9.1.5 Fallback Function Implementations

Due to the huge number of Unix varieties in common use today, many of the C library functions that you take for granted on your prefered development platform are very likely missing from some of the architectures you would like your code to compile on. Fundamentally there are two ways to cope with this:

• Use only the few library calls that are available everywhere. In reality this is not actually possible because there are two lowest common denominators with mutually exclusive APIs, one rooted in BSD Unix (`bcopy', `rindex') and the other in SYSV Unix (`memcpy', `strrchr'). The only way to deal with this is to define one API in terms of the other using the preprocessor. The newer POSIX standard deprecates many of the BSD originated calls (with exceptions such as the BSD socket API). Even on non-POSIX platforms, there has been so much cross pollination that often both varieties of a given call may be provided, however you would be wise to write your code using POSIX endorsed calls, and where they are missing, define them in terms of whatever the host platform provides.

  This approach requires a lot of knowledge about various system libraries and standards documents, and can leave you with reams of preprocessor code to handle the differences between APIS. You will also need to perform a lot of checking in `configure.in' to figure out which calls are available. For example, to allow the rest of your code to use the `strcpy' call with impunity, you would need the following code in `configure.in':

  AC_CHECK_FUNCS(strcpy bcopy)

  And the following preprocessor code in a header file that is seen by every source file:

  #if !HAVE_STRCPY # if HAVE_BCOPY # define strcpy(dest, src) bcopy (src, dest, 1 + strlen (src)) # else /* !HAVE_BCOPY */ error no strcpy or bcopy # endif /* HAVE_BCOPY */ #endif /* HAVE_STRCPY */

• Alternatively you could provide your own fallback implementations of function calls you know are missing on some platforms. In practice you don't need to be as knowledgable about problematic functions when using this approach. You can look in GNU libiberty(9) or François Pinard's libit project(10) to see for which functions other GNU developers have needed to implement fallback code. The libit project is especially useful in this respect as it comprises canonical versions of fallback functions, and suitable Autoconf macros assembled from across the entire GNU project. I won't give an example of setting up your package to use this approach, since that is how I have chosen to structure the project described in this chapter.

Rather than writing code to the lowest common denominator of system libraries, I am a strong advocate of the latter school of thought in the majority of cases. As with all things it pays to take a pragmatic approach; don't be afraid of the middle ground -- weigh the options on a case by case basis.