I am writing C program in Linux. I compile it with gcc. I wish to
define the variable value of top_or_pack according to the second
argument. I have the following code:

printf("\nargv[2] = %s\t\ttop_or_pack = %d\n", argv[2], top_or_pack);
if(argv[2] == "-top"){
  top_or_pack=1;
}else{
   if(argv[2] == "-pack"){
    top_or_pack=2;
   }else{
    help_message();
    return 0;
  }
}

See how it runs here:


# my_program   file_a   -top   file_b

argv[2] = -top          top_or_pack = 0

    Display help message


# my_program   file_a   -pack   file_b

argv[2] = -pack         top_or_pack = 0

    Display help message


# my_program   file_a   a   file_b

argv[2] = a             top_or_pack = 0

    Display help message


#

The value of argv[2] is successfully got from command line. But the
flow control judgement always assumes argv[2] is neither "-top" nor "-
pack".

I tried adding & or *, which means changing  if(argv[2] ==  into  if
(&argv[2] ==  or  if(*argv[2] == . But the result is still like this.

What's the error in my code? How to fix it? Thanks.

On Jun 10, 6:42=A0pm, Kuhl <chen_zhi...@yahoo.com> wrote:

> if(argv[2] =3D=3D "-top"){

> =A0 =A0if(argv[2] =3D=3D "-pack"){

What type is 'argv[2]'? What type is "-top" or "-pack"?

Consider:
int j=3D3;
int i=3D3;
if(&j =3D=3D &i) ...

This will be false all the time, right?

Don't compare pointers with '=3D=3D'. Use the appropriate function to
compare their contents, which in this case is probably 'strcmp'.

DS

Kuhl <chen_zhitao@yahoo.com> writes:
<snip>
>... I have the following code:
>
> printf("\nargv[2] = %s\t\ttop_or_pack = %d\n", argv[2], top_or_pack);
> if(argv[2] == "-top"){
>   top_or_pack=1;
> }else{
>    if(argv[2] == "-pack"){
>     top_or_pack=2;
>    }else{
>     help_message();
>     return 0;
>   }
> }
<snip>
> What's the error in my code? How to fix it? Thanks.

Use strcmp to compare strings.

The test argv[2] == "-top" tests two pointers.  The pointer in argv[2]
will never compare equal to the pointer that results from the string
literal "-top".

-- 
Ben.

Hi, strcmp works. But there's a further question. I did not include
any file in my code, however strcmp still works. There's no definition
of strcmp in my code. So is strcmp a key word that can be recognized
by gcc directly? Thanks.

Kuhl wrote:
> Hi, strcmp works. But there's a further question. I did not include
> any file in my code, however strcmp still works. There's no definition
> of strcmp in my code. So is strcmp a key word that can be recognized
> by gcc directly? Thanks.

No it is not, it is just implicitly declared (in your code) and subsequently 
found in a standard library.
Crank up the compiler's warning level...

Bye, Jojo 

Kuhl  <chen_zhitao@yahoo.com> wrote:
>Hi, strcmp works. But there's a further question. I did not include
>any file in my code, however strcmp still works.

You should #include <string.h> to use strcmp() ("man strcmp" will tell
you which headers you need.) 

Plus this is a great example of why you should turn up your warning
level on your compiler:

$ gcc foo.c

vs:

$ gcc -Wall foo.c
foo.c: In function 'main':
foo.c:3: warning: implicit declaration of function 'strcmp'

-Beej

On Jun 10, 8:43=A0pm, Kuhl <chen_zhi...@yahoo.com> wrote:

> Hi, strcmp works. But there's a further question. I did not include
> any file in my code, however strcmp still works. There's no definition
> of strcmp in my code. So is strcmp a key word that can be recognized
> by gcc directly? Thanks.

It doesn't work. It should generate an error, but it doesn't. So you
broke the rules, and things didn't work. No big surprise there.

DS

On 11 June, 13:16, David Schwartz <dav...@webmaster.com> wrote:
> On Jun 10, 8:43 pm, Kuhl <chen_zhi...@yahoo.com> wrote:
>
> > Hi, strcmp works. But there's a further question. I did not include
> > any file in my code, however strcmp still works. There's no definition
> > of strcmp in my code. So is strcmp a key word that can be recognized
> > by gcc directly? Thanks.
>
> It doesn't work. It should generate an error, but it doesn't.

What do you mean should , like some standard says that it
should ?

> So you
> broke the rules, and things didn't work. No big surprise there.

But he said that things did work.

--
Who's your mama?