here is some problem in structures

[ashu]

lets take a look at the following code:-


#include<stdio.h>
#include<conio.h>
struct tag{

int age;
char *name;
}a;
void main()
{
clrscr();
puts("enter name ");
scanf("%s",a.name);
fflush(stdin);
puts("enter");
scanf("%d",&a.age);
printf("%s\t%d",a.name,a.age);
getch();
}

its look ok to me, but as i run this, i face some problem, i m not able
to assign value to name
but as i declare a structure's instance i.e, "a" within the main
program, i m able to assign a value in a.name.
why this is so,
plz help me i m very confused

 

[Chris Dollin]

lets take a look at the following code:-


#include<stdio.h>
#include<conio.h>

Nonstandard header. Avoid.

struct tag{

int age;
char *name;
}a;

`a.name` will be initialised with a null pointer.

void main()

Nonstandard declaration for main. All bets are off. Avoid. `main`
should return `int`.

{
clrscr();

Nonstandard function. Also pointless. Avoid.

puts("enter name ");
scanf("%s",a.name);

Attempt to store characters through a null pointer. All bets are off.
If you're lucky, you'll get some kind of crash at this point. Also
scanf is NOT a wise choice for reading user input.

fflush(stdin);

fflush takes an /output/ stream argument. Undefined behaviour. Avoid.
If you want to eat characters until end of line, eat characters.

puts("enter");
scanf("%d",&a.age);

scanf is NOT a wise choice for reading user input. Consider what might
happen if the user responds "some cake", "deadbeef", "12345678987654321".

printf("%s\t%d",a.name,a.age);

No newline output, so result may not appear.

getch();

Non-standard function. Avoid.

}

its look ok to me, but as i run this, i face some problem, i m not able
to assign value to name
but as i declare a structure's instance i.e, "a" within the main
program, i m able to assign a value in a.name.

If `a` is declared within `main`, then `a.name` will have some random
undefined value and, if you're unlucky, may point somewhere so the
machine doesn't catch it and crash.

why this is so,
plz help me i m very confused

It's a fine concoction of undefined behaviours you have there. I'd
advice picking up a decent C book and starting through it or, if the
above commentary gives you the willies, try a less spikey language.

 

[Mike Wahler]

lets take a look at the following code:-


#include<stdio.h>
#include<conio.h>

This is not a standard header. Please omit such
from code posted here. It's not germane to your
question anyway.

struct tag{

int age;
char *name;
}a;

Note that 'a.name' is a pointer. It can store
an address. But you've not assigned it the value
of any address. If you do, it must be the address
of memory which is part of your program.

void main()

int main(void)

{
clrscr();

This is a nonstandard function. Please omit such from
code posted here. It's not germane to your question
anyway.

puts("enter name ");
scanf("%s",a.name);

This gives undefined behavior. 'scanf()' will try
to store data at the address stored in 'a.name'.
But 'a.name' does not contain the address of memory
which is part of your program.

fflush(stdin);

This is more undefined behavior. 'fflush()' behavior
is only defined for output and update streams, not
input streams.

puts("enter");
scanf("%d",&a.age);

This is "OK" (other than that you didn't check for
errors), since 'a.age' does provide storage for a
type 'int' object.

printf("%s\t%d",a.name,a.age);

More undefined behavior. You've passed an invalid pointer
('a.name') to 'printf()'.

getch();

This is a nonstandard function. Please omit such from
code posted here. It's not germane to your question
anyway.

}

its look ok to me,

It is *not* OK. It is riddled with errors.

but as i run this, i face some problem,

The problem is undefined behavior, which can range from
'seems to work', to a crash, or anything in between.

i m not able
to assign value to name

A value can be assigned to 'name'. But 'name' is a *pointer*.
The only valid values which can be assigned are addresses of
type 'char' objects. But that's not what your code does.

but as i declare a structure's instance i.e, "a" within the main
program, i m able to assign a value in a.name.

