wasm-linker: implement -fno-entry and correctly pass --shared and --pie when given

doc/langref.html.in

@@ -11336,12 +11336,12 @@ all your base are belong to us{#end_shell_samp#}
       {#header_open|WebAssembly#}
       <p>Zig supports building for WebAssembly out of the box.</p>
       {#header_open|Freestanding#}
-      <p>For host environments like the web browser and nodejs, build as a dynamic library using the freestanding
+      <p>For host environments like the web browser and nodejs, build as an executable using the freestanding
       OS target. Here's an example of running Zig code compiled to WebAssembly with nodejs.</p>
-      {#code_begin|lib|math#}
+      {#code_begin|exe|math#}
       {#target_wasm#}
-      {#link_mode_dynamic#}
-      {#additonal_option|-rdynamic#}
+      {#additonal_option|-fno-entry#}
+      {#additonal_option|--export=add#}
 extern fn print(i32) void;
 
 export fn add(a: i32, b: i32) void {

lib/std/Build/Step/Compile.zig

@@ -189,7 +189,8 @@ dll_export_fns: ?bool = null,
 
 subsystem: ?std.Target.SubSystem = null,
 
-entry_symbol_name: ?[]const u8 = null,
+/// How the linker must handle the entry point of the executable.
+entry: Entry = .default,
 
 /// List of symbols forced as undefined in the symbol table
 /// thus forcing their resolution by the linker.
@@ -304,6 +305,18 @@ const FrameworkLinkInfo = struct {
     weak: bool = false,
 };
 
+const Entry = union(enum) {
+    /// Let the compiler decide whether to make an entry point and what to name
+    /// it.
+    default,
+    /// The executable will have no entry point.
+    disabled,
+    /// The executable will have an entry point with the default symbol name.
+    enabled,
+    /// The executable will have an entry point with the specified symbol name.
+    symbol_name: []const u8,
+};
+
 pub const IncludeDir = union(enum) {
     path: LazyPath,
     path_system: LazyPath,
@@ -1418,9 +1431,13 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
         try zig_args.append(try std.fmt.allocPrint(b.allocator, "-ofmt={s}", .{@tagName(ofmt)}));
     }
 
-    if (self.entry_symbol_name) |entry| {
-        try zig_args.append("--entry");
-        try zig_args.append(entry);
+    switch (self.entry) {
+        .default => {},
+        .disabled => try zig_args.append("-fno-entry"),
+        .enabled => try zig_args.append("-fentry"),
+        .symbol_name => |entry_name| {
+            try zig_args.append(try std.fmt.allocPrint(b.allocator, "-fentry={s}", .{entry_name}));
+        },
     }
 
     {

lib/std/start.zig

@@ -82,11 +82,15 @@ comptime {
                     .reactor => "_initialize",
                     .command => "_start",
                 };
-                if (!@hasDecl(root, wasm_start_sym)) {
+                if (!@hasDecl(root, wasm_start_sym) and @hasDecl(root, "main")) {
+                    // Only call main when defined. For WebAssembly it's allowed to pass `-fno-entry` in which
+                    // case it's not required to provide an entrypoint such as main.
                     @export(wasi_start, .{ .name = wasm_start_sym });
                 }
             } else if (native_arch.isWasm() and native_os == .freestanding) {
-                if (!@hasDecl(root, start_sym_name)) @export(wasm_freestanding_start, .{ .name = start_sym_name });
+                // Only call main when defined. For WebAssembly it's allowed to pass `-fno-entry` in which
+                // case it's not required to provide an entrypoint such as main.
+                if (!@hasDecl(root, start_sym_name) and @hasDecl(root, "main")) @export(wasm_freestanding_start, .{ .name = start_sym_name });
             } else if (native_os != .other and native_os != .freestanding) {
                 if (!@hasDecl(root, start_sym_name)) @export(_start, .{ .name = start_sym_name });
             }

src/link/Wasm.zig

@@ -2817,15 +2817,11 @@ fn setupExports(wasm: *Wasm) !void {
 }
 
 fn setupStart(wasm: *Wasm) !void {
-    const entry_name = wasm.base.options.entry orelse "_start";
+    // do not export entry point if user set none or no default was set.
+    const entry_name = wasm.base.options.entry orelse return;
 
     const symbol_loc = wasm.findGlobalSymbol(entry_name) orelse {
-        if (wasm.base.options.output_mode == .Exe) {
-            if (wasm.base.options.wasi_exec_model == .reactor) return; // Not required for reactors
-        } else {
-            return; // No entry point needed for non-executable wasm files
-        }
-        log.err("Entry symbol '{s}' missing", .{entry_name});
+        log.err("Entry symbol '{s}' missing, use '-fno-entry' to suppress", .{entry_name});
         return error.MissingSymbol;
     };
 
