In file included from /usr/include/stdio.h:33:0,
                 from /in/foo.c:2:
/usr/lib/gcc/x86_64-linux-gnu/6/include/stddef.h:216:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'typedef'
 typedef __SIZE_TYPE__ size_t;
 ^~~~~~~
In file included from /usr/include/stdio.h:74:0,
                 from /in/foo.c:2:
/usr/include/libio.h:302:3: error: unknown type name 'size_t'
   size_t __pad5;
   ^~~~~~
/usr/include/libio.h:305:67: error: 'size_t' undeclared here (not in a function)
   char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
                                                                   ^~~~~~
/usr/include/libio.h:333:62: error: expected declaration specifiers or '...' before 'size_t'
 typedef __ssize_t __io_read_fn (void *__cookie, char *__buf, size_t __nbytes);
                                                              ^~~~~~
/usr/include/libio.h:342:6: error: expected declaration specifiers or '...' before 'size_t'
      size_t __n);
      ^~~~~~
/usr/include/libio.h:464:19: error: expected '=', ',', ';', 'asm' or '__attribute__' before '_IO_sgetn'
 extern _IO_size_t _IO_sgetn (_IO_FILE *, void *, _IO_size_t);
                   ^~~~~~~~~
In file included from /in/foo.c:2:0:
/usr/include/stdio.h:339:20: error: expected declaration specifiers or '...' before 'size_t'
       int __modes, size_t __n) __THROW;
                    ^~~~~~
/usr/include/stdio.h:388:44: error: expected declaration specifiers or '...' before 'size_t'
 extern int snprintf (char *__restrict __s, size_t __maxlen,
                                            ^~~~~~
/usr/include/stdio.h:392:45: error: expected declaration specifiers or '...' before 'size_t'
 extern int vsnprintf (char *__restrict __s, size_t __maxlen,
                                             ^~~~~~
/usr/include/stdio.h:711:15: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'fread'
 extern size_t fread (void *__restrict __ptr, size_t __size,
               ^~~~~
/usr/include/stdio.h:717:15: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'fwrite'
 extern size_t fwrite (const void *__restrict __ptr, size_t __size,
               ^~~~~~