Compilation error with Mojave: error: ‘_Atomic’ does not name a type

Question

After updating to Mojave 10.14.4 and Xcode 10.2, I fail to compile GCC 8.3.0. The error message is

In file included from /usr/include/sys/sysctl.h:83,
                 from ../../gcc-8.3.0/gcc/config/darwin-driver.c:30:
/usr/include/sys/ucred.h:94:2: error: ‘_Atomic’ does not name a type
  _Atomic u_long          cr_ref;  /* reference count */
  ^~~~~~~

I have not found any clue how this can be fixed. The header in question is

struct ucred {
    LIST_ENTRY(ucred)       cr_link; /* never modify this without KAUTH_CRED_HASH_LOCK */
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
    _Atomic u_long          cr_ref;  /* reference count */
#elif defined(__cplusplus) && __cplusplus >= 201103L
    _Atomic u_long          cr_ref;  /* reference count */   // <--- this line
#else
    volatile u_long         cr_ref;  /* reference count */
#endif

Any ideas are highly appreciated!

Answer 1

I encountered the same problem myself. The error is actually reported by g++, a C++ compiler. On MacOS 10.14.4, the header file <sys/ucred.h> uses the _Atomic keyword for C++, although it is only defined in the C standard. (I assume that Apple's compiler clang++ defines this keyword as extension to the C++ standard.)

The proper fix is to tell gcc to "fixinclude" the header file <sys/ucred.h>. This patch does this. This is a patch that needs to be applied to the source code of GCC 8.3.0 before building it.

Answer 2

You need to use a C11 compatible compiler in order to have support for the _Atomic keyword.

Ensure that you are compiling with a recent, modern compiler, and that STDC_NO_ATOMICS is not defined.

It might be easier for you to use HomeBrew in order to compile it without having to configure anything yourself. After installing HomeBrew, you can install gcc with:

brew install gcc

Answer 3

I had the same issue and I found a solution here.

macOS Mojave changed the location of the system headers, this broke the GCC 8 build process. In order to build GCC install the required header files in the old location:

The solution is to install the package found in the following directory.

/Library/Developer/CommandLineTools/Packages/