constructing header

src/shell.zig

@@ -38,6 +38,8 @@ pub const ShellWriter = struct {
     };
 
     pub const ShellSections = struct {
+        // len is the number of shells in this section.
+        len: u8,
         // index points the i'th shell to it's offset in blob. The last
         // byte of the i'th shell is index[i+1].
         index: BoundedArray(u16, max_shells),
@@ -55,6 +57,7 @@ pub const ShellWriter = struct {
             shells: BoundedArray([]const u8, max_shells),
         ) error{ Overflow, OutOfMemory }!ShellSections {
             var self = ShellSections{
+                .len = @intCast(u8, shells.len),
                 .index = try BoundedArray(u16, max_shells).init(shells.len),
                 .blob = try BoundedArray(u8, (max_shells + 1) * max_shell_len).init(0),
                 .shell2idx = StringHashMap(u8).init(allocator),