No Clang warning for uninitialized field in struct

Question

Consider the following struct:

typedef struct foo {
    int a;
    int b;
} foo;

My compiler issues no warning for the following statement:

foo m = {300}

Why is no warning emitted? I expected a warning, since I have not provided any value for the final field of the struct.

Here is my compiler invocation:

gcc -Wall -Wpadded -Wuninitialized -g bar.c

Here is my gcc version:

Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin15.0.0
Thread model: posix

Will the flipped_move field just contain garbage? Is this safe?

Answer 1

-Wuninitialized warns about uninitialised variables. But your compound literal very well is completely initialised.

C does not support partial initialisation. Fields in structs and array initialisers or a compound literal which are not explicitly given are automatically set to 0/0.0/null pointer (depending on type). So there is no need for the compiler to warn.

Maybe it is better practice in modern C to use designated initialisers, instead of the positional ones:

(move){orig_c, curr_c, ... }

Will store values to the first, second, etc. field, no matter which names it has and its type. This will cause trouble if you e.g. remove a field, but forget to remove the value:

// from field removed
(move){orig_c, curr_c, ... }

will set to and the following field. While this will generate an error for your struct, imagine the types were compatible.

Designated initialisers don't have this problem:

(move){ .from = orig_c, .to = curr_c, ... }

If you remove from, you will get an error about an unknown field name if you forget to change that list, too. Also you easier will see if you forgot a field. This is especially helpful for larger structs.

---

Sidenote: Your compiler is not gcc, but llvm/clang. Apple unfortunately linked gcc to that compiler when they changed the toolchain for compatibility reasons. Bad idea, as the compilers are not fully compatible.

Answer 2

-Wmissing-field-initializers will warn you about missing field initializers in your struct initialization.

However, it looks like on clang that flag won't warn you if you choose to initialize a struct with no values. For example

struct foo{
  int a;
};

int main(int argc, char **argv) {
  struct foo f = {};
  return 0;
}

Will emit no warnings even with -Wmissing-field-initializers on. If you'd like warnings in this case as well you should add the -Wgnu-empty-initializer.

GCC

Although you asked about clang, this also works in GCC
GCC will emit warnings for both cases when you pass in -Wmissing-field-initializers. There is no -Wgnu-empty-initializer flag for GCC.