diff --git a/lib/std/Thread.zig b/lib/std/Thread.zig
index 0244eb6d41..aa21a8a0ea 100644
--- a/lib/std/Thread.zig
+++ b/lib/std/Thread.zig
@@ -589,7 +589,7 @@ const WindowsThreadImpl = struct {
             fn_args: Args,
             thread: ThreadCompletion,
 
-            fn entryFn(raw_ptr: windows.PVOID) callconv(.C) windows.DWORD {
+            fn entryFn(raw_ptr: windows.PVOID) callconv(.winapi) windows.DWORD {
                 const self: *@This() = @ptrCast(@alignCast(raw_ptr));
                 defer switch (self.thread.completion.swap(.completed, .seq_cst)) {
                     .running => {},
@@ -749,7 +749,7 @@ const PosixThreadImpl = struct {
         const allocator = std.heap.c_allocator;
 
         const Instance = struct {
-            fn entryFn(raw_arg: ?*anyopaque) callconv(.C) ?*anyopaque {
+            fn entryFn(raw_arg: ?*anyopaque) callconv(.c) ?*anyopaque {
                 const args_ptr: *Args = @ptrCast(@alignCast(raw_arg));
                 defer allocator.destroy(args_ptr);
                 return callFn(f, args_ptr.*);
@@ -1363,7 +1363,7 @@ const LinuxThreadImpl = struct {
             fn_args: Args,
             thread: ThreadCompletion,
 
-            fn entryFn(raw_arg: usize) callconv(.C) u8 {
+            fn entryFn(raw_arg: usize) callconv(.c) u8 {
                 const self = @as(*@This(), @ptrFromInt(raw_arg));
                 defer switch (self.thread.completion.swap(.completed, .seq_cst)) {
                     .running => {},
diff --git a/lib/std/os/windows.zig b/lib/std/os/windows.zig
index dbd6df46b7..6903bbf9a8 100644
--- a/lib/std/os/windows.zig
+++ b/lib/std/os/windows.zig
@@ -2101,8 +2101,8 @@ pub fn UnlockFile(
 
 /// This is a workaround for the C backend until zig has the ability to put
 /// C code in inline assembly.
-extern fn zig_x86_windows_teb() callconv(.C) *anyopaque;
-extern fn zig_x86_64_windows_teb() callconv(.C) *anyopaque;
+extern fn zig_x86_windows_teb() callconv(.c) *anyopaque;
+extern fn zig_x86_64_windows_teb() callconv(.c) *anyopaque;
 
 pub fn teb() *TEB {
     return switch (native_arch) {
@@ -3567,7 +3567,7 @@ pub const MEM_RESERVE_PLACEHOLDERS = 0x2;
 pub const MEM_DECOMMIT = 0x4000;
 pub const MEM_RELEASE = 0x8000;
 
-pub const PTHREAD_START_ROUTINE = *const fn (LPVOID) callconv(.C) DWORD;
+pub const PTHREAD_START_ROUTINE = *const fn (LPVOID) callconv(.winapi) DWORD;
 pub const LPTHREAD_START_ROUTINE = PTHREAD_START_ROUTINE;
 
 pub const WIN32_FIND_DATAW = extern struct {
@@ -3745,7 +3745,7 @@ pub const IMAGE_TLS_DIRECTORY = extern struct {
 pub const IMAGE_TLS_DIRECTORY64 = IMAGE_TLS_DIRECTORY;
 pub const IMAGE_TLS_DIRECTORY32 = IMAGE_TLS_DIRECTORY;
 
-pub const PIMAGE_TLS_CALLBACK = ?*const fn (PVOID, DWORD, PVOID) callconv(.C) void;
+pub const PIMAGE_TLS_CALLBACK = ?*const fn (PVOID, DWORD, PVOID) callconv(.winapi) void;
 
 pub const PROV_RSA_FULL = 1;
 
@@ -3843,7 +3843,7 @@ pub const RTL_QUERY_REGISTRY_ROUTINE = ?*const fn (
     ULONG,
     ?*anyopaque,
     ?*anyopaque,
-) callconv(WINAPI) NTSTATUS;
+) callconv(.winapi) NTSTATUS;
 
 /// Path is a full path
 pub const RTL_REGISTRY_ABSOLUTE = 0;
@@ -3940,7 +3940,7 @@ pub const FILE_ACTION_MODIFIED = 0x00000003;
 pub const FILE_ACTION_RENAMED_OLD_NAME = 0x00000004;
 pub const FILE_ACTION_RENAMED_NEW_NAME = 0x00000005;
 
