WASI & darwin: support env variables based on libc

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

src/syscall/syscall_libc.go

@@ -1,4 +1,4 @@
-// +build darwin nintendoswitch
+// +build darwin nintendoswitch wasi
 
 package syscall
 
@@ -6,6 +6,12 @@ import (
 	"unsafe"
 )
 
+type sliceHeader struct {
+	buf *byte
+	len uintptr
+	cap uintptr
+}
+
 func Close(fd int) (err error) {
 	return ENOSYS // TODO
 }
@@ -48,18 +54,32 @@ func Getpid() (pid int) {
 }
 
 func Getenv(key string) (value string, found bool) {
-	return "", false // TODO
+	data := append([]byte(key), 0)
+	raw := libc_getenv(&data[0])
+	if raw == nil {
+		return "", false
+	}
+
+	ptr := uintptr(unsafe.Pointer(raw))
+	for size := uintptr(0); ; size++ {
+		v := *(*byte)(unsafe.Pointer(ptr))
+		if v == 0 {
+			src := *(*[]byte)(unsafe.Pointer(&sliceHeader{buf: raw, len: size, cap: size}))
+			return string(src), true
+		}
+		ptr += unsafe.Sizeof(byte(0))
+	}
 }
 
 func splitSlice(p []byte) (buf *byte, len uintptr) {
-	slice := (*struct {
-		buf *byte
-		len uintptr
-		cap uintptr
-	})(unsafe.Pointer(&p))
+	slice := (*sliceHeader)(unsafe.Pointer(&p))
 	return slice.buf, slice.len
 }
 
 // ssize_t write(int fd, const void *buf, size_t count)
 //export write
 func libc_write(fd int32, buf *byte, count uint) int
+
+// char *getenv(const char *name);
+//export getenv
+func libc_getenv(name *byte) *byte