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Issues in Writing Portable C

One important portability issue is library function calls. Though the C programming language was fairly well defined by the original K&R book (Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language [Englewood Cliffs, NJ.: Prentice-Hall, 1978]), the run-time library implementation was left totally up to the whims of the implementor. Fortunately, the American National Standards Institute was able to complete the C language specification in 1990, and the result was published as ANSI standard XJ11.34. This standard not only expanded and pinned down the original K&R language specification, but it also took on the definition of a standard C run-time library. This makes it much easier to write code that works the same way from machine to machine. The code in this book will be written with the intention of using only ANSI C library calls. Compiler-dependent extensions to either the language or the library will be avoided wherever possible.

Given the standardization of the libraries, the remaining portability issues center around two things: sizes of the basic data types and dealing with noncompliant compilers. The majority of data-type conflicts arise when switching between 16- and 32-bit machines.

Fortunately, it is fairly easy to manage the change between 16- and 32-bit machines. Though the basic integer data type switches between 16- and 32-bits, both machines have a 16-bit “short int” data type. Once again, a “long int” is generally 32 bits on both machines. So in cases where the size of an integer clearly matters, it can be pinned down to either 16-or 32-bits with the appropriate declaration.

On the vast majority of machines used in the world today, the C compiler implementation of the “char” data type is 8 bits wide. In this book, we will gloss over the possibility that any other size exists and stick with 8-bit characters. In general, porting a program shown here to a machine with an unusual char size is not too difficult, but spending too much time on it will obscure the important point of the programs here, which is data compression.

The final issue to deal with when writing portable code is the problem of noncompliant compilers. In the MS-DOS world, most C compilers undergo major releases and upgrades every two years or so. This means that most compiler vendors have been able to release new versions of their compilers that now conform closely to the ANSI C standard. But this is not the case for users of many other operating systems. In particular, UNIX users will frequently be using a C compiler which came with their system and which conforms to the older K&R language definition. While the ANSI C committee went to great lengths to make ANSI C upwardly compatible from K&R C, we need to watch out for a few problems.

The first problem lies in the use of function prototypes. Under K&R C, function prototypes were generally used only when necessary. The compiler assumed that any unseen function returned an integer, and it accepted this without complaint. If a function returned something unusual—a pointer or a long, for instance—the programmer would write a function prototype to inform the compiler.

long locate_string();

Here, the prototype told the compiler to generate code that assumes that the function returned a long instead of an int. Function prototypes didn’t have much more use than that. Because of this, many C programmers working under a K&R regime made little or no use of function prototypes, and their appearance in a program was something of an oddity.

While the ANSI C committee tried not to alter the basic nature of C, they were unable to pass up the potential improvements to the language that were possible through the expansion of the prototyping facility. Under ANSI C, a function prototype defines not only the return type of a function, but also the type of all the arguments as well. The function shown earlier, for example, might have the following prototype with an ANSI C compiler:

long locate_string( FILE *input_file, char *string );

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