diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4ded1c29fb..b1306d732b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -204,6 +204,10 @@ install(FILES ${C_HEADERS} DESTINATION ${C_HEADERS_DEST})
 install(FILES "${CMAKE_SOURCE_DIR}/std/bootstrap.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/build.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/builtin.zig" DESTINATION "${ZIG_STD_DEST}")
+install(FILES "${CMAKE_SOURCE_DIR}/std/c/darwin.zig" DESTINATION "${ZIG_STD_DEST}/c")
+install(FILES "${CMAKE_SOURCE_DIR}/std/c/index.zig" DESTINATION "${ZIG_STD_DEST}/c")
+install(FILES "${CMAKE_SOURCE_DIR}/std/c/linux.zig" DESTINATION "${ZIG_STD_DEST}/c")
+install(FILES "${CMAKE_SOURCE_DIR}/std/c/windows.zig" DESTINATION "${ZIG_STD_DEST}/c")
 install(FILES "${CMAKE_SOURCE_DIR}/std/compiler_rt.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/cstr.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/darwin.zig" DESTINATION "${ZIG_STD_DEST}")
@@ -231,6 +235,7 @@ install(FILES "${CMAKE_SOURCE_DIR}/std/rand.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/rand_test.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/sort.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/test_runner.zig" DESTINATION "${ZIG_STD_DEST}")
+install(FILES "${CMAKE_SOURCE_DIR}/std/windows.zig" DESTINATION "${ZIG_STD_DEST}")
 
 add_executable(run_tests ${TEST_SOURCES})
 target_link_libraries(run_tests)
diff --git a/src/all_types.hpp b/src/all_types.hpp
index 35ed95fbdb..e1ebf50e69 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -246,6 +246,7 @@ enum TldId {
 
 enum TldResolution {
     TldResolutionUnresolved,
+    TldResolutionResolving,
     TldResolutionInvalid,
     TldResolutionOk,
 };
diff --git a/src/analyze.cpp b/src/analyze.cpp
index 1a0f0b8fe8..631707c978 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -2423,8 +2423,8 @@ Tld *find_decl(CodeGen *g, Scope *scope, Buf *name) {
         AstNode *use_decl_node = import->use_decls.at(i);
         if (use_decl_node->data.use.resolution == TldResolutionUnresolved) {
             preview_use_decl(g, use_decl_node);
+            resolve_use_decl(g, use_decl_node);
         }
-        resolve_use_decl(g, use_decl_node);
     }
 
     while (scope) {
@@ -2795,14 +2795,18 @@ static void add_symbols_from_import(CodeGen *g, AstNode *src_use_node, AstNode *
 void resolve_use_decl(CodeGen *g, AstNode *node) {
     assert(node->type == NodeTypeUse);
 
-    if (node->data.use.resolution != TldResolutionUnresolved)
+    if (node->data.use.resolution == TldResolutionOk ||
+        node->data.use.resolution == TldResolutionInvalid)
+    {
         return;
+    }
     add_symbols_from_import(g, node, node);
 }
 
 void preview_use_decl(CodeGen *g, AstNode *node) {
     assert(node->type == NodeTypeUse);
 
+    node->data.use.resolution = TldResolutionResolving;
     IrInstruction *result = analyze_const_value(g, &node->owner->decls_scope->base,
         node->data.use.expr, g->builtin_types.entry_namespace, nullptr);
 
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 3babaac0e4..03629b2da1 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -272,7 +272,17 @@ static LLVMValueRef fn_llvm_value(CodeGen *g, FnTableEntry *fn_table_entry) {
     }
 
     TypeTableEntry *fn_type = fn_table_entry->type_entry;
-    fn_table_entry->llvm_value = LLVMAddFunction(g->module, buf_ptr(symbol_name), fn_type->data.fn.raw_type_ref);
+    LLVMTypeRef fn_llvm_type = fn_type->data.fn.raw_type_ref;
+    if (!fn_table_entry->internal_linkage && fn_table_entry->body_node == nullptr) {
+        LLVMValueRef existing_llvm_fn = LLVMGetNamedFunction(g->module, buf_ptr(symbol_name));
+        if (existing_llvm_fn) {
+            fn_table_entry->llvm_value = LLVMConstBitCast(existing_llvm_fn, LLVMPointerType(fn_llvm_type, 0));
+        } else {
+            fn_table_entry->llvm_value = LLVMAddFunction(g->module, buf_ptr(symbol_name), fn_llvm_type);
+        }
+    } else {
+        fn_table_entry->llvm_value = LLVMAddFunction(g->module, buf_ptr(symbol_name), fn_llvm_type);
+    }
 
     switch (fn_table_entry->fn_inline) {
         case FnInlineAlways:
diff --git a/std/bootstrap.zig b/std/bootstrap.zig
index 074f06557e..b490989a4d 100644
--- a/std/bootstrap.zig
+++ b/std/bootstrap.zig
@@ -4,7 +4,7 @@
 const root = @import("@root");
 const std = @import("std");
 
