 insertions(+), 22 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index 55345aa99..ba9002260 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -187,24 +187,22 @@ static void ipc_send(HANDLE hPipe, unsigned char *buffer, ssize_t len, const HAN
 
 static void ipc_send(int fd, unsigned char *buffer, ssize_t len, const int fds[ATLEAST 3])
 {
-  char msg_control[CMSG_SPACE(3 * sizeof(int))];
+  union {
+    char buf[CMSG_SPACE(3 * sizeof(int))];
+    struct cmsghdr align;
+  } u;
   struct iovec iov = { .iov_base = buffer, .iov_len = len };
-  struct msghdr msg = {
-    .msg_iov = &iov, .msg_iovlen = 1,
-    .msg_controllen = CMSG_SPACE(3 * sizeof(int)),
-  };
-  msg.msg_control = &msg_control;
-  memset(msg.msg_control, 0, msg.msg_controllen);
+  struct msghdr msg = { 0 };
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+  msg.msg_control = &u.buf;
+  msg.msg_controllen = sizeof(u.buf);
 
   struct cmsghdr *cm = CMSG_FIRSTHDR(&msg);
   cm->cmsg_level = SOL_SOCKET;
   cm->cmsg_type = SCM_RIGHTS;
   cm->cmsg_len = CMSG_LEN(3 * sizeof(int));
-
-  int *fds0 = (int*)CMSG_DATA(cm);
-  fds0[0] = fds[0];
-  fds0[1] = fds[1];
-  fds0[2] = fds[2];
+  memcpy(CMSG_DATA(cm), fds, 3 * sizeof(int));
 
   ssize_t sent;
   NO_EINTR(sent, sendmsg(fd, &msg, 0));
diff --git a/src/platform/os_ipc_stub.c b/src/platform/os_ipc_stub.c
index be61f6990..b90052134 100644
--- a/src/platform/os_ipc_stub.c
+++ b/src/platform/os_ipc_stub.c
@@ -81,14 +81,16 @@ static unsigned char buffer[BUFFER_SIZE];
 
 static ssize_t recv_buffer(int fd, int fds[ATLEAST 3])
 {
-  char msg_control[CMSG_SPACE(3 * sizeof(int))];
+  union {
+    char buf[CMSG_SPACE(3 * sizeof(int))];
+    struct cmsghdr align;
+  } u;
   struct iovec iov = { .iov_base = buffer, .iov_len = sizeof(buffer) };
-  struct msghdr msg = {
-    .msg_iov = &iov, .msg_iovlen = 1,
-    .msg_controllen = CMSG_SPACE(3 * sizeof(int)),
-  };
-  msg.msg_control = &msg_control;
-  memset(msg.msg_control, 0, msg.msg_controllen);
+  struct msghdr msg = { 0 };
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+  msg.msg_control = &u.buf;
+  msg.msg_controllen = sizeof(u.buf);
 
   ssize_t recvd;
   NO_EINTR(recvd, recvmsg(fd, &msg, 0));
@@ -134,20 +136,19 @@ static ssize_t recv_buffer(int fd, int fds[ATLEAST 3])
     perror("recvmsg");
     return -1;
   }
-  int *fds0 = (int*)CMSG_DATA(cm);
   int nfds = (cm->cmsg_len - CMSG_LEN(0)) / sizeof(int);
+  memcpy(fds, CMSG_DATA(cm), nfds * sizeof(int));
 
   /* Check malformed packet */
   if (nfds != 3 || recvd != target || buffer[recvd-1] != '\0')
   {
     for (int i = 0; i < nfds; ++i)
-      close(fds0[i]);
+      close(fds[i]);
     return -1;
   }
 
   for (int i = 0; i < 3; ++i)
   {
-    fds[i] = fds0[i];
     if (fcntl(fds[i], F_SETFD, FD_CLOEXEC) == -1)
       perror("fcntl");
   }

From 50af671ff5f4d28aaf91da073e5a8581010ba446 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 17:48:17 +0100
Subject: [PATCH 20/25] ocamlmerlin.c: fix mingw-w64 format-truncation warnings

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index ba9002260..b9cd22590 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -340,8 +340,11 @@ static void start_server(const char *socketname, const char* eventname, const ch
   PROCESS_INFORMATION pi;
   STARTUPINFO si;
   HANDLE hEvent = CreateEvent(NULL, FALSE, FALSE, eventname);
+  int len;
 
