5.7 Calling System Functions

C implementations differ substantially. Standard C reduces but does not eliminate the incompatibilities; meanwhile, many GNU packages still support pre-standard compilers because this is not hard to do. This chapter gives recommendations for how to use the more-or-less standard C library functions to avoid unnecessary loss of portability.

• Don't use the return value of sprintf. It returns the number of characters written on some systems, but not on all systems.
• Be aware that vfprintf is not always available.
• main should be declared to return type int. It should terminate either by calling exit or by returning the integer status code; make sure it cannot ever return an undefined value.

• Don't declare system functions explicitly.

  Almost any declaration for a system function is wrong on some system. To minimize conflicts, leave it to the system header files to declare system functions. If the headers don't declare a function, let it remain undeclared.

  While it may seem unclean to use a function without declaring it, in practice this works fine for most system library functions on the systems where this really happens; thus, the disadvantage is only theoretical. By contrast, actual declarations have frequently caused actual conflicts.

• If you must declare a system function, don't specify the argument types. Use an old-style declaration, not a Standard C prototype. The more you specify about the function, the more likely a conflict.
• In particular, don't unconditionally declare malloc or realloc.

  Most GNU programs use those functions just once, in functions conventionally named xmalloc and xrealloc. These functions call malloc and realloc, respectively, and check the results.

  Because xmalloc and xrealloc are defined in your program, you can declare them in other files without any risk of type conflict.

  On most systems, int is the same length as a pointer; thus, the calls to malloc and realloc work fine. For the few exceptional systems (mostly 64-bit machines), you can use conditionalized declarations of malloc and realloc—or put these declarations in configuration files specific to those systems.

• The string functions require special treatment. Some Unix systems have a header file string.h; others have strings.h. Neither file name is portable. There are two things you can do: use Autoconf to figure out which file to include, or don't include either file.
• If you don't include either strings file, you can't get declarations for the string functions from the header file in the usual way.

  That causes less of a problem than you might think. The newer standard string functions should be avoided anyway because many systems still don't support them. The string functions you can use are these:

            strcpy   strncpy   strcat   strncat
            strlen   strcmp    strncmp
            strchr   strrchr
  

  The copy and concatenate functions work fine without a declaration as long as you don't use their values. Using their values without a declaration fails on systems where the width of a pointer differs from the width of int, and perhaps in other cases. It is trivial to avoid using their values, so do that.

  The compare functions and strlen work fine without a declaration on most systems, possibly all the ones that GNU software runs on. You may find it necessary to declare them conditionally on a few systems.

  The search functions must be declared to return char *. Luckily, there is no variation in the data type they return. But there is variation in their names. Some systems give these functions the names index and rindex; other systems use the names strchr and strrchr. Some systems support both pairs of names, but neither pair works on all systems.

  You should pick a single pair of names and use it throughout your program. (Nowadays, it is better to choose strchr and strrchr for new programs, since those are the standard names.) Declare both of those names as functions returning char *. On systems which don't support those names, define them as macros in terms of the other pair. For example, here is what to put at the beginning of your file (or in a header) if you want to use the names strchr and strrchr throughout:

            #ifndef HAVE_STRCHR
            #define strchr index
            #endif
            #ifndef HAVE_STRRCHR
            #define strrchr rindex
            #endif
            
            char *strchr ();
            char *strrchr ();
  

Here we assume that HAVE_STRCHR and HAVE_STRRCHR are macros defined in systems where the corresponding functions exist. One way to get them properly defined is to use Autoconf.