I found it odd and problematic that two glibc headers seem to compete with each
other regarding definition of int64_t (and probably uint64_t as well). It looks
like it's either incomplete, or some update never made it's way into the source
tree, or, there is some rarer side issue which comes up and that they headers
are this way on purpose.

Here are details:

1) Header file /usr/include/sys/types.h defines int64_t as follows:

/* These types are defined by the ISO C99 header <inttypes.h>. */
# ifndef __int8_t_defined
#  define __int8_t_defined
typedef	char int8_t;
typedef	short int int16_t;
typedef	int int32_t;
#  if __WORDSIZE == 64
typedef long int int64_t;
#  elif __GLIBC_HAVE_LONG_LONG
__extension__ typedef long long int int64_t;
#  endif
# endif

2) Header /usr/include/stdint.h also defines int64_t but slightly differently:

/* There is some amount of overlap with <sys/types.h> as known by inet code */
#ifndef __int8_t_defined
# define __int8_t_defined
typedef signed char		int8_t;
typedef short int		int16_t;
typedef int			int32_t;
# if __WORDSIZE == 64
typedef long int		int64_t;
# else
__extension__
typedef long long int		int64_t;
# endif
#endi
# endif

As the comment states, this file does repeat some type definitions.
However, definition of int64_t is different, it is not guarded with
__GLIBC_HAVE_LONG_LONG preprocessor definition.

This difference causes problems, for example, when using some of libraries from
Boost C++ collection and compiling with Comeau C/C++ Compiler front-end.
Here is original report I submitted to Boost
(https://svn.boost.org/trac/boost/ticket/3548).


Here are small tests that present how use of sys/types.h vs stdint.h influence
compilation result if int64_t type is used:

1) Include sys/types.h and compilation fails:

////// $ cat bad.cpp ///////////////////////////////
#include <sys/types.h>
#include <boost/cstdint.hpp>
int main()
{
    boost::int64_t a(0);
    return 0;
}

$ como -I/home/mloskot/dev/boost/_svn/trunk bad.cpp 
Comeau C/C++ 4.3.10.1 (May  7 2008 12:23:21) for LINUX_INTEL_ELF_Beta
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:non-strict warnings C++ noC++0x_extensions

"/home/mloskot/dev/boost/_svn/trunk/boost/cstdint.hpp", line 111: error: the
          global scope has no "int64_t"
    using ::int64_t;            
            ^

"bad.cpp", line 5: error: namespace "boost" has no member "int64_t"
      boost::int64_t a(0);
             ^


2) 1) Include stdint.h and compilation fails:

////// $ cat good.cpp //////////////////////////////
#include <stdint.h>
#include <boost/cstdint.hpp>
int main()
{
    boost::int64_t a(0);
    return 0;
}

$ como -I/home/mloskot/dev/boost/_svn/trunk good.cpp 
Comeau C/C++ 4.3.10.1 (May  7 2008 12:23:21) for LINUX_INTEL_ELF_Beta
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:non-strict warnings C++ noC++0x_extensions

"good.cpp", line 5: warning: variable "a" was declared but never referenced
      boost::int64_t a(0);
                     ^


It is important to notice that int64_t is defined only if __GLIBC_HAVE_LONG_LONG.
I checked the two headers in current glibc git repository and it still includes
those definitions of int64_t, thus I decided to submit bug report directly to glibc
instead of Ubuntu team (I use glibc from Ubuntu packages):
(http://sourceware.org/git/?p=glibc.git;a=blob;f=posix/sys/types.h#l160)
(http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/generic/stdint.h#l34)


Environment:
* OS: Linux / Ubuntu 9.10
* GCC: gcc(Ubuntu 4.4.1-4ubuntu8) 4.4.1
* glibc: 2.10.1-0ubuntu15 (http://packages.ubuntu.com/karmic/libc6-dev)