Doubt in structure

Consider the below program

Code:

struct str { char *p; int i; }t; main() { scanf("%s %d",t.p,&t.i); printf("%s",t.p); }

the output is : Runtimer Error

The reason is given that as we are trying to store the string at the address to which str(of t) points to. Since the pointer is not initialized it holds garbage value and so the reslult will be undefined. Hence the error.

Then what about this

Code:

void main() { char *p; scanf("%s",p); printf("%s",p); }

Here, though the pointer variable p is not initialized, it is accepting the input from the user and printing it without showing any error..

May I know the reason in detail??

That is undefined behaviour. It may work sometimes, it may not other times. I wouldn't depend it. Always allocate space for your variables!

The first one is probably trying to write to NULL, since it's a global.
You got lucky in the second one. In that one it might not be NULL, and might point to somewhere in a valid memory page.

Main must return int, not void.

Having a guess at this, in the first case *p is declared globally which means it will be initilaised to 0000, pointing to the first address in memory I think, possibly what is called a 'Null pointer', depends how it is defined internally, but I expect it is 0000.
That is bound to generate a runtime error I think, segmentation violation??

In the second case the pointer is declared as a local variable on the stack, which will not be automatically set to zero, it will be some pretty random value, depending, I think on what was previously on the stack.
So.. either it just happens to write to a 'safe area' or what was previously on the stack was a pointer which pointed to a safe area.

If you compiler it to produce the assembler code you might be able to work out what was on the stack, and where it was writing.

Incidently when I run the second example on my machine the program crashed. :)

I compiled it as well, it segfaulted, which is what I expected.

You need to include stdio.h
You need to change your main function to return int.
You need to allocate room in *p.

In the above structure code, in scanf statement are we trying to store the sting in the pointer or at the address pointed by the pointer.....

If we declare

Code:

char *p; p = 12; *p = 20;

it means that we are storing 12 in p at 2nd stmt and 20 is being stored at address pointed by the p i.e at address 12.

So, I think in the scanf statement when we are trying to store the string in t.p, the address of the string will be stored in t.p... right?.....

If not please tell me the correct one...in detail...please

I included stdio.h, but I didn't return int, none of my other progs reurn int and work fine.
I changed it to return int and it still fails, it didn't say segfault, just
"prog.exe has encountered a problem and needs to close".

Anyway I compiled it to produce the assembler code, so I am sure you can see
exactly what the problem is now <cough> :)

Code:

.file "null.c" .def ___main; .scl 2; .type 32; .endef .section .rdata,"dr" LC0: .ascii "%s\0" .text .globl _main .def _main; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp, %ebp subl $24, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax movl %eax, -8(%ebp) movl -8(%ebp), %eax call __alloca call ___main movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _scanf movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf leave ret .def _printf; .scl 2; .type 32; .endef .def _scanf; .scl 2; .type 32; .endef

Just to make it easier I compiler a version which works

Code:

#include <stdio.h> int main() { char p[100]; scanf("%s",p); printf("%s",p); }

Assembler

Code:

.file "null.c" .def ___main; .scl 2; .type 32; .endef .section .rdata,"dr" LC0: .ascii "%s\0" .text .globl _main .def _main; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp, %ebp subl $152, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax movl %eax, -124(%ebp) movl -124(%ebp), %eax call __alloca call ___main leal -120(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _scanf leal -120(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf leave ret .def _printf; .scl 2; .type 32; .endef .def _scanf; .scl 2; .type 32; .endef

In the above structure code, in scanf statement are we trying to store the sting in the pointer or at the address pointed by the pointer.....

If we declare

Code:

char *p; p = 12; *p = 20;

it means that we are storing 12 in p at 2nd stmt and 20 is being stored at address pointed by the p i.e at address 12.

So, I think in the scanf statement when we are trying to store the string in t.p, the address of the string will be stored in t.p... right?.....

If not please tell me the correct one...in detail...please

Incidentl I am not sure how the numbers add up, looking at how the code has changed

Code:

subl $152, %esp subl $24, %esp# and movl %eax, -8(%ebp) movl %eax, -124(%ebp)

I would expect to see a difference of 100 somewhere, of course 100 in hex is 64 so
possibly 2 time 64 = 128??

Note 128
plus 24
is = 152 :)
so I am not sure why it is twice 100, I guess it is down to word sizeor something like that.

Quote:

Originally Posted by karthik537

In the above structure code, in scanf statement are we trying to store the sting in the pointer or at the address pointed by the pointer.....

If we declare

Code:

char *p; p = 12; *p = 20;

it means that we are storing 12 in p at 2nd stmt and 20 is being stored at address pointed by the p i.e at address 12.

So, I think in the scanf statement when we are trying to store the string in t.p, the address of the string will be stored in t.p... right?.....

If not please tell me the correct one...in detail...please

I would say in the first p is pointing to 12, which would be a constant(?).
In the second case the pointer is being set to the memory address 12, so whatever
is at memory address 12 will be p, probably that wil be some fairly random data which
will be treated as character codes if you try to print p. More likely the addess will be
invalid (illegal value, out of bounds) and the program will produce a fault of some sort.

Quote:

Originally Posted by esbo

Just to make it easier I compiler a version which works

Code:

#include <stdio.h> int main() { char p[100]; scanf("%s",p); printf("%s",p); }

It won't work for strings over 99 non-whitespace characters long. Change it to:

Code:

#include <stdio.h> int main() { char p[100]; scanf("%99s",p); printf("%s",p); }

It's designed to crash if the string is too long ;)

Quote:

Originally Posted by esbo

...in the first case *p is declared globally...

Quote:

Originally Posted by carrotcake1029

You need to allocate room in *p.

Whaddareya talkin' about? There is no *p declared anywhere, just p ;)
(Actually, that's true to some degree.)

Quote:

Originally Posted by esbo

It's designed to crash if the string is too long ;)

No no, esbo that is not true. It's designed to create a security flaw and undefined behavior if it's too long. Perhaps even format the entire hard drive.
Scanf woes