diff --git a/compat/compiler.h b/compat/compiler.h
index e9ad9db84f..e12e426404 100644
--- a/compat/compiler.h
+++ b/compat/compiler.h
@@ -9,7 +9,7 @@
 
 static inline void get_compiler_info(struct strbuf *info)
 {
-	int len = info->len;
+	size_t len = info->len;
 #ifdef __clang__
 	strbuf_addf(info, "clang: %s\n", __clang_version__);
 #elif defined(__GNUC__)
@@ -27,7 +27,7 @@ static inline void get_compiler_info(struct strbuf *info)
 
 static inline void get_libc_info(struct strbuf *info)
 {
-	int len = info->len;
+	size_t len = info->len;
 
 #ifdef __GLIBC__
 	strbuf_addf(info, "glibc: %s\n", gnu_get_libc_version());
diff --git a/compat/mingw.c b/compat/mingw.c
index 0e851ecae2..5293f4cdae 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -782,7 +782,7 @@ static inline void filetime_to_timespec(const FILETIME *ft, struct timespec *ts)
  */
 static int has_valid_directory_prefix(wchar_t *wfilename)
 {
-	int n = wcslen(wfilename);
+	ssize_t n = wcslen(wfilename); /*can become negative*/
 
 	while (n > 0) {
 		wchar_t c = wfilename[--n];
@@ -891,7 +891,7 @@ static int do_lstat(int follow, const char *file_name, struct stat *buf)
  */
 static int do_stat_internal(int follow, const char *file_name, struct stat *buf)
 {
-	int namelen;
+	size_t namelen; /* contains length of a string*/
 	char alt_name[PATH_MAX];
 
 	if (!do_lstat(follow, file_name, buf))
@@ -1274,7 +1274,8 @@ static const char *parse_interpreter(const char *cmd)
 {
 	static char buf[100];
 	char *p, *opt;
-	int n, fd;
+	ssize_t n; /* read() can return negativ values */
+	int fd;
 
 	/* don't even try a .exe */
 	n = strlen(cmd);
@@ -1339,7 +1340,7 @@ static char *path_lookup(const char *cmd, int exe_only)
 {
 	const char *path;
 	char *prog = NULL;
-	int len = strlen(cmd);
+	size_t len = strlen(cmd);
 	int isexe = len >= 4 && !strcasecmp(cmd+len-4, ".exe");
 
 	if (strpbrk(cmd, "/\\"))
@@ -1956,7 +1957,7 @@ char *mingw_getenv(const char *name)
 #define GETENV_MAX_RETAIN 64
 	static char *values[GETENV_MAX_RETAIN];
 	static int value_counter;
-	int len_key, len_value;
+	size_t len_key, len_value; /* lengt of strings */
 	wchar_t *w_key;
 	char *value;
 	wchar_t w_value[32768];
@@ -1968,7 +1969,8 @@ char *mingw_getenv(const char *name)
 	/* We cannot use xcalloc() here because that uses getenv() itself */
 	w_key = calloc(len_key, sizeof(wchar_t));
 	if (!w_key)
-		die("Out of memory, (tried to allocate %u wchar_t's)", len_key);
+		die("Out of memory, (tried to allocate %"PRIuMAX" wchar_t's)",
+			(uintmax_t)len_key);
 	xutftowcs(w_key, name, len_key);
 	/* GetEnvironmentVariableW() only sets the last error upon failure */
 	SetLastError(ERROR_SUCCESS);
@@ -1983,7 +1985,8 @@ char *mingw_getenv(const char *name)
 	/* We cannot use xcalloc() here because that uses getenv() itself */
 	value = calloc(len_value, sizeof(char));
 	if (!value)
-		die("Out of memory, (tried to allocate %u bytes)", len_value);
+		die("Out of memory, (tried to allocate %"PRIuMAX" bytes)",
+			(uintmax_t)len_value);
 	xwcstoutf(value, w_value, len_value);
 
 	/*
@@ -2001,7 +2004,7 @@ char *mingw_getenv(const char *name)
 
 int mingw_putenv(const char *namevalue)
 {
-	int size;
+	size_t size; /* lengt of a string */
 	wchar_t *wide, *equal;
 	BOOL result;
 
@@ -2011,7 +2014,8 @@ int mingw_putenv(const char *namevalue)
 	size = strlen(namevalue) * 2 + 1;
 	wide = calloc(size, sizeof(wchar_t));
 	if (!wide)
-		die("Out of memory, (tried to allocate %u wchar_t's)", size);
+		die("Out of memory, (tried to allocate %" PRIuMAX " wchar_t's)",
+		    (uintmax_t)size);
 	xutftowcs(wide, namevalue, size);
 	equal = wcschr(wide, L'=');
 	if (!equal)
@@ -3085,7 +3089,8 @@ static void maybe_redirect_std_handles(void)
  */
 int wmain(int argc, const wchar_t **wargv)
 {
-	int i, maxlen, exit_status;
+	int i, exit_status;
+	size_t maxlen; /*contains length os arguments*/
 	char *buffer, **save;
 	const char **argv;
 
diff --git a/compat/vcbuild/include/unistd.h b/compat/vcbuild/include/unistd.h
index 3a959d124c..1c0096ab21 100644
--- a/compat/vcbuild/include/unistd.h
+++ b/compat/vcbuild/include/unistd.h
@@ -14,7 +14,11 @@ typedef _mode_t	mode_t;
 
 #ifndef _SSIZE_T_
 #define _SSIZE_T_
+#ifdef _WIN64
+typedef __int64 _ssize_t;
+#else
 typedef long _ssize_t;
+#endif // _AMD64
 
 #ifndef	_OFF_T_
 #define	_OFF_T_
