Fix lseek in emscripten

CHANGELOG.md

@@ -5,6 +5,7 @@ All PRs to the Wasmer repository must add to this file.
 Blocks of changes will separated by version increments.
 
 ## **[Unreleased]**
+- [#513](https://github.com/wasmerio/wasmer/pull/510) Fix emscripten lseek implementation.
 - [#510](https://github.com/wasmerio/wasmer/pull/510) Simplify construction of floating point constants in LLVM backend. Fix LLVM assertion failure due to definition of %ctx.
 
 ## 0.5.1 - 2019-06-24

lib/emscripten/src/syscalls/mod.rs

@@ -50,7 +50,6 @@ use super::env;
 use std::cell::Cell;
 #[allow(unused_imports)]
 use std::io::Error;
-use std::mem;
 use std::slice;
 
 /// exit
@@ -437,23 +436,21 @@ pub fn ___syscall140(ctx: &mut Ctx, _which: i32, mut varargs: VarArgs) -> i32 {
     // -> c_int
     debug!("emscripten::___syscall140 (lseek) {}", _which);
     let fd: i32 = varargs.get(ctx);
-    let _ = varargs.get::<i32>(ctx); // ignore high offset
+    let _offset_high: i32 = varargs.get(ctx); // We don't use the offset high as emscripten skips it
     let offset_low: i32 = varargs.get(ctx);
-    let result_ptr_value = varargs.get::<i32>(ctx);
+    let result_ptr_value: WasmPtr<i64> = varargs.get(ctx);
     let whence: i32 = varargs.get(ctx);
     let offset = offset_low as off_t;
-    let ret = unsafe { lseek(fd, offset, whence) as i32 };
-    #[allow(clippy::cast_ptr_alignment)]
-    let result_ptr = emscripten_memory_pointer!(ctx.memory(0), result_ptr_value) as *mut i32;
-    assert_eq!(8, mem::align_of_val(&result_ptr));
-    unsafe {
-        *result_ptr = ret;
-    }
+    let ret = unsafe { lseek(fd, offset, whence) };
+
+    let result_ptr = result_ptr_value.deref(ctx.memory(0)).unwrap();
+    result_ptr.set(ret);
+
     debug!(
-        "=> fd: {}, offset: {}, result_ptr: {}, whence: {} = {}\nlast os error: {}",
+        "=> fd: {}, offset: {}, result: {}, whence: {} = {}\nlast os error: {}",
         fd,
         offset,
-        result_ptr_value,
+        ret,
         whence,
         0,
         Error::last_os_error(),