-  sprintf(buf, "%s server %s %s", exec_path, socketname, eventname);
+  len = snprintf(buf, countof(buf), "%s server %s %s", exec_path, socketname, eventname);
+  if (len < 0 || (unsigned)len > countof(buf) - 1)
+    failwith("arguments array was truncated");
   ZeroMemory(&si, sizeof(si));
   si.cb = sizeof(si);
   ZeroMemory(&pi, sizeof(pi));
@@ -623,14 +626,18 @@ static void compute_socketname(char socketname[ATLEAST PATHSZ], char eventname[A
   if (! user_sid_string)
     user_sid_string = LocalAlloc(LPTR, 1);
 
+  int len;
   // @@DRA Need to use Windows API functions to get meaningful values for st_dev and st_ino
-  snprintf(eventname, PATHSZ,
-      "ocamlmerlin_%s_%lx_%llx",
-      user_sid_string,
-      info.dwVolumeSerialNumber,
-      ((__int64)info.nFileIndexHigh) << 32 | ((__int64)info.nFileIndexLow));
-  snprintf(socketname, PATHSZ,
-      "\\\\.\\pipe\\%s", eventname);
+  len = snprintf(eventname, PATHSZ, "ocamlmerlin_%s_%lx_%llx",
+                 user_sid_string,
+                 info.dwVolumeSerialNumber,
+                 ((__int64)info.nFileIndexHigh) << 32 | ((__int64)info.nFileIndexLow));
+  if (len < 0 || (unsigned) len > PATHSZ - 1)
+    failwith("event name was truncated");
+
+  len = snprintf(socketname, PATHSZ, "\\\\.\\pipe\\%s", eventname);
+  if (len < 0 || (unsigned) len > PATHSZ - 1)
+    failwith("socket name was truncated");
 
   LocalFree(user_sid_string);
 }

From 782afce3b0406a5b358633c55838bf9039a50e7e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 11:04:17 +0100
Subject: [PATCH 21/25] ocamlmerlin: use Dune to generate the C compiler
 invocation

---
 src/frontend/ocamlmerlin/dune           | 49 +++++++++++++++++++------
 src/frontend/ocamlmerlin/gen_ccflags.ml | 17 ---------
 2 files changed, 38 insertions(+), 28 deletions(-)
 delete mode 100644 src/frontend/ocamlmerlin/gen_ccflags.ml

diff --git a/src/frontend/ocamlmerlin/dune b/src/frontend/ocamlmerlin/dune
index 8b17d252d..a3c44c7f6 100644
--- a/src/frontend/ocamlmerlin/dune
+++ b/src/frontend/ocamlmerlin/dune
@@ -14,7 +14,6 @@
   -open Merlin_analysis
   -open Merlin_kernel
   -open Merlin_commands)
- (modules (:standard \ gen_ccflags))
  (libraries merlin-lib.config yojson merlin-lib.analysis merlin-lib.kernel
             merlin-lib.utils merlin-lib.os_ipc merlin-lib.ocaml_parsing
             merlin-lib.query_protocol merlin-lib.query_commands
@@ -22,20 +21,48 @@
             merlin-lib.commands
             unix str))
 
-(executable
- (name      gen_ccflags)
- (modules   gen_ccflags)
- (libraries str))
+; Write one flag per line in the output file
+
+(rule
+ (enabled_if (= %{ocaml-config:ccomp_type} "cc"))
+ (action
+  (progn
+   (write-file ldlibs "")
+   (write-file outputexe "-o"))))
+
+(rule
+ (enabled_if (= %{ocaml-config:ccomp_type} "msvc"))
+ (action
+  (progn
+   (write-file ldlibs "advapi32.lib")
+   (write-file outputexe "-Fe"))))
+
+(rule
+ (enabled_if
+  (and
+   (= %{ocaml-config:ccomp_type} "cc")
+   (= %{profile} "dev")))
+ (action
+  (write-file cflags "-Wextra\n-Werror")))
+
+(rule
+ (enabled_if
+  (and
+   (= %{ocaml-config:ccomp_type} "msvc")
+   (= %{profile} "dev")))
+ (action
+  (write-file cflags "-WX")))
 
 (rule
- (targets pre-flags post-flags)
- (deps    gen_ccflags.exe)
- (action  (run %{deps} "%{ocaml-config:ccomp_type}" %{targets})))
+ (enabled_if
+  (<> %{profile} "dev"))
+ (action
+  (write-file cflags "")))
 
