diff --git a/vlib/builtin/cfns.c.v b/vlib/builtin/cfns.c.v
index 20f31dc449..75fdbb8bac 100644
--- a/vlib/builtin/cfns.c.v
+++ b/vlib/builtin/cfns.c.v
@@ -7,7 +7,7 @@ fn C.memcmp(byteptr, byteptr, int) int
 
 fn C.memmove(byteptr, byteptr, int) voidptr
 
-fn C.calloc(int) byteptr
+fn C.calloc(int, int) byteptr
 
 fn C.malloc(int) byteptr
 
@@ -48,16 +48,16 @@ fn C.printf(byteptr, ...byteptr) int
 
 fn C.puts(byteptr) int
 
-fn C.fputs(byteptr) int
+fn C.fputs(str byteptr, stream &C.FILE) int
 
-fn C.fflush(byteptr) int
+fn C.fflush(&C.FILE) int
 
 // TODO define args in these functions
 fn C.fseek() int
 
-fn C.fopen() voidptr
+fn C.fopen(filename charptr, mode charptr) &C.FILE
 
-fn C.fileno(voidptr) int
+fn C.fileno(&C.FILE) int
 
 fn C.fread(ptr voidptr, item_size size_t, items size_t, stream &C.FILE) size_t
 
@@ -86,7 +86,7 @@ fn C.waitpid(pid int, status &int, options int) int
 
 fn C.kill(pid int, sig int) int
 
-fn C.setenv(charptr) int
+fn C.setenv(charptr, charptr, int) int
 
 fn C.unsetenv(charptr) int
 
@@ -100,7 +100,7 @@ fn C.chdir() int
 
 fn C.rewind() int
 
-fn C.stat(charptr) int
+fn C.stat(charptr, voidptr) int
 
 fn C.lstat() int
 
@@ -125,7 +125,7 @@ fn C.sleep(int) int
 
 fn C.usleep() int
 
-fn C.opendir() voidptr
+fn C.opendir(charptr) voidptr
 
 fn C.closedir() int
 
diff --git a/vlib/json/json_primitives.v b/vlib/json/json_primitives.v
index df7672b58c..b2fa5a153c 100644
--- a/vlib/json/json_primitives.v
+++ b/vlib/json/json_primitives.v
@@ -105,22 +105,22 @@ fn decode_string(root &C.cJSON) string {
 	return tos_clone(root.valuestring) // , _strlen(root.valuestring))
 }
 
-fn C.cJSON_IsTrue() bool
+fn C.cJSON_IsTrue(voidptr) bool
 
 
-fn C.cJSON_CreateNumber() &C.cJSON
+fn C.cJSON_CreateNumber(int) &C.cJSON
 
 
-fn C.cJSON_CreateBool() &C.cJSON
+fn C.cJSON_CreateBool(bool) &C.cJSON
 
 
-fn C.cJSON_CreateString() &C.cJSON
+fn C.cJSON_CreateString(charptr) &C.cJSON
 
 
-fn C.cJSON_Parse() &C.cJSON
+fn C.cJSON_Parse(charptr) &C.cJSON
 
 
-fn C.cJSON_PrintUnformatted() byteptr
+fn C.cJSON_PrintUnformatted(voidptr) byteptr
 
 
 fn decode_bool(root &C.cJSON) bool {
diff --git a/vlib/os/os_c.v b/vlib/os/os_c.v
index 9ad7e7d10b..fe40e3a4f8 100644
--- a/vlib/os/os_c.v
+++ b/vlib/os/os_c.v
@@ -16,7 +16,7 @@ fn C.ftell(fp voidptr) int
 
 fn C.sigaction(int, voidptr, int)
 
-fn C.open(charptr, int, int) int
+fn C.open(charptr, int, ...int) int
 
 fn C.fdopen(int, string) voidptr
 
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 21e26e5a5e..d22a1d3f91 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1781,6 +1781,15 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 		call_expr.should_be_skipped = true
 	}
 	if f.language != .v || call_expr.language != .v {
+		// ignore C function of type `fn()`, assume untyped
+		// For now don't check C functions that are variadic, underscored, capitalized
+		// or have no params and return int
+		if f.language == .c && f.params.len != call_expr.args.len && !f.is_variadic
+			&& f.name[2] != `_` && !f.name[2].is_capital()
+			&& (f.params.len != 0 || f.return_type !in [table.void_type, table.int_type]) {
+			// change to error later
+			c.warn('expected $f.params.len arguments, but got $call_expr.args.len', call_expr.pos)
+		}
 		for arg in call_expr.args {
 			c.expr(arg.expr)
 		}
diff --git a/vlib/v/checker/tests/c_fn_surplus_args.out b/vlib/v/checker/tests/c_fn_surplus_args.out
new file mode 100644
index 0000000000..e3aa25a967
--- /dev/null
+++ b/vlib/v/checker/tests/c_fn_surplus_args.out
@@ -0,0 +1,27 @@
+vlib/v/checker/tests/c_fn_surplus_args.vv:7:4: error: expected 1 arguments, but got 0
+    5 | fn main() {
+    6 |     C.no(1) // allowed
+    7 |     C.y1()
+      |       ~~~~
+    8 |     C.y1(1) // ok
+    9 |     C.y1(1, 2)
+vlib/v/checker/tests/c_fn_surplus_args.vv:9:4: error: expected 1 arguments, but got 2
+    7 |     C.y1()
+    8 |     C.y1(1) // ok
+    9 |     C.y1(1, 2)
+      |       ~~~~~~~~
+   10 |     C.ret() // ok
+   11 |     C.ret(1)
+vlib/v/checker/tests/c_fn_surplus_args.vv:11:4: error: expected 0 arguments, but got 1
+    9 |     C.y1(1, 2)
+   10 |     C.ret() // ok
+   11 |     C.ret(1)
+      |       ~~~~~~
+   12 |     // avoid cgen whilst warning, later above should error
+   13 |     main()
+vlib/v/checker/tests/c_fn_surplus_args.vv:13:2: error: the `main` function cannot be called in the program
+   11 |     C.ret(1)
+   12 |     // avoid cgen whilst warning, later above should error
+   13 |     main()
+      |     ~~~~~~
+   14 | }
diff --git a/vlib/v/checker/tests/c_fn_surplus_args.vv b/vlib/v/checker/tests/c_fn_surplus_args.vv
new file mode 100644
index 0000000000..7e6aa38e25
--- /dev/null
+++ b/vlib/v/checker/tests/c_fn_surplus_args.vv
@@ -0,0 +1,14 @@
+fn C.no() // untyped
+fn C.y1(int)
+fn C.ret()byte
+
+fn main() {
+	C.no(1) // allowed
+	C.y1()
+	C.y1(1) // ok
+	C.y1(1, 2)
+	C.ret() // ok
+	C.ret(1)
+	// avoid cgen whilst warning, later above should error
+	main()
+}