@@ -4535,6 +4531,8 @@ fn linkWithLLD(wasm: *Wasm, comp: *Compilation, prog_node: *std.Progress.Node) !
         if (wasm.base.options.entry) |entry| {
             try argv.append("--entry");
             try argv.append(entry);
+        } else {
+            try argv.append("--no-entry");
         }
 
         // Increase the default stack size to a more reasonable value of 1MB instead of
@@ -4544,24 +4542,17 @@ fn linkWithLLD(wasm: *Wasm, comp: *Compilation, prog_node: *std.Progress.Node) !
         const arg = try std.fmt.allocPrint(arena, "stack-size={d}", .{stack_size});
         try argv.append(arg);
 
-        if (wasm.base.options.output_mode == .Exe) {
-            if (wasm.base.options.wasi_exec_model == .reactor) {
-                // Reactor execution model does not have _start so lld doesn't look for it.
-                try argv.append("--no-entry");
-                // Make sure "_initialize" and other used-defined functions are exported if this is WASI reactor.
-                // If rdynamic is true, it will already be appended, so only verify if the user did not specify
-                // the flag in which case, we ensure `--export-dynamic` is called.
-                if (!wasm.base.options.rdynamic) {
-                    try argv.append("--export-dynamic");
-                }
-            }
-        } else if (wasm.base.options.entry == null) {
-            try argv.append("--no-entry"); // So lld doesn't look for _start.
-        }
         if (wasm.base.options.import_symbols) {
             try argv.append("--allow-undefined");
         }
 
+        if (wasm.base.options.output_mode == .Lib and wasm.base.options.link_mode == .Dynamic) {
+            try argv.append("--shared");
+        }
+        if (wasm.base.options.pie) {
+            try argv.append("--pie");
+        }
+
         // XXX - TODO: add when wasm-ld supports --build-id.
         // if (wasm.base.options.build_id) {
         //     try argv.append("--build-id=tree");

src/main.zig

@@ -509,7 +509,9 @@ const usage_build_generic =
     \\  --dynamic-linker [path]        Set the dynamic interpreter path (usually ld.so)
     \\  --sysroot [path]               Set the system root directory (usually /)
     \\  --version [ver]                Dynamic library semver
-    \\  --entry [name]                 Set the entrypoint symbol name
+    \\  -fentry                        Enable entry point with default symbol name
+    \\  -fentry=[name]                 Override the entry point symbol name
+    \\  -fno-entry                     Do not output any entry point
     \\  --force_undefined [name]       Specify the symbol must be defined for the link to succeed
     \\  -fsoname[=name]                Override the default SONAME value
     \\  -fno-soname                    Disable emitting a SONAME
@@ -835,6 +837,7 @@ fn buildOutputType(
     var linker_import_symbols: bool = false;
     var linker_import_table: bool = false;
     var linker_export_table: bool = false;
+    var linker_force_entry: ?bool = null;
     var linker_initial_memory: ?u64 = null;
     var linker_max_memory: ?u64 = null;
     var linker_shared_memory: bool = false;
@@ -1071,8 +1074,8 @@ fn buildOutputType(
                         subsystem = try parseSubSystem(args_iter.nextOrFatal());
                     } else if (mem.eql(u8, arg, "-O")) {
                         optimize_mode_string = args_iter.nextOrFatal();
-                    } else if (mem.eql(u8, arg, "--entry")) {
-                        entry = args_iter.nextOrFatal();
+                    } else if (mem.startsWith(u8, arg, "-fentry=")) {
+                        entry = arg["-fentry=".len..];
                     } else if (mem.eql(u8, arg, "--force_undefined")) {
                         try force_undefined_symbols.put(gpa, args_iter.nextOrFatal(), {});
                     } else if (mem.eql(u8, arg, "--stack")) {
@@ -1503,6 +1506,10 @@ fn buildOutputType(
                         }
                     } else if (mem.eql(u8, arg, "--import-memory")) {
                         linker_import_memory = true;
+                    } else if (mem.eql(u8, arg, "-fentry")) {
+                        linker_force_entry = true;
+                    } else if (mem.eql(u8, arg, "-fno-entry")) {
+                        linker_force_entry = false;
                     } else if (mem.eql(u8, arg, "--export-memory")) {
                         linker_export_memory = true;
                     } else if (mem.eql(u8, arg, "--import-symbols")) {
@@ -2134,6 +2141,8 @@ fn buildOutputType(
                     linker_import_table = true;
                 } else if (mem.eql(u8, arg, "--export-table")) {
                     linker_export_table = true;
+                } else if (mem.eql(u8, arg, "--no-entry")) {
+                    linker_force_entry = false;
                 } else if (mem.eql(u8, arg, "--initial-memory")) {
                     const next_arg = linker_args_it.nextOrFatal();
                     linker_initial_memory = std.fmt.parseUnsigned(u32, eatIntPrefix(next_arg, 16), 16) catch |err| {
@@ -2596,6 +2605,23 @@ fn buildOutputType(
             link_libcpp = true;
     }
 
