i need help with sorting an array

**brewbuck** · 04-18-2007

Originally Posted by dwks

I've learned something today. And now I can correct people whenever they use this.

In case you're wondering... The reason you must not declare main(void) is because of parameter passing conventions. Under most ABI's, C parameters are evaluated and passed right-to-left, with the CALLER of the function being responsible for managing the stack. However it is conceivable that on some platform somewhere it is the CALLEE's responsibility to clean the stack.

The problem is, if the function is specifically declared to take a void argument list, the compiler (rightly) assumes that it will never be passed any arguments, so it will not produce the code necessary to fix the stack if arguments had been passed. Then, when the main(void) gets invoked, the C library DOES pass arguments (argc, argv), but main is not aware of this. So when main() returns, the stack becomes corrupt and things go wrong.

**dwks** · 04-18-2007

The FAQ contradicts what you say: http://faq.cprogramming.com/cgi-bin/...&id=1043284376

For C

Under C89, main() is acceptable, although it is advisable to use the C99 standard, under which only these are acceptable:

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

Slight variations of the above are acceptable, where int can be replaced by a typedef name defined as int, or the type of argv can be written as char ** argv, and so on.

The first option is used when you do not require access to the command line arguments.

The names argc and argv are identifiers that can be changed if you so desire, but sticking to argc/argv is convention.

The return type of main() must always be an int, this allows a return code to be passed to the invoker.

Under C89, the return statement at the end of main() is required, whereas under C99 if no return statement is present, return 0 is implied. However, it is good programming practice to always use a return statement, even if you don't have to.

That's not the first error anyone's found in the FAQ . . . .

I'll read the page linked to by this and see if it is mentioned. [edit] It isn't. [/edit]

[edit=2] It makes sense once I think about it, though. Why else could main be declared with argc and argv (and sometimes envp) or without parameters? I guess I just never thought about it. [/edit]

**robatino** · 04-18-2007

The quote from the C standard in the following link says that "int main(void)" is one of the allowed declarations.

http://www.thescripts.com/forum/thread212123.html

Compiling with gcc with -W -Wall -ansi -pedantic, and in either C89 or C99 mode (-std=c99), either of the following compiles without any warnings:

Code:

int main(void) {
  return 0;
}

Code:

int main() {
  return 0;
}

If I give a bogus argument:

Code:

int main(float x) {
  return 0;
}

I get the following warnings in either C89 or C99 mode:

foo.c:2: warning: first argument of ‘main’ should be ‘int’
foo.c:2: warning: ‘main’ takes only zero or two arguments
foo.c:1: warning: unused parameter ‘x’

**brewbuck** · 04-18-2007

Originally Posted by robatino

The quote from the C standard in the following link says that "int main(void)" is one of the allowed declarations.

http://www.thescripts.com/forum/thread212123.html

I admit that it's been a while since I looked at this part of the standard. But as an implementor, I can see that there are serious problems with declaring a function to take a void argument list when in fact it takes arguments. This is the sort of thing that creates massive headaches for compiler writers. "It would be so easy if it weren't for this darn section 1.5.12!" Etc.

**robatino** · 04-18-2007

Under the hood, the compiler could just treat main(void) as a special case and interpret it differently. It does have the advantage that to the user it makes main() appear more similar to other functions.

**stubin87** · 04-19-2007

hehe nice....so we all learned something about useing main(void)

Like i said, i didn't use it in my code, and i never use it..... But can anyone tell me, why i see "void main(void)" in some codes???

**itsme86** · 04-19-2007

But can anyone tell me, why i see "void main(void)" in some codes???

Pick at least 1:
- Ignorance
- Stubbornness
- Laziness

**KONI** · 04-20-2007

I always wrote "int main()" but then I remember a thread (that was lost during the reset) where someone explained in great detail why we should write all "int main(void)" and how that would be the correct way ... guess it's back to the empty braces.

**robatino** · 04-20-2007

As the quote from the C99 standard from the above URL indicates, "int main(void)" is explicitly blessed (5.1.2.2.1 Program startup). I'm not sure about "int main()", although I wasn't able to get gcc to emit any errors or warnings with either version.

http://www.google.com/search?hl=en&q...=Google+Search

**brewbuck** · 04-20-2007

Originally Posted by robatino

As the quote from the C99 standard from the above URL indicates, "int main(void)" is explicitly blessed (5.1.2.2.1 Program startup). I'm not sure about "int main()", although I wasn't able to get gcc to emit any errors or warnings with either version.

It's okay in all cases, not only for main(). Following code is warning-free:

Code:

void foo(void);
void foo()
{
}

**swoopy** · 04-20-2007

>The FAQ contradicts what you say
And the FAQ is correct.

**redche** · 04-20-2007

here is another solution

Code:

#include<stdio.h>
#define MAX 20

int main(void)
{
    int tab[MAX],nb_item,i,j,tmp;
    
    do{
        printf("Enter the number of elements ");
        scanf("&#37;d",&nb_item);
        
    }while( nb_item<1 && nb_item>=MAX );
    
    for( i=0 ; i<nb_item ; i++)
    {
         printf("T[%d]=",i+1);
         scanf("%d",&tab[i]);
    }
    
    //the sorting start here
    for( i=0 ; i<nb_item ; i++)
    {
         if(tab[i]>=0)
         {
             tmp=tab[i];
             for(j=i-1 ; j>=0 && tab[j]<0 ; j--)
             {
                   tab[j+1]=tab[j];  
             }
             tab[j+1]=tmp;
         }
     }
     
     for( i=0 ; i<nb_item ; i++)
     printf("%d  ",tab[i]);
     
    
     return 0;
}

**Dave_Sinkula** · 04-20-2007

Originally Posted by redche

here is another solution

Type something like "oops" at the prompt.

**redche** · 04-20-2007

what do you mean???

**MacGyver** · 04-20-2007

1) Run program.
2) When prompted for input, enter the string "oops".
3) Watch program run in circles like The Six Million Dollar Man.

I believe he's just pointing out that your program cannot accept every kind of input.

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