Yes, a type 'char*' value. That's not what you wrote, though.

why this is so,
plz help me i m very confused

You need to provide storage for your 'name' pointer to point to.
You have a couple options:

Change your pointer to an array:

#define SOME_SIZE 100 /* you decide what this value should be */
struct tag
{
int age;
char name[SOME_SIZE];
} a;

(this will fix at compile time the maximum number of
characters that 'name' can store).

or:

#include <stdlib.h>

struct tag
{
int age;
char *name;
} a;

size_t some_size = 100; /* Again, this is my arbitrary value. */
/* You may want to calculate this value */
int main(void)
{

a.name = malloc(some_size);
if(a.name)
{
/* store characters at address 'a.name' */
}

free(a.name);
return 0;
}

(this will allow you to determine at run-time how
many characters to allocate for your data).


In both cases above, if you're wanting to store a
string, be sure to allow an extra byte for the
string terminator.

Finally:

Repeat to yourself until it sinks in:
"A pointer is not an array. An array is not a pointer."

Which C book(s) are you reading?

-Mike

 

[Vladimir S. Oka]

lets take a look at the following code:-

Let's...

First observation: don't use TABs, use spaces; while you're at it, feel
free to add some horizontal whitespace -- it makes reading /much/
easier.

#include<stdio.h>

#include<conio.h>

Non standard header. You don't even need it, as clrscr( ), and getch()
I happen to know it provides, are not essential to your code.

struct tag{

int age;
char *name;
}a;

So far, so good...

void main()

Standard C allows only:

int main(void)

or

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

{
clrscr();

Non-standard function, and not required.

puts("enter name ");
scanf("%s",a.name);

Now, here's where the problem lies (at least the one that you're
complaining about -- others abound). Where have you allocated the space
to hold your string. In the struct, `name` is just a pointer to a
`char`. This is usually expected to be a number of `char`s holding a
string, and terminated by a 0 character, but you need to allocate space
for it first.

fflush(stdin);

Behaviour of fflush() for `stdin` is undefined. It may work as you
expected, but it may also flush your toilet for you every second
Thursday.

puts("enter");
scanf("%d",&a.age);
printf("%s\t%d",a.name,a.age);

You just love TABs, don't you. There's nothing wrong with them per se,
but can you be sure how big they are on any given system your program
may happen to run on?

getch();

Another non-standard function.

}

its look ok to me, but as i run this, i face some problem, i m not able
to assign value to name
but as i declare a structure's instance i.e, "a" within the main
program, i m able to assign a value in a.name.

Now, this is not true. You can assign a value to `a.name`, it's just
not the value you'd like. Although your sentence is rather confusing, I
suspect you tried:

a.name = "a";

and it worked fine. That would be because the compiler creates a
(nameless) string literal "a", and then assigns its /address/ to
`a.name`.

Go back to your textbook and read more about pointers, and C strings.

why this is so,
plz help me i m very confused

That, you most certainly are...

PS
I must have omitted to point at at least some errors, and probably
wasn't precise enough in all instances, but others may feel like
filling in the gaps...

 

[pablo reda]

Hey ashu, try this...

struct tag{
int age;
char name[40]
}a;

 

[Keith Thompson]

lets take a look at the following code:-


#include<stdio.h>
#include<conio.h>

Nonstandard header. Avoid.
[snip]

clrscr();

Nonstandard function. Also pointless. Avoid.
[snip]

getch();

Non-standard function. Avoid.

I would place a slightly different emphasis on that. Non-standard
headers and functions should be avoided *unless* there's a good reason
to use them. They should also be avoided in code posted to this
newsgroup.

There's nothing wrong with using non-standard features *when it's
necessary*.

In this case, though, it's really not necessary. There's no standard
and portable way to clear the screen (there may not be a screen), but
there's no need for this program to clear the screen at all. Programs
that clear my screen unnecessarily are seriously annoying; that's *my*
information being erased. Likewise, if you need to wait for user
input before terminating the program, a call to the standard getchar()
should work as well as the non-standard getch(). I think getch() will
return without waiting for a newline, but that's not a good enough
reason to use it in this case.

 

[Hemant Mohan]

you have not allocated any memeoy to name

 

[Hemant Mohan]

You have not allocated any memory to name. This code should give you a
crash.
Ofcourse you need to avoid usage of non-standard headers and functions.

 

[Nelu]

You have not allocated any memory to name. This code should give you a
crash.
Ofcourse you need to avoid usage of non-standard headers and functions.

Please read http://cfaj.freeshell.org/google/
before posting again.

Thank you very much!