+    if (linker_force_entry) |force| {
+        if (!force) {
+            entry = null;
+        } else if (entry == null and output_mode == .Exe) {
+            entry = switch (target_info.target.ofmt) {
+                .coff => "wWinMainCRTStartup",
+                .macho => "_main",
+                .elf, .plan9 => "_start",
+                .wasm => defaultWasmEntryName(wasi_exec_model),
+                else => |tag| fatal("No default entry point available for output format {s}", .{@tagName(tag)}),
+            };
+        }
+    } else if (entry == null and target_info.target.isWasm() and output_mode == .Exe) {
+        // For WebAssembly the compiler defaults to setting the entry name when no flags are set.
+        entry = defaultWasmEntryName(wasi_exec_model);
+    }
+
     if (target_info.target.ofmt == .coff) {
         // Now that we know the target supports resources,
         // we can add the res files as link objects.
@@ -2618,6 +2644,23 @@ fn buildOutputType(
         if (single_threaded == null) {
             single_threaded = true;
         }
+        if (link_mode) |mode| {
+            if (mode == .Dynamic) {
+                if (linker_export_memory != null and linker_export_memory.?) {
+                    fatal("flags '-dynamic' and '--export-memory' are incompatible", .{});
+                }
+                // User did not supply `--export-memory` which is incompatible with -dynamic, therefore
+                // set the flag to false to ensure it does not get enabled by default.
+                linker_export_memory = false;
+            }
+        }
+        if (wasi_exec_model != null and wasi_exec_model.? == .reactor) {
+            if (entry) |entry_name| {
+                if (!mem.eql(u8, "_initialize", entry_name)) {
+                    fatal("the entry symbol of the reactor model must be '_initialize', but found '{s}'", .{entry_name});
+                }
+            }
+        }
         if (linker_shared_memory) {
             if (output_mode == .Obj) {
                 fatal("shared memory is not allowed in object files", .{});
@@ -7214,3 +7257,11 @@ fn createDependenciesModule(
     try main_mod.deps.put(arena, "@dependencies", deps_mod);
     return deps_mod;
 }
+
+fn defaultWasmEntryName(exec_model: ?std.builtin.WasiExecModel) []const u8 {
+    const model = exec_model orelse .command;
+    if (model == .reactor) {
+        return "_initialize";
+    }
+    return "_start";
+}

test/link/elf.zig

@@ -651,7 +651,7 @@ fn testEntryPoint(b: *Build, opts: Options) *Step {
         const exe = addExecutable(b, "main", opts);
         exe.addObject(a_o);
         exe.addObject(b_o);
-        exe.entry_symbol_name = "foo";
+        exe.entry = .{ .symbol_name = "foo" };
 
         const check = exe.checkObject();
         check.checkStart();
@@ -667,7 +667,7 @@ fn testEntryPoint(b: *Build, opts: Options) *Step {
         const exe = addExecutable(b, "other", opts);
         exe.addObject(a_o);
         exe.addObject(b_o);
-        exe.entry_symbol_name = "bar";
+        exe.entry = .{ .symbol_name = "bar" };
 
         const check = exe.checkObject();
         check.checkStart();

