improvements to zig's implementation of libc and WebAssembly

 * rename std/special/builtin.zig to std/special/c.zig
   not to be confused with @import("builtin") which is entirely
   different, this is zig's multi-target libc implementation.
 * WebAssembly: build-exe is for executables which have a main().
   build-lib is for building libraries of functions to use from,
   for example, a web browser environment.
   - for now pass --export-all for libraries when there are any
     C objects because we have no way to detect the list of exports
     when compiling C code.
   - stop passing --no-entry for executables. if you want --no-entry
     then use build-lib.
 * make the "musl" ABI the default ABI for wasm32-freestanding.
 * zig provides libc for wasm32-freestanding-musl.

1 files changed, 18 insertions, 2 deletions

diff --git a/std/special/builtin.zig b/std/special/c.zig
index 3776cc22bf..d5de52a6a3 100644
--- a/std/special/builtin.zig
+++ b/std/special/c.zig
@@ -1,10 +1,26 @@
-// These functions are provided when not linking against libc because LLVM
-// sometimes generates code that calls them.
+// This is Zig's multi-target implementation of libc.
+// When builtin.link_libc is true, we need to export all the functions and
+// provide an entire C API.
+// Otherwise, only the functions which LLVM generates calls to need to be generated,
+// such as memcpy, memset, and some math functions.
 
 const std = @import("std");
 const builtin = @import("builtin");
 const maxInt = std.math.maxInt;
 
+const is_wasm = switch (builtin.arch) { .wasm32, .wasm64 => true, else => false};
+const is_freestanding = switch (builtin.os) { .freestanding => true, else => false };
+comptime {
+    if (is_freestanding and is_wasm) {
+        @export("_start", wasm_start, .Strong);
+    }
+}
+
+extern fn main(argc: c_int, argv: [*][*]u8) c_int;
+extern fn wasm_start() c_int {
+    return main(0, undefined);
+}
+
 // Avoid dragging in the runtime safety mechanisms into this .o file,
 // unless we're trying to test this file.
 pub fn panic(msg: []const u8, error_return_trace: ?*builtin.StackTrace) noreturn {