function declaration isn t a prototype

Question

I have a library I created   mylib c    include  lt mylib h gt  int testlib         printf  Hello world n        return  0       mylib h    include  lt stdio h gt  extern int testlib      In my program  I ve attempted to call this library function   myprogram c    include  lt mylib h gt   int main  int argc  char  argv          testlib        return  0       When I attempt to compile this program I get the following error   In file included from myprogram c 1 mylib h 2 warning  function declaration isn t a prototype  I m using  gcc  GCC  3 4 5 20051201  Red Hat 3 4 5-2   My question is  what is the proper way to declare a function prototype

User · Answer

Try   extern int testlib void

User · Answer

Quick answer  change int testlib   to int testlib void  to specify that the function takes no arguments   A prototype is by definition a function declaration that specifies the type s  of the function s argument s    A non-prototype function declaration like  int foo      is an old-style declaration that does not specify the number or types of arguments   Prior to the 1989 ANSI C standard  this was the only kind of function declaration available in the language   You can call such a function with any arbitrary number of arguments  and the compiler isn t required to complain -- but if the call is inconsistent with the definition  your program has undefined behavior   For a function that takes one or more arguments  you can specify the type of each argument in the declaration   int bar int x  double y     Functions with no arguments are a special case  Logically  empty parentheses would have been a good way to specify that an argument but that syntax was already in use for old-style function declarations  so the ANSI C committee invented a new syntax using the void keyword   int foo void      foo takes no arguments      A function definition  which includes code for what the function actually does  also provides a declaration  In your case  you have something similar to   int testlib            code that implements testlib        This provides a non-prototype declaration for testlib  As a definition  this tells the compiler that testlib has no parameters  but as a declaration  it only tells the compiler that testlib takes some unspecified but fixed number and type s  of arguments   If you change    to  void  the declaration becomes a prototype   The advantage of a prototype is that if you accidentally call testlib with one or more arguments  the compiler will diagnose the error    C   has slightly different rules  C   doesn t have old-style function declarations  and empty parentheses specifically mean that a function takes no arguments  C   supports the  void  syntax for consistency with C  But unless you specifically need your code to compile both as C and as C    you should probably use the    in C   and the  void  syntax in C

User · Answer

In C int foo   and int foo void  are different functions  int foo   accepts an arbitrary number of arguments  while int foo void  accepts 0 arguments  In C   they mean the same thing  I suggest that you use void consistently when you mean no arguments   If you have a variable a  extern int a  is a way to tell the compiler that a is a symbol that might be present in a different translation unit  C compiler speak for source file   don t resolve it until link time  On the other hand  symbols which are function names are anyway resolved at link time  The meaning of a storage class specifier on a function  extern  static  only affects its visibility and extern is the default  so extern is actually unnecessary   I suggest removing the extern  it is extraneous and is usually omitted

[c] function declaration isn't a prototype

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