Unknown type name 'u_int'

I ran across the error message (Unknown type name 'u_int', 'u_long', 'u_char' etc.) when trying to incorporate some old C code into a more modern project. It shows up in system headers, like sys/attr.h, and I was confused. After quite some time I figured out what the problem was, so I thought I’d share.

Simplified example

The source code, when simplified, looked something like this:

$ cat test.c
#define _XOPEN_SOURCE 500
#include <sys/attr.h>
#include <stdio.h>

int main(void)
{
    printf("Hello, world!");
}
$

Compiler output

When compiled in GCC, on OS X, you get errors from the included file sys/attr.h:

$ gcc-8 test.c -o test.o
In file included from /usr/include/sys/attr.h:42,
                 from test.c:2:
/usr/include/sys/ucred.h:91:2: error: unknown type name 'u_long'
  u_long cr_ref;   /* reference count */
  ^~~~~~
/usr/include/sys/ucred.h:133:9: error: unknown type name 'u_int'
         u_int   cr_version;             /* structure layout version */
         ^~~~~
In file included from test.c:2:
/usr/include/sys/attr.h:77:2: error: unknown type name 'u_short'
  u_short bitmapcount;   /* number of attr. bit sets in list (should be 5) */
  ^~~~~~~
/usr/include/sys/attr.h:541:2: error: unknown type name 'u_long'
  u_long    maxmatches;
  ^~~~~~
/usr/include/sys/attr.h:554:2: error: unknown type name 'u_char'
  u_char    ss_fsstate[548]; // fs private
  ^~~~~~

And the same in clang:

$ clang test.c -o test.o
In file included from test.c:2:
In file included from /usr/include/sys/attr.h:42:
/usr/include/sys/ucred.h:91:2: error: unknown type name 'u_long'; did you mean 'long'?
        u_long  cr_ref;                 /* reference count */
        ^
/usr/include/sys/ucred.h:133:9: error: unknown type name 'u_int'
        u_int   cr_version;             /* structure layout version */
        ^
In file included from test.c:2:
/usr/include/sys/attr.h:77:2: error: unknown type name 'u_short'; did you mean 'short'?
        u_short bitmapcount;                    /* number of attr. bit sets in list (should be 5) */
        ^
/usr/include/sys/attr.h:541:2: error: unknown type name 'u_long'; did you mean 'long'?
        u_long                          maxmatches;
        ^
/usr/include/sys/attr.h:554:2: error: unknown type name 'u_char'; did you mean 'char'?
        u_char                          ss_fsstate[548];        // fs private
        ^
5 errors generated.

Explanation and solution

I had never seen the macro _XOPEN_SOURCE before. It turns out, this specifies the X/OPEN or POSIX standard you are writing for.

#define _XOPEN_SOURCE 500

The above value (500) corresponds to POSIX 1995. Many headers, (at least on OS X), expect types which are not available in this mode. The fix was to remove the line.