cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

src/runtime/baremetal.go

@@ -86,3 +86,16 @@ func AdjustTimeOffset(offset int64) {
 	// TODO: do this atomically?
 	timeOffset += offset
 }
+
+// Picolibc is not configured to define its own errno value, instead it calls
+// __errno_location.
+// TODO: a global works well enough for now (same as errno on Linux with
+// -scheduler=tasks), but this should ideally be a thread-local variable stored
+// in task.Task.
+// Especially when we add multicore support for microcontrollers.
+var errno int32
+
+//export __errno_location
+func libc_errno_location() *int32 {
+	return &errno
+}

src/runtime/os_darwin.go

@@ -151,7 +151,7 @@ func syscall_rawSyscall(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	r1 = uintptr(result)
 	if result == -1 {
 		// Syscall returns -1 on failure.
-		err = uintptr(*libc___error())
+		err = uintptr(*libc_errno_location())
 	}
 	return
 }
@@ -161,7 +161,7 @@ func syscall_syscallX(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	r1 = call_syscallX(fn, a1, a2, a3)
 	if int64(r1) == -1 {
 		// Syscall returns -1 on failure.
-		err = uintptr(*libc___error())
+		err = uintptr(*libc_errno_location())
 	}
 	return
 }
@@ -171,7 +171,7 @@ func syscall_syscallPtr(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	r1 = call_syscallX(fn, a1, a2, a3)
 	if r1 == 0 {
 		// Syscall returns a pointer on success, or NULL on failure.
-		err = uintptr(*libc___error())
+		err = uintptr(*libc_errno_location())
 	}
 	return
 }
@@ -182,7 +182,7 @@ func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 	r1 = uintptr(result)
 	if result == -1 {
 		// Syscall returns -1 on failure.
-		err = uintptr(*libc___error())
+		err = uintptr(*libc_errno_location())
 	}
 	return
 }
@@ -192,7 +192,7 @@ func syscall_syscall6X(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 	r1 = call_syscall6X(fn, a1, a2, a3, a4, a5, a6)
 	if int64(r1) == -1 {
 		// Syscall returns -1 on failure.
-		err = uintptr(*libc___error())
+		err = uintptr(*libc_errno_location())
 	}
 	return
 }
@@ -216,7 +216,7 @@ func libc_getpagesize() int32
 //	}
 //
 //export __error
-func libc___error() *int32
+func libc_errno_location() *int32
 
 //export tinygo_syscall
 func call_syscall(fn, a1, a2, a3 uintptr) int32

src/runtime/os_windows.go

@@ -113,3 +113,6 @@ func syscall_Getpagesize() int {
 	_GetSystemInfo(unsafe.Pointer(&info))
 	return int(info.dwpagesize)
 }
+
+//export _errno
+func libc_errno_location() *int32

src/runtime/runtime.go

@@ -127,3 +127,8 @@ func write(fd uintptr, p unsafe.Pointer, n int32) int32 {
 func getAuxv() []uintptr {
 	return nil
 }
+
+// Called from cgo to obtain the errno value.
+func cgo_errno() uintptr {
+	return uintptr(*libc_errno_location())
+}

src/runtime/runtime_nintendoswitch.go

@@ -313,3 +313,9 @@ func hardwareRand() (n uint64, ok bool) {
 	// TODO: see whether there is a RNG and use it.
 	return 0, false
 }
+
+func libc_errno_location() *int32 {
+	// CGo is unavailable, so this function should be unreachable.
+	runtimePanic("runtime: no cgo errno")
+	return nil
+}

src/runtime/runtime_tinygowasm.go

@@ -112,3 +112,8 @@ func hardwareRand() (n uint64, ok bool) {
 //
 //export arc4random
 func libc_arc4random() uint32
+
+// int *__errno_location(void);
+//
+//export __errno_location
+func libc_errno_location() *int32

src/runtime/runtime_tinygowasm_unknown.go

@@ -51,3 +51,9 @@ func procUnpin() {
 func hardwareRand() (n uint64, ok bool) {
 	return 0, false
 }
+
+func libc_errno_location() *int32 {
+	// CGo is unavailable, so this function should be unreachable.
+	runtimePanic("runtime: no cgo errno")
+	return nil
+}

src/runtime/runtime_tinygowasmp2.go

@@ -81,3 +81,9 @@ func procUnpin() {
 func hardwareRand() (n uint64, ok bool) {
 	return random.GetRandomU64(), true
 }
+
+func libc_errno_location() *int32 {
+	// CGo is unavailable, so this function should be unreachable.
+	runtimePanic("runtime: no cgo errno")
+	return nil
+}