-const want_main_symbol = std.build.linkingLibrary("c");
+const want_main_symbol = std.build.linking_libc;
 const want_start_symbol = !want_main_symbol;
 
 const exit = switch(@compileVar("os")) {
@@ -18,6 +18,9 @@ var argv: &&u8 = undefined;
 
 export nakedcc fn _start() -> unreachable {
     @setFnVisible(this, want_start_symbol);
+    if (!want_start_symbol) {
+        @unreachable();
+    }
 
     switch (@compileVar("arch")) {
         Arch.x86_64 => {
@@ -49,6 +52,9 @@ fn callMainAndExit() -> unreachable {
 
 export fn main(c_argc: i32, c_argv: &&u8) -> i32 {
     @setFnVisible(this, want_main_symbol);
+    if (!want_main_symbol) {
+        @unreachable();
+    }
 
     argc = usize(c_argc);
     argv = c_argv;
diff --git a/std/build.zig b/std/build.zig
index a21baa387d..f6b6f7177d 100644
--- a/std/build.zig
+++ b/std/build.zig
@@ -1,5 +1,7 @@
 const mem = @import("mem.zig");
 
+pub const linking_libc = linkingLibrary("c");
+
 pub fn linkingLibrary(lib_name: []const u8) -> bool {
     // TODO shouldn't need this if
     if (@compileVar("link_libs").len != 0) {
diff --git a/std/c/darwin.zig b/std/c/darwin.zig
new file mode 100644
index 0000000000..2095139b91
--- /dev/null
+++ b/std/c/darwin.zig
@@ -0,0 +1,4 @@
+pub extern fn getrandom(buf_ptr: &u8, buf_len: usize) -> c_int;
+
+extern fn __error() -> &c_int;
+pub const _errno = __error;
diff --git a/std/c/index.zig b/std/c/index.zig
new file mode 100644
index 0000000000..84b54cfc8e
--- /dev/null
+++ b/std/c/index.zig
@@ -0,0 +1,13 @@
+pub use @import("errno.zig");
+
+pub use switch(@compileVar("os")) {
+    Os.linux => @import("c/linux.zig"),
+    Os.windows => @import("c/windows.zig"),
+    Os.darwin, Os.macosx, Os.ios => @import("c/darwin.zig"),
+    else => empty_import,
+};
+
+pub extern fn abort() -> unreachable;
+
+
+const empty_import = @import("empty.zig");
diff --git a/std/c/linux.zig b/std/c/linux.zig
new file mode 100644
index 0000000000..fe5476a20f
--- /dev/null
+++ b/std/c/linux.zig
@@ -0,0 +1,4 @@
+pub extern fn getrandom(buf_ptr: &u8, buf_len: usize, flags: c_uint) -> c_int;
+
+extern fn __errno_location() -> &c_int;
+pub const _errno = __errno_location;
diff --git a/std/c/windows.zig b/std/c/windows.zig
new file mode 100644
index 0000000000..de77f67f6c
--- /dev/null
+++ b/std/c/windows.zig
@@ -0,0 +1 @@
+pub extern fn _errno() -> &c_int;
diff --git a/std/debug.zig b/std/debug.zig
index d03f22b4c9..7fe1ef4d4b 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -78,13 +78,13 @@ pub fn writeStackTrace(out_stream: &io.OutStream) -> %void {
             }
         },
         ObjectFormat.coff => {
-            out_stream.write("(stack trace unavailable for COFF object format)\n");
+            %return out_stream.write("(stack trace unavailable for COFF object format)\n");
         },
         ObjectFormat.macho => {
             %return out_stream.write("(stack trace unavailable for Mach-O object format)\n");
         },
         ObjectFormat.unknown => {
-            out_stream.write("(stack trace unavailable for unknown object format)\n");
+            %return out_stream.write("(stack trace unavailable for unknown object format)\n");
         },
     }
 }
diff --git a/std/os.zig b/std/os.zig
index bfb8daecb3..b50aa021f9 100644
--- a/std/os.zig
+++ b/std/os.zig
@@ -1,20 +1,49 @@
-const system = switch(@compileVar("os")) {
+const posix = switch(@compileVar("os")) {
     Os.linux => @import("linux.zig"),
-    Os.darwin => @import("darwin.zig"),
+    Os.darwin, Os.macosx, Os.ios => @import("darwin.zig"),
     else => @compileError("Unsupported OS"),
 };
+const windows = @import("windows.zig");
 const errno = @import("errno.zig");
+const linking_libc = @import("build.zig").linking_libc;
+const c = @import("c/index.zig");
 
