If a function prototype parameter field is left empty does it assume an int?

This is a discussion on If a function prototype parameter field is left empty does it assume an int? within the C Programming forums, part of the General Programming Boards category; Take this code as an example Code: #include <stdio.h> void function1(); void function2( void ); int main() { // function1( ...

  1. #1
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    If a function prototype parameter field is left empty does it assume an int?

    Take this code as an example
    Code:
    #include <stdio.h>
    
    void function1();
    void function2( void );
    
    int main()
    {
    //	function1( 5 );
    
    	function1();
    //	function2( 5 );
    
    }
    
    /*
    void function1( int x )
    {
    
    	return;
    
    }
    */
    
    void function1()
    {
    
    	return;
    
    }
    
    
    
    /*
    void function2( int x )
    {
    
    	return;
    
    }
    */

    The parameter field for the prototype of function1 is left blank. This lets me create either a function1 that takes an int parameter or a function1 that takes no parameters and nothing else. Could someone explain this to me?

  2. #2
    cas
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    When you supply no parameters, you're not actually creating a prototype, just a declaration. An empty parameter lists means an unspecified number of arguments. You can pass one int, but you could also pass, say, 5 ints. The compiler does not check whether the call is correct because it can't.

    You should never use an empty parameter list. It's a holdover from the pre-prototype days of C, and prototypes are better, because they allow the compiler to make sure you're calling a function with the proper types.

  3. #3
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by yougene
    The parameter field for the prototype of function1 is left blank. This lets me create either a function1 that takes an int parameter or a function1 that takes no parameters and nothing else. Could someone explain this to me?
    The empty parameter list means that the number and types of the parameters are unspecified in that declaration.
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  4. #4
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    Thanks for all the helpful replies


    @laserlight
    if the types of parameters are unspecified why does this produce an error for me?
    Code:
    #include <stdio.h>
    
    void function1();
    
    int main()
    {
    
    	function1( 'p' );
    
    }
    
    
    void function1( char x )
    {
    
    	return;
    
    }



    Code:
    eugene@eugene-laptop:~/cfiles$ cc -Wall -pedantic testprototypes.c
    testprototypes.c:8:1: warning: C++ style comments are not allowed in ISO C90
    testprototypes.c:8:1: warning: (this will be reported only once per input file)
    testprototypes.c: In function ‘main’:
    testprototypes.c:13: warning: control reaches end of non-void function
    testprototypes.c: At top level:
    testprototypes.c:25: error: conflicting types for ‘function1’
    testprototypes.c:25: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
    testprototypes.c:3: error: previous declaration of ‘function1’ was here
    eugene@eugene-laptop:~/cfiles$
    It didn't work with char types for some reason.

  5. #5
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by yougene
    It didn't work with char types for some reason.
    According to the error message, "an argument type that has a default promotion can’t match an empty parameter name list declaration". So the problem is that char would be promoted to int, which would then cause a mismatch with the function definition's char parameter. (That said, I am not entirely sure why the error message was phrased that way since 'p' is of type int, so there should not have been a type promotion, from what I see.)
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  6. #6
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by yougene View Post
    It didn't work with char types for some reason.
    Fascinating. This is something I haven't seen before. The spec has the answer...
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    mmmm, C voodoo

    Thanks guys

  8. #8
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    Quote Originally Posted by yougene View Post
    mmmm, C voodoo

    Thanks guys
    What compiler are you using to test this?

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    Looks like gcc's output, and I think that's what it is because of this thread. Never seen this syntax before. Help?
    dwk

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  10. #10
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    Yeah, it's GCC

  11. #11
    Captain Crash brewbuck's Avatar
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    I think it's interesting, because you first declare a prototype with unspecified arguments, then you define the function with specified arguments -- apparently, gcc allows the definition to override the previously specified prototype.

    The reason that's interesting is because in all other cases it would be an error (well, a different error). If you did this:

    Code:
    int function( int x );
    
    int function( char y )
    {
    }
    You'd get an error because the prototype does not match the definition.

    I'm curious whether this behavior is specific to gcc, or something which is prescribed by the standard. It's almost like the prototype with unspecified arguments is "weak".
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  12. #12
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    So is it an implementation-specific symptom or is it enforced by the C standard because the C++ compiler (I have) barfs but it's no problemo for the C compiler.

  13. #13
    Frequently Quite Prolix dwks's Avatar
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    That's probably because in C++, if you leave out a parameter list in a prototype, it's exactly like saying (void). In other words, C++ doesn't have the old K&R provision that () means "unspecified parameters" like C does.

    Anyway, you shouldn't be using K&R prototypes if you can absolutely help it.

    [edit] For what it's worth:
    Code:
    $ cat emptyproto.c
    #include <stdio.h>
    
    void function1();
    
    int main()
    {
    
            function1( 'p' );
    
    }
    
    
    void function1( char x )
    {
    
            return;
    
    }
    $ gcc-4.1 -W -Wall -ansi -pedantic emptyproto.c -o emptyproto
    emptyproto.c: In function ‘main’:
    emptyproto.c:10: warning: control reaches end of non-void function
    emptyproto.c: At top level:
    emptyproto.c:14: error: conflicting types for ‘function1’
    emptyproto.c:14: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
    emptyproto.c:3: error: previous declaration of ‘function1’ was here
    emptyproto.c:13: warning: unused parameter ‘x’
    $ gcc-4.2 -W -Wall -ansi -pedantic emptyproto.c -o emptyproto
    emptyproto.c: In function ‘main’:
    emptyproto.c:10: warning: control reaches end of non-void function
    emptyproto.c: At top level:
    emptyproto.c:14: error: conflicting types for ‘function1’
    emptyproto.c:14: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
    emptyproto.c:3: error: previous declaration of ‘function1’ was here
    emptyproto.c:13: warning: unused parameter ‘x’
    $ gcc-4.3 -W -Wall -ansi -pedantic emptyproto.c -o emptyproto
    emptyproto.c: In function ‘main’:
    emptyproto.c:10: warning: control reaches end of non-void function
    emptyproto.c: At top level:
    emptyproto.c:13: error: conflicting types for ‘function1’
    emptyproto.c:14: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
    emptyproto.c:3: error: previous declaration of ‘function1’ was here
    emptyproto.c:13: warning: unused parameter ‘x’
    $ gcc-4.4 -W -Wall -ansi -pedantic emptyproto.c -o emptyproto
    emptyproto.c:13: error: conflicting types for ‘function1’
    emptyproto.c:14: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
    emptyproto.c:3: note: previous declaration of ‘function1’ was here
    emptyproto.c: In function ‘function1’:
    emptyproto.c:13: warning: unused parameter ‘x’
    $
    This is a 64-bit Debian GNU/Linux system. [/edit]
    Last edited by dwks; 06-12-2009 at 03:01 PM.
    dwk

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  14. #14
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by itCbitC View Post
    So is it an implementation-specific symptom or is it enforced by the C standard because the C++ compiler (I have) barfs but it's no problemo for the C compiler.
    The C standard explicitly allows a definition to override a () empty parameter list.

  15. #15
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    Quote Originally Posted by tabstop View Post
    The C standard explicitly allows a definition to override a () empty parameter list.
    Well, that makes sense, since you obviously have to be able to specify the actual arguments when defining the function.

    But why does it change the type promotion rules so that the call with a char is not valid? Weird.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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