Commit 25c3878c00

@@ -1048,12 +1048,27 @@ pub const File = struct {
     /// Returns the number of bytes read. If the number read is smaller than the total bytes
     /// from all the buffers, it means the file reached the end. Reaching the end of a file
     /// is not an error condition.
-    /// The `iovecs` parameter is mutable because this function needs to mutate the fields in
-    /// order to handle partial reads from the underlying OS layer.
-    /// See https://github.com/ziglang/zig/issues/7699
+    ///
+    /// The `iovecs` parameter is mutable because:
+    /// * This function needs to mutate the fields in order to handle partial
+    ///   reads from the underlying OS layer.
+    /// * The OS layer expects pointer addresses to be inside the application's address space
+    ///   even if the length is zero. Meanwhile, in Zig, slices may have undefined pointer
+    ///   addresses when the length is zero. So this function modifies the iov_base fields
+    ///   when the length is zero.
+    ///
+    /// Related open issue: https://github.com/ziglang/zig/issues/7699
     pub fn readvAll(self: File, iovecs: []os.iovec) ReadError!usize {
         if (iovecs.len == 0) return 0;
 
+        // We use the address of this local variable for all zero-length
+        // vectors so that the OS does not complain that we are giving it
+        // addresses outside the application's address space.
+        var garbage: [1]u8 = undefined;
+        for (iovecs) |*v| {
+            if (v.iov_len == 0) v.iov_base = &garbage;
+        }
+
         var i: usize = 0;
         var off: usize = 0;
         while (true) {

@@ -766,6 +766,9 @@ pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
 /// This operation is non-atomic on the following systems:
 /// * Windows
 /// On these systems, the read races with concurrent writes to the same file descriptor.
+///
+/// This function assumes that all zero-length vectors have a pointer within the address
+/// space of the application.
 pub fn readv(fd: fd_t, iov: []const iovec) ReadError!usize {
     if (builtin.os.tag == .windows) {
         // TODO improve this to use ReadFileScatter