A list of the original clang-tidy warnings for reference.

clang-tidy-warnings.txt

To run clang-tidy, you need a compilation "database" (which is a JSON file listing how to compile each individual file).
Generate this by setting -DCMAKE_EXPORT_COMPILE_COMMANDS=ON in the cmake command used to configure libmongocrypt. E.g.

cd cmake-build
cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ../

Then the cmake-build directory should contain the compilation database: compile_commands.json

Then, run clang-tidy by passing a list of file names and the path to the directory containing the compilation database:
find ./src/ -name "*.c" | xargs ~/bin/clang+llvm-8.0.0-x86_64-apple-darwin/bin/clang-tidy -p ./cmake-build 

Users/kevinalbertson/code/libmongocrypt/src//crypto/cng.c:20:10: error: 'bcrypt.h' file not found [clang-diagnostic-error]
#include <bcrypt.h>
         ^
/Users/kevinalbertson/code/libmongocrypt/src//crypto/openssl.c:28:10: error: 'openssl/crypto.h' file not found [clang-diagnostic-error]
#include <openssl/crypto.h>
         ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_mutex.c:22:4: warning: implicit declaration of function 'InitializeCriticalSection' is invalid in C99 [clang-diagnostic-implicit-function-declaration]
   InitializeCriticalSection (mutex);
   ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_mutex.c:28:4: warning: implicit declaration of function 'DeleteCriticalSection' is invalid in C99 [clang-diagnostic-implicit-function-declaration]
   DeleteCriticalSection (mutex);
   ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_mutex.c:34:4: warning: implicit declaration of function 'EnterCriticalSection' is invalid in C99 [clang-diagnostic-implicit-function-declaration]
   EnterCriticalSection (mutex);
   ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_mutex.c:40:4: warning: implicit declaration of function 'LeaveCriticalSection' is invalid in C99 [clang-diagnostic-implicit-function-declaration]
   LeaveCriticalSection (mutex);
   ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_once.c:21:8: error: unknown type name 'INIT_ONCE' [clang-diagnostic-error]
static INIT_ONCE once_control = INIT_ONCE_STATIC_INIT;
       ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_once.c:21:33: error: use of undeclared identifier 'INIT_ONCE_STATIC_INIT' [clang-diagnostic-error]
static INIT_ONCE once_control = INIT_ONCE_STATIC_INIT;
                                ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_once.c:23:8: error: unknown type name 'BOOL' [clang-diagnostic-error]
static BOOL WINAPI
       ^
/Users/kevinalbertson/code/libmongocrypt/src//os_win/os_once.c:23:19: error: expected ';' after top level declarator [clang-diagnostic-error]
static BOOL WINAPI
                  ^
                  ;
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ciphertext.c:144:4: warning: Value stored to 'offset' is never read [clang-analyzer-deadcode.DeadStores]
   offset += ciphertext->data.len;
   ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ciphertext.c:144:4: note: Value stored to 'offset' is never read
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:41:27: warning: Access to field 'status' results in a dereference of a null pointer (loaded from variable 'ctx') [clang-analyzer-core.NullDereference]
   _mongocrypt_set_error (ctx->status,
                          ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:481:8: note: Assuming 'ctx' is null
   if (!ctx || !ctx->initialized) {
       ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:481:13: note: Left side of '||' is true
   if (!ctx || !ctx->initialized) {
            ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:482:42: note: Passing null pointer value via 1st parameter 'ctx'
      return _mongocrypt_ctx_fail_w_msg (ctx, "ctx NULL or uninitialized");
                                         ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:482:14: note: Calling '_mongocrypt_ctx_fail_w_msg'
      return _mongocrypt_ctx_fail_w_msg (ctx, "ctx NULL or uninitialized");
             ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-ctx.c:41:27: note: Access to field 'status' results in a dereference of a null pointer (loaded from variable 'ctx')
   _mongocrypt_set_error (ctx->status,
                          ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key-broker.c:706:25: warning: Value stored to 'status' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
   mongocrypt_status_t *status = kb->status;
                        ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key-broker.c:706:25: note: Value stored to 'status' during its initialization is never read
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key.c:470:17: warning: va_arg() is called on an uninitialized va_list [clang-analyzer-valist.Uninitialized]
      arg_ptr = va_arg (args, const char *);
                ^
/Users/kevinalbertson/bin/clang+llvm-8.0.0-x86_64-apple-darwin/bin/../lib/clang/8.0.0/include/stdarg.h:35:29: note: expanded from macro 'va_arg'
#define va_arg(ap, type)    __builtin_va_arg(ap, type)
                            ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key.c:457:4: note: Loop condition is true.  Entering loop body
   while (arg_ptr) {
   ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key.c:464:7: note: Taking true branch
      if (!prev) {
      ^
/Users/kevinalbertson/code/libmongocrypt/src/mongocrypt-key.c:470:17: note: va_arg() is called on an uninitialized va_list
      arg_ptr = va_arg (args, const char *);
                ^
/Users/kevinalbertson/bin/clang+llvm-8.0.0-x86_64-apple-darwin/bin/../lib/clang/8.0.0/include/stdarg.h:35:29: note: expanded from macro 'va_arg'
#define va_arg(ap, type)    __builtin_va_arg(ap, type)
                            ^
Suppressed 50 warnings (50 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
Found compiler error(s).