Are You Ready For C99?


Dare Obasanjo

ANSI and the ISO ratified the newest draft of the C standard in 1999 and unleashed the biggest changes to the language to date. New keywords, library functions, and macros are just the tip of the iceberg when it comes to the new improved C. In fact, the language is so different it is no longer compatible with C++. This article attempts to give an overview of the major changes made to the C language and also attempts to discover why no one yet seems to be affected by the new standard.

Interesting New Features

A list of features that should[0] have made it into the C99 standard is available on Thomas Wolf's webpage. Listed below are the changes that most developers would notice or care about at first use.
  1. Increased identifier size limits: specifically 63 significant initial characters in an internal identifier or macro name, 31 significant initial characters in an external identifier, and 4095 characters in a logical source line. These values were 31, 6, and 509, respectively, in C89.

    The small identifier size is the reason so many ANSI functions had terse names (strcpy, strstr, strchr, etc) since the standard only guaranteed that the first six characters would be used to uniquely identify the functions.

  2. C++ style/line comments: The characters '//' can now be used to delineate a line of commented text, just like in C++.

  3. Macros take variable arguments denoted by elipsees: Function-like macros will accept variable arguments denoted by using the ellipsis (...) notation. For replacement, the variable arguments (including the separating commas) are "collected" into one single extra argument that can be referenced as __VA_ARGS__ within the macro's replacement list.

  4. Inline functions: The C language now supports the inline keyword which allows functions to be defined as inline, which is a hint to the compiler that invocations of such functions can be replaced with inline code expansions rather than actual function calls.

  5. Restricted pointers: The C language now supports the restrict keyword which allows pointers to be defined as restricted, which is a hint to the compiler to disallow two restricted pointers from being aliases to the same object which allows for certain optimizations when dealing with the said pointers.

  6. _Bool Macro: There is a _Bool type which is a actually two valued integer type. True is defined as
    #define true (_Bool)1
    while false is defined as
    #define false (_Bool)0
  7. Variable Declarations can appear anywhere in the code block: No longer do variables have to be defined at the top of the code block.

  8. Variable length arrays: These are arrays whose size is determined at runtime.

  9. Variable declarations in for loops: Variables can now be declared and initialized in for loops just like in C++ and Java.

  10. Named initialization of structs: The members of a struct can now be initialized by name such as is done in the code block below
    struct {float x, y, z;} s = { .y = 1.0, .x = 3.5, .z = 12.8};
  11. New long long type: There is a new type called long long which is at least 64 bits and can be both signed or unsigned. The new suffixes "LL" or "ll" (and "ULL" or "ull") are used for constants of the new long long type.

  12. Functions must declare a return value: Function return types no longer defaults to int if the function declares no return type.

  13. Last member of a struct may be an incomplete array type. : This is to support the "struct hack" which works on most existing C compilers already. A code example of the struct hack is shown on Thomas's site.

  14. Addition of _Complex and _Imaginary number types: A boon for programmers doing any sort of advanced math in their programs.

  15. Multiple uses of a type qualifier are ignored after the first occurence: If a type qualifier appears several times (either directly or indirectly through typedefs) in a type specification, it's treated as if it appeared only once. E.g.
    const const int x;
    is the same as
    const int x;
C++ Incompatibilities

The aforementioned features are rather impressive but one soon realizes that this means that C is no longer a subset of C++. There is a list of the major incompatibilities between the current ISO standards for C and C++ on David Tribble's webpage. At this point it is still too early to see if either C or C++ will be harmed by this development but it is clear that there will be some growing pains once C99 adoption becomes widespread.

Bjarne Stroustrup mentioned in a recent interview with LinuxWorld that he would have liked for both languages to be compatible and would favor a technical committee whose express purpose would be integrating both languages, but doubts the possibility of this coming to pass. Stroustrup also contrasted how C's technical commitee decided to implement most of the added functionality via changes to the actual C language against the C++ approach which was by adding additional libraries.

Compiler support

After describing all the exciting new features of C99, one would expect there to be more awareness of the standard by now but there isn't. The primary reason for the lack of awareness of C99 is the fact that compiler support at the current time is practically non-existent. Here is the status of the major compiler vendors for the development platforms that I'm interested in:
  1. Microsoft: A search on Microsoft's site for C99 draws a total blank. It looks like Microsoft does not plan to upgrade the C compiler that ships with Visual C++ to cover the C99 standard.

  2. Borland: A search on Borland's site for C99 also draws a blank. Again it looks like exclusive C++ development has won over keeping their C compiler up to date.

  3. Comeau Computing: The most recent version of their compiler (version which is available for online testing claims to support a large number of features from C99. The compiler has not yet been released but can be tested online by submitting code snippets in an HTML form.

  4. SAS Institute: SAS/C version 7.0 supports the "long long" type, inline functions and a few of the preprocessor directives.

  5. Edison Design Group: The EDG C++ front end supposedly supports both C89 and C99 as well as Microsoft C++ and ANSI/ISO C++.

  6. GNU: GCC has a status page of C99 compliance which shows that they are quite close to becoming fully compliant with the C99 standard.

Where To Get It

If you are interested in being the first kid on your block who knows the finer points of restricted pointers and variable argument macros, you can purchase the C99 standard online from ANSI. Finally no discussion of C99 is complete without a reference to Dennis Ritchie's opinion of the C99 standard.

[0] I didn't buy a copy of the standard, so I cannot guarantee that everything on that site made it into the standard although there is a good chance that everything did.

© 2001 Dare Obasanjo