diff --git a/Makefile b/Makefile
index 9999a09490..411669e8bf 100644
--- a/Makefile
+++ b/Makefile
@@ -355,7 +355,7 @@ GIT-VERSION-FILE: FORCE
 
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
-CFLAGS = -g -O2 -Wall
+CFLAGS = -g -O2 -Wall -Werror
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
diff --git a/compat/mingw.c b/compat/mingw.c
index efe840c1e2..ab2957dec3 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -8,6 +8,8 @@
 
 #undef isatty
 
+#define HCAST(type, handle) ((type)(intptr_t)handle)
+
 static const int delay[] = { 0, 1, 10, 20, 40 };
 unsigned int _CRT_fmode = _O_BINARY;
 
@@ -728,13 +730,13 @@ int pipe(int filedes[2])
 		errno = err_win_to_posix(GetLastError());
 		return -1;
 	}
-	filedes[0] = _open_osfhandle((int)h[0], O_NOINHERIT);
+	filedes[0] = _open_osfhandle(HCAST(int, h[0]), O_NOINHERIT);
 	if (filedes[0] < 0) {
 		CloseHandle(h[0]);
 		CloseHandle(h[1]);
 		return -1;
 	}
-	filedes[1] = _open_osfhandle((int)h[1], O_NOINHERIT);
+	filedes[1] = _open_osfhandle(HCAST(int, h[1]), O_NOINHERIT);
 	if (filedes[0] < 0) {
 		close(filedes[0]);
 		CloseHandle(h[1]);
@@ -1958,7 +1960,8 @@ void mingw_open_html(const char *unixpath)
 		die("cannot run browser");
 
 	printf("Launching default browser to display HTML ...\n");
-	r = (int)ShellExecute(NULL, "open", htmlpath, NULL, "\\", SW_SHOWNORMAL);
+	r = HCAST(int, ShellExecute(NULL, "open", htmlpath,
+				NULL, "\\", SW_SHOWNORMAL));
 	FreeLibrary(shell32);
 	/* see the MSDN documentation referring to the result codes here */
 	if (r <= 32) {
@@ -2364,8 +2367,8 @@ int mingw_isatty(int fd) {
 			char buffer[64];
 
 			for (i = 0; i < ARRAY_SIZE(is_tty); i++) {
-				sprintf(buffer, " %ld ", (unsigned long)
-					GetStdHandle(id[i]));
+				sprintf(buffer, " %" PRIuMAX " ",
+					HCAST(uintmax_t, GetStdHandle(id[i])));
 				is_tty[i] = !!strstr(env, buffer);
 			}
 		}
diff --git a/compat/poll/poll.c b/compat/poll/poll.c
index db4e03ed79..b10adc780f 100644
--- a/compat/poll/poll.c
+++ b/compat/poll/poll.c
@@ -76,7 +76,7 @@
 
 #ifdef WIN32_NATIVE
 
-#define IsConsoleHandle(h) (((long) (h) & 3) == 3)
+#define IsConsoleHandle(h) (((long) (intptr_t) (h) & 3) == 3)
 
 static BOOL
 IsSocketHandle (HANDLE h)
diff --git a/compat/regex/regcomp.c b/compat/regex/regcomp.c
index 06f3088708..d8bde06f1a 100644
--- a/compat/regex/regcomp.c
+++ b/compat/regex/regcomp.c
@@ -2577,7 +2577,7 @@ parse_dup_op (bin_tree_t *elem, re_string_t *regexp, re_dfa_t *dfa,
     old_tree = NULL;
 
   if (elem->token.type == SUBEXP)
-    postorder (elem, mark_opt_subexp, (void *) (long) elem->token.opr.idx);
+    postorder (elem, mark_opt_subexp, (void *) (intptr_t) elem->token.opr.idx);
 
