Different ways to declare main in C

Question

Possible Duplicate:
Alternate C syntax for function declaration use cases

I see this very different definition of main() in an old C-program. (It does compile, with gcc)

main(argc, argv)
   int argc;
   char *argv[]; 
   {
   ...
   ...

   }

What kind of variables are being declared and why are they being done before the beginning of the function brace?

Not voting to close because if anything this one should be kept as it has a more searchable, specific title. — djechlin, Nov 08 '12 at 18:33
@djechlin: Only because the other title is crap. We should adapt this title into that question and close this one as a dup. — Lightness Races in Orbit, Nov 08 '12 at 18:36

score 3 · Accepted Answer · answered Nov 08 '12 at 18:29

3

This is old style of function definition in C. This non-prototype style is marked as obsolescent in the C Standard and should not be used.

And omitting the return type (implicitly int) is no longer permitted in C.

answered Nov 08 '12 at 18:29

ouah

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djechlin · Answer 2 · 2012-11-08T23:04:09.703

1

It implicitly returns int per the C spec (although per other answers in this and the linked question this was obsoleted in C99?), as is the case with functions with undeclared return type.

This is an old style of declaration, it comes from Fortran.

main(argc, argv)
   int argc;
   char *argv[];

Tells you exactly what you'd expect - that argc is an int and argv is an arryay of char *.

edited Nov 08 '12 at 23:04

answered Nov 08 '12 at 18:29

djechlin

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Are you sure the implicit return type is true for ISO C? C99 5.1.2.2.1/1 suggests not; 6.7.2/2 says it explicitly, by my interpretation. – Lightness Races in Orbit Nov 08 '12 at 22:45
Not sure, edited accordingly. – djechlin Nov 08 '12 at 23:04
Would be interested to see a citation from some standard about the implicit return. – Lightness Races in Orbit Nov 08 '12 at 23:07

score 1 · Answer 3 · answered Nov 08 '12 at 18:29

1

This is called a K&R-style definition. You basically just list the parameters and declare them between the closing argument bracket and the opening curly bracket.

answered Nov 08 '12 at 18:29

fuz

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score 1 · Answer 4 · answered Nov 08 '12 at 18:29

1

That's an old KnR C style, pre 1989 C (non-standard). It was a way to pass arguments to functions.. not only with main() that was with every other function as well.

For example:

void non_main(argx, argy)
int argx,argy;
{
   /*do something here*/
}

answered Nov 08 '12 at 18:29

Aniket Inge

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It's still standard, even in the 2011 ISO C standard, but it's obsolescent and should not be used. – Keith Thompson Nov 08 '12 at 23:20

Brian Campbell · Answer 5 · 2012-11-08T18:35:20.997

1

This is simply an alternative way of specifying the types of the arguments to main. It is exactly equivalent to main(int argc, char *argv[]). This is known as K&R style, as it's the original style used in the original C compiler and K&R books. It is considered obsolete; it is not compatible with C++. There's a GCC option to warn about it, -Wold-style-definition, but you will still see it in some old code.

edited Nov 08 '12 at 18:35

answered Nov 08 '12 at 18:29

Brian Campbell

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It is not equivalent. For example if you have a function call `main(a, b)` and `a` is an object of type `double`, `a` will not be converted to `int` as with prototype definition. – ouah Nov 08 '12 at 18:33
@ouah Thanks for the clarification; I hadn't thought of that. – Brian Campbell Nov 10 '12 at 00:08

Different ways to declare main in C

5 Answers5