Are there any well-established/standardized ways to use fixed-width integers in C89?

Some background:

the header stdint.h is part of the C standard since C99. It includes typedefs that are ensured to be 8, 16, 32, and 64-bit long integers, both signed and unsigned. This header is not part of the C89 standard, though, and I haven't yet found any straightforward way to ensure that my datatypes have a known length.

Getting to the actual topic

The following code is how SQLite (written in C89) defines 64-bit integers, but I don't find it convincing. That is, I don't think it's going to work everywhere. Worst of all, it could fail silently:

/*
** CAPI3REF: 64-Bit Integer Types
** KEYWORDS: sqlite_int64 sqlite_uint64
**
** Because there is no cross-platform way to specify 64-bit integer types
** SQLite includes typedefs for 64-bit signed and unsigned integers.
*/
#ifdef SQLITE_INT64_TYPE
  typedef SQLITE_INT64_TYPE sqlite_int64;
  typedef unsigned SQLITE_INT64_TYPE sqlite_uint64;
#elif defined(_MSC_VER) || defined(__BORLANDC__)
  typedef __int64 sqlite_int64;
  typedef unsigned __int64 sqlite_uint64;
#else
  typedef long long int sqlite_int64;
  typedef unsigned long long int sqlite_uint64;
#endif
typedef sqlite_int64 sqlite3_int64;
typedef sqlite_uint64 sqlite3_uint64;

So, this is what I've been doing so far:

Checking that the "char" data type is 8 bits long, since it's not guaranteed to be. If the preprocessor variable "CHAR_BIT" is not equal to 8, compilation fails
Now that "char" is guaranteed to be 8 bits long, I create a struct containing an array of several unsigned chars, which correspond to several bytes in the integer.
I write "operator" functions for my datatypes. Addition, multiplication, division, modulo, conversion from/to string, etc.

I have abstracted this process in a header file, which is the best I can do with what I know, but I wonder if there is a more straightforward way to achieve this.

I'm asking because I want to write a portable C library.

Solution

First, you should ask yourself whether you really need to support implementations that don't provide <stdint.h>. It was standardized in 1999, and even many pre-C99 implementations are likely to provide it as an extension.

Assuming you really need this, Doug Gwyn, a member of the ISO C standard committee, created an implementation of several of the new headers for C9x (as C99 was then known), compatible with C89/C90. The headers are in the public domain and should be reasonably portable.

http://www.lysator.liu.se/(nobg)/c/q8/index.html

(As I understand it, the name "q8" has no particular meaning; he just chose it as a reasonably short and unique search term.)