   tree = create_tree (dfa, elem, NULL, (end == -1 ? OP_DUP_ASTERISK : OP_ALT));
   if (BE (tree == NULL, 0))
@@ -3806,7 +3806,7 @@ create_token_tree (re_dfa_t *dfa, bin_tree_t *left, bin_tree_t *right,
 static reg_errcode_t
 mark_opt_subexp (void *extra, bin_tree_t *node)
 {
-  int idx = (int) (long) extra;
+  int idx = (int) (intptr_t) extra;
   if (node->token.type == SUBEXP && node->token.opr.idx == idx)
     node->token.opt_subexp = 1;
 
diff --git a/compat/win32/pthread.h b/compat/win32/pthread.h
index 1ade961769..62151d7550 100644
--- a/compat/win32/pthread.h
+++ b/compat/win32/pthread.h
@@ -18,7 +18,10 @@
  */
 #define pthread_mutex_t CRITICAL_SECTION
 
-#define pthread_mutex_init(a,b) (InitializeCriticalSection((a)), 0)
+static inline int return_0(int i) {
+	return 0;
+}
+#define pthread_mutex_init(a,b) return_0((InitializeCriticalSection((a)), 0))
 #define pthread_mutex_destroy(a) DeleteCriticalSection((a))
 #define pthread_mutex_lock EnterCriticalSection
 #define pthread_mutex_unlock LeaveCriticalSection
diff --git a/compat/winansi.c b/compat/winansi.c
index 71fb0a4f25..23dec64007 100644
--- a/compat/winansi.c
+++ b/compat/winansi.c
@@ -456,7 +456,8 @@ static HANDLE duplicate_handle(HANDLE hnd)
 	HANDLE hresult, hproc = GetCurrentProcess();
 	if (!DuplicateHandle(hproc, hnd, hproc, &hresult, 0, TRUE,
 			DUPLICATE_SAME_ACCESS))
-		die_lasterr("DuplicateHandle(%li) failed", (long) hnd);
+		die_lasterr("DuplicateHandle(%li) failed",
+			(long) (intptr_t) hnd);
 	return hresult;
 }
 
diff --git a/config.mak.uname b/config.mak.uname
index 06c47af02b..98aad70555 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -547,6 +547,7 @@ else
 			BASIC_LDFLAGS += -Wl,--large-address-aware
 		endif
 		CC = gcc
+		COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0
 		INSTALL = /bin/install
 		NO_R_TO_GCC_LINKER = YesPlease
 		INTERNAL_QSORT = YesPlease
diff --git a/git-compat-util.h b/git-compat-util.h
index 6373d93679..fc66006b76 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -516,7 +516,7 @@ extern int git_lstat(const char *, struct stat *);
 #endif
 
 #define DEFAULT_PACKED_GIT_LIMIT \
-	((1024L * 1024L) * (sizeof(void*) >= 8 ? 8192 : 256))
+	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? 8192 : 256))
 
 #ifdef NO_PREAD
 #define pread git_pread
diff --git a/pack-revindex.c b/pack-revindex.c
index 5c8376e978..e542ea7703 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -21,7 +21,7 @@ static int pack_revindex_hashsz;
 
 static int pack_revindex_ix(struct packed_git *p)
 {
-	unsigned long ui = (unsigned long)p;
+	unsigned long ui = (unsigned long)(intptr_t)p;
 	int i;
 
 	ui = ui ^ (ui >> 16); /* defeat structure alignment */
diff --git a/sha1_file.c b/sha1_file.c
index 88f06bac92..77efc0e8a9 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -2017,7 +2017,7 @@ static unsigned long pack_entry_hash(struct packed_git *p, off_t base_offset)
 {
 	unsigned long hash;
 
-	hash = (unsigned long)p + (unsigned long)base_offset;
+	hash = (unsigned long)(intptr_t)p + (unsigned long)base_offset;
 	hash += (hash >> 8) + (hash >> 16);
 	return hash % MAX_DELTA_CACHE;
 }