 error Unexpected;
 
+/// Fills `buf` with random bytes. If linking against libc, this calls the
+/// appropriate OS-specific library call. Otherwise it uses the zig standard
+/// library implementation.
 pub fn getRandomBytes(buf: []u8) -> %void {
     while (true) {
-        const ret = switch (@compileVar("os")) {
-            Os.linux => system.getrandom(buf.ptr, buf.len, 0),
-            Os.darwin => system.getrandom(buf.ptr, buf.len),
-            else => @compileError("unsupported os"),
+        const err = switch (@compileVar("os")) {
+            Os.linux => {
+                if (linking_libc) {
+                    if (c.getrandom(buf.ptr, buf.len, 0) == -1) *c._errno() else 0
+                } else {
+                    posix.getErrno(posix.getrandom(buf.ptr, buf.len, 0))
+                }
+            },
+            Os.darwin, Os.macosx, Os.ios => {
+                if (linking_libc) {
+                    if (posix.getrandom(buf.ptr, buf.len) == -1) *c._errno() else 0
+                } else {
+                    posix.getErrno(posix.getrandom(buf.ptr, buf.len))
+                }
+            },
+            Os.windows => {
+                var hCryptProv: windows.HCRYPTPROV = undefined;
+                if (!windows.CryptAcquireContext(&hCryptProv, null, null, windows.PROV_RSA_FULL, 0)) {
+                    return error.Unexpected;
+                }
+                defer _ = windows.CryptReleaseContext(hCryptProv, 0);
+
+                if (!windows.CryptGenRandom(hCryptProv, windows.DWORD(buf.len), buf.ptr)) {
+                    return error.Unexpected;
+                }
+                return;
+            },
+            else => @compileError("Unsupported OS"),
         };
-        const err = system.getErrno(ret);
         if (err > 0) {
             return switch (err) {
                 errno.EINVAL => @unreachable(),
@@ -27,13 +56,19 @@ pub fn getRandomBytes(buf: []u8) -> %void {
     }
 }
 
+/// Raises a signal in the current kernel thread, ending its execution.
+/// If linking against libc, this calls the abort() libc function. Otherwise
+/// it uses the zig standard library implementation.
 pub coldcc fn abort() -> unreachable {
+    if (linking_libc) {
+        c.abort();
+    }
     switch (@compileVar("os")) {
-        Os.linux, Os.darwin => {
-            _ = system.raise(system.SIGABRT);
-            _ = system.raise(system.SIGKILL);
+        Os.linux => {
+            _ = posix.raise(posix.SIGABRT);
+            _ = posix.raise(posix.SIGKILL);
             while (true) {}
         },
-        else => @compileError("unsupported os"),
+        else => @compileError("Unsupported OS"),
     }
 }
diff --git a/std/windows.zig b/std/windows.zig
new file mode 100644
index 0000000000..f713f61abe
--- /dev/null
+++ b/std/windows.zig
@@ -0,0 +1,17 @@
+pub extern fn CryptAcquireContext(phProv: &HCRYPTPROV, pszContainer: LPCTSTR,
+    pszProvider: LPCTSTR, dwProvType: DWORD, dwFlags: DWORD) -> bool;
+
+pub extern fn CryptReleaseContext(hProv: HCRYPTPROV, dwFlags: DWORD) -> bool;
+
+pub extern fn CryptGenRandom(hProv: HCRYPTPROV, dwLen: DWORD, pbBuffer: &BYTE) -> bool;
+
+pub const PROV_RSA_FULL = 1;
+
+
+pub const BYTE = u8;
+pub const DWORD = u32;
+// TODO something about unicode WCHAR vs char
+pub const TCHAR = u8;
+pub const LPCTSTR = ?&const TCHAR;
+pub const ULONG_PTR = usize;
+pub const HCRYPTPROV = ULONG_PTR;