-pub const LPOVERLAPPED_COMPLETION_ROUTINE = ?*const fn (DWORD, DWORD, *OVERLAPPED) callconv(.C) void;
+pub const LPOVERLAPPED_COMPLETION_ROUTINE = ?*const fn (DWORD, DWORD, *OVERLAPPED) callconv(.winapi) void;
 
 pub const FileNotifyChangeFilter = packed struct(DWORD) {
     file_name: bool = false,
@@ -4003,7 +4003,7 @@ pub const RTL_CRITICAL_SECTION = extern struct {
 pub const CRITICAL_SECTION = RTL_CRITICAL_SECTION;
 pub const INIT_ONCE = RTL_RUN_ONCE;
 pub const INIT_ONCE_STATIC_INIT = RTL_RUN_ONCE_INIT;
-pub const INIT_ONCE_FN = *const fn (InitOnce: *INIT_ONCE, Parameter: ?*anyopaque, Context: ?*anyopaque) callconv(.C) BOOL;
+pub const INIT_ONCE_FN = *const fn (InitOnce: *INIT_ONCE, Parameter: ?*anyopaque, Context: ?*anyopaque) callconv(.winapi) BOOL;
 
 pub const RTL_RUN_ONCE = extern struct {
     Ptr: ?*anyopaque,
@@ -4467,7 +4467,7 @@ pub const EXCEPTION_POINTERS = extern struct {
     ContextRecord: *CONTEXT,
 };
 
-pub const VECTORED_EXCEPTION_HANDLER = *const fn (ExceptionInfo: *EXCEPTION_POINTERS) callconv(WINAPI) c_long;
+pub const VECTORED_EXCEPTION_HANDLER = *const fn (ExceptionInfo: *EXCEPTION_POINTERS) callconv(.winapi) c_long;
 
 pub const EXCEPTION_DISPOSITION = i32;
 pub const EXCEPTION_ROUTINE = *const fn (
@@ -4475,7 +4475,7 @@ pub const EXCEPTION_ROUTINE = *const fn (
     EstablisherFrame: PVOID,
     ContextRecord: *(Self.CONTEXT),
     DispatcherContext: PVOID,
-) callconv(WINAPI) EXCEPTION_DISPOSITION;
+) callconv(.winapi) EXCEPTION_DISPOSITION;
 
 pub const UNWIND_HISTORY_TABLE_SIZE = 12;
 pub const UNWIND_HISTORY_TABLE_ENTRY = extern struct {
@@ -4851,7 +4851,7 @@ pub const RTL_DRIVE_LETTER_CURDIR = extern struct {
     DosPath: UNICODE_STRING,
 };
 
-pub const PPS_POST_PROCESS_INIT_ROUTINE = ?*const fn () callconv(.C) void;
+pub const PPS_POST_PROCESS_INIT_ROUTINE = ?*const fn () callconv(.winapi) void;
 
 pub const FILE_DIRECTORY_INFORMATION = extern struct {
     NextEntryOffset: ULONG,
@@ -4905,7 +4905,7 @@ pub fn FileInformationIterator(comptime FileInformationType: type) type {
     };
 }
 
-pub const IO_APC_ROUTINE = *const fn (PVOID, *IO_STATUS_BLOCK, ULONG) callconv(.C) void;
+pub const IO_APC_ROUTINE = *const fn (PVOID, *IO_STATUS_BLOCK, ULONG) callconv(.winapi) void;
 
 pub const CURDIR = extern struct {
     DosPath: UNICODE_STRING,
@@ -5009,8 +5009,8 @@ pub const ENUM_PAGE_FILE_INFORMATION = extern struct {
     PeakUsage: SIZE_T,
 };
 
-pub const PENUM_PAGE_FILE_CALLBACKW = ?*const fn (?LPVOID, *ENUM_PAGE_FILE_INFORMATION, LPCWSTR) callconv(.C) BOOL;
-pub const PENUM_PAGE_FILE_CALLBACKA = ?*const fn (?LPVOID, *ENUM_PAGE_FILE_INFORMATION, LPCSTR) callconv(.C) BOOL;
+pub const PENUM_PAGE_FILE_CALLBACKW = ?*const fn (?LPVOID, *ENUM_PAGE_FILE_INFORMATION, LPCWSTR) callconv(.winapi) BOOL;
+pub const PENUM_PAGE_FILE_CALLBACKA = ?*const fn (?LPVOID, *ENUM_PAGE_FILE_INFORMATION, LPCSTR) callconv(.winapi) BOOL;
 
 pub const PSAPI_WS_WATCH_INFORMATION_EX = extern struct {
     BasicInfo: PSAPI_WS_WATCH_INFORMATION,
@@ -5122,7 +5122,7 @@ pub const CTRL_CLOSE_EVENT: DWORD = 2;
 pub const CTRL_LOGOFF_EVENT: DWORD = 5;
 pub const CTRL_SHUTDOWN_EVENT: DWORD = 6;
 
-pub const HANDLER_ROUTINE = *const fn (dwCtrlType: DWORD) callconv(WINAPI) BOOL;
+pub const HANDLER_ROUTINE = *const fn (dwCtrlType: DWORD) callconv(.winapi) BOOL;
 
 /// Processor feature enumeration.
 pub const PF = enum(DWORD) {