test/link/macho/entry/build.zig

@@ -20,7 +20,7 @@ fn add(b: *std.Build, test_step: *std.Build.Step, optimize: std.builtin.Optimize
     });
     exe.addCSourceFile(.{ .file = .{ .path = "main.c" }, .flags = &.{} });
     exe.linkLibC();
-    exe.entry_symbol_name = "_non_main";
+    exe.entry = .{ .symbol_name = "_non_main" };
 
     const check_exe = exe.checkObject();
 

test/link/macho/entry_in_dylib/build.zig

@@ -30,7 +30,7 @@ fn add(b: *std.Build, test_step: *std.Build.Step, optimize: std.builtin.Optimize
     exe.addCSourceFile(.{ .file = .{ .path = "main.c" }, .flags = &.{} });
     exe.linkLibrary(lib);
     exe.linkLibC();
-    exe.entry_symbol_name = "_bootstrap";
+    exe.entry = .{ .symbol_name = "_bootstrap" };
     exe.forceUndefinedSymbol("_my_main");
 
     const check_exe = exe.checkObject();

test/link/wasm/archive/build.zig

@@ -15,12 +15,13 @@ pub fn build(b: *std.Build) void {
 fn add(b: *std.Build, test_step: *std.Build.Step, optimize: std.builtin.OptimizeMode) void {
     // The code in question will pull-in compiler-rt,
     // and therefore link with its archive file.
-    const lib = b.addSharedLibrary(.{
+    const lib = b.addExecutable(.{
         .name = "main",
         .root_source_file = .{ .path = "main.zig" },
         .optimize = optimize,
         .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
     });
+    lib.entry = .disabled;
     lib.use_llvm = false;
     lib.use_lld = false;
     lib.strip = false;

test/link/wasm/basic-features/build.zig

@@ -4,7 +4,7 @@ pub const requires_stage2 = true;
 
 pub fn build(b: *std.Build) void {
     // Library with explicitly set cpu features
-    const lib = b.addSharedLibrary(.{
+    const lib = b.addExecutable(.{
         .name = "lib",
         .root_source_file = .{ .path = "main.zig" },
         .optimize = .Debug,
@@ -15,6 +15,7 @@ pub fn build(b: *std.Build) void {
             .os_tag = .freestanding,
         },
     });
+    lib.entry = .disabled;
     lib.use_llvm = false;
     lib.use_lld = false;
 

test/link/wasm/bss/build.zig

@@ -14,12 +14,13 @@ pub fn build(b: *std.Build) void {
 
 fn add(b: *std.Build, test_step: *std.Build.Step, optimize_mode: std.builtin.OptimizeMode, is_safe: bool) void {
     {
-        const lib = b.addSharedLibrary(.{
+        const lib = b.addExecutable(.{
             .name = "lib",
             .root_source_file = .{ .path = "lib.zig" },
             .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
             .optimize = optimize_mode,
         });
+        lib.entry = .disabled;
         lib.use_llvm = false;
         lib.use_lld = false;
         lib.strip = false;
@@ -60,12 +61,13 @@ fn add(b: *std.Build, test_step: *std.Build.Step, optimize_mode: std.builtin.Opt
 
     // verify zero'd declaration is stored in bss for all optimization modes.
     {
-        const lib = b.addSharedLibrary(.{
+        const lib = b.addExecutable(.{
             .name = "lib",
             .root_source_file = .{ .path = "lib2.zig" },
             .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
             .optimize = optimize_mode,
         });
+        lib.entry = .disabled;
         lib.use_llvm = false;
         lib.use_lld = false;
         lib.strip = false;

test/link/wasm/export-data/build.zig

@@ -9,12 +9,13 @@ pub fn build(b: *std.Build) void {
         return;
     }
 
-    const lib = b.addSharedLibrary(.{
+    const lib = b.addExecutable(.{
         .name = "lib",
         .root_source_file = .{ .path = "lib.zig" },
         .optimize = .ReleaseSafe, // to make the output deterministic in address positions
         .target = .{ .cpu_arch = .wasm32, .os_tag = .freestanding },
     });
+    lib.entry = .disabled;
     lib.use_lld = false;
     lib.export_symbol_names = &.{ "foo", "bar" };
     lib.global_base = 0; // put data section at address 0 to make data symbols easier to parse