 (rule
- (targets ocamlmerlin.exe)
- (deps    (:c ocamlmerlin.c) pre-flags post-flags)
- (action  (run %{cc} "%{read-lines:pre-flags}%{targets}" %{c} %{read-lines:post-flags})))
+ (target ocamlmerlin.exe)
+ (deps   (:c ocamlmerlin.c))
+ (action (run %{cc} %{read-strings:cflags} %{read:outputexe}%{target} %{c} %{read-strings:ldlibs})))
 
 (install
  (package merlin)
diff --git a/src/frontend/ocamlmerlin/gen_ccflags.ml b/src/frontend/ocamlmerlin/gen_ccflags.ml
deleted file mode 100644
index 5bbf386b2..000000000
--- a/src/frontend/ocamlmerlin/gen_ccflags.ml
+++ /dev/null
@@ -1,17 +0,0 @@
-let ccomp_type = Sys.argv.(1)
-let pre_flags_f = Sys.argv.(2)
-let post_flags_f = Sys.argv.(3)
-
-let pre_flags, post_flags =
-  if Str.string_match (Str.regexp "msvc") ccomp_type 0 then
-    ("/Fe", "advapi32.lib")
-  else ("-o", "")
-
-let write_lines f s =
-  let oc = open_out f in
-  output_string oc s;
-  close_out oc
-
-let () =
-  write_lines pre_flags_f pre_flags;
-  write_lines post_flags_f post_flags

From b36f7099247f3e9ebe86722a0a622311a90df978 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 18:03:13 +0100
Subject: [PATCH 22/25] os_ipc_stub: fix unused variable warning

---
 src/platform/os_ipc_stub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/platform/os_ipc_stub.c b/src/platform/os_ipc_stub.c
index b90052134..416ba208b 100644
--- a/src/platform/os_ipc_stub.c
+++ b/src/platform/os_ipc_stub.c
@@ -161,13 +161,13 @@ value ml_merlin_server_setup(value path, value strfd)
 {
   CAMLparam2(path, strfd);
   CAMLlocal2(payload, ret);
-  char *endptr = NULL;
   int fd;
 
 #ifdef _WIN32
   fd = 0;
   ret = strfd;
 #else
+  char *endptr = NULL;
   fd = strtol(String_val(strfd), &endptr, 0);
   if (!endptr || *endptr != '\0')
     fd = -1;

From 226400a1f54ca4e59b92f9e62cf5b0baac848fa6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 17:55:42 +0100
Subject: [PATCH 23/25] Don't run Make targets in parallel

https://www.gnu.org/software/make/manual/html_node/Parallel-Disable.html
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index 920fcf0f6..18dc627a0 100644
--- a/Makefile
+++ b/Makefile
@@ -26,3 +26,4 @@ bench:
 	jq . merl-an_bench/bench.json
 
 .PHONY: all build dev clean test promote bench bench
+.NOTPARALLEL:

From 0528a3406cd41d3dc915939a762589f9939d0483 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Tue, 25 Nov 2025 11:36:50 +0100
Subject: [PATCH 24/25] Update changes

---
 CHANGES.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/CHANGES.md b/CHANGES.md
index a1b98ec3d..31a41585f 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,8 @@
 unreleased
 ==========
 
+  + merlin binary
+    - Fix a plethora of minor issues with the C code (#1998)
   + merlin library
     - Fix completion not working for inlined records labels (#1978, fixes #1977)
     - Perform buffer indexing only if the query requires it (#1990 and #1991)

From 4e51ecbc428ee35d85edf81ce2d369919fd3e58c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonin=20D=C3=A9cimo?= <antonin@tarides.com>
Date: Mon, 1 Dec 2025 15:37:02 +0000
Subject: [PATCH 25/25] ocamlmerlin.c: we're going to need a bigger buffer

---
 src/frontend/ocamlmerlin/ocamlmerlin.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/frontend/ocamlmerlin/ocamlmerlin.c b/src/frontend/ocamlmerlin/ocamlmerlin.c
index b9cd22590..8f7130cd9 100644
--- a/src/frontend/ocamlmerlin/ocamlmerlin.c
+++ b/src/frontend/ocamlmerlin/ocamlmerlin.c
@@ -336,7 +336,7 @@ static int connect_socket(const char *socketname, bool fail)
 #ifdef _WIN32
 static void start_server(const char *socketname, const char* eventname, const char *exec_path)
 {
-  char buf[PATHSZ], lpSystemDir[PATHSZ];
+  char buf[32767], lpSystemDir[PATHSZ];
   PROCESS_INFORMATION pi;
   STARTUPINFO si;
   HANDLE hEvent = CreateEvent(NULL, FALSE, FALSE, eventname);