add execvp

lib/emscripten/src/exec.rs

@@ -0,0 +1,40 @@
+use libc::execvp;
+use std::cell::Cell;
+use std::ffi::CString;
+use wasmer_runtime_core::vm::Ctx;
+
+pub fn _execvp(ctx: &mut Ctx, command_name_offset: u32, argv_offset: u32) -> i32 {
+    // a single reference to re-use
+    let emscripten_memory = ctx.memory(0);
+
+    // read command name as string
+    let command_name_string_vec: Vec<u8> = emscripten_memory.view()
+        [(command_name_offset as usize)..]
+        .iter()
+        .map(|cell| cell.get())
+        .take_while(|&byte| byte != 0)
+        .collect();
+    let command_name_string = CString::new(command_name_string_vec).unwrap();
+
+    // get the array of args
+    let mut argv: Vec<*const i8> = emscripten_memory.view()[((argv_offset / 4) as usize)..]
+        .iter()
+        .map(|cell: &Cell<u32>| cell.get())
+        .take_while(|&byte| byte != 0)
+        .map(|offset| {
+            let p: *const i8 = (emscripten_memory.view::<u8>()[(offset as usize)..])
+                .iter()
+                .map(|cell| cell.as_ptr() as *const i8)
+                .collect::<Vec<*const i8>>()[0];
+            p
+        })
+        .collect();
+
+    // push a nullptr on to the end of the args array
+    argv.push(std::ptr::null());
+
+    // construct raw pointers and hand them to `execvp`
+    let command_pointer = command_name_string.as_ptr() as *const i8;
+    let args_pointer = argv.as_ptr();
+    unsafe { execvp(command_pointer, args_pointer) }
+}