Thread: How i will get this output

Dear Allo,
Recently i was attended one interview i got one question like this

Code:

#include<stdio.h>
#define MAX_STR1_LEN 15
#define MAX_STR2_LEN 16
void main()
{
    char a,b,c,d,e,f,g,h,j,k,l,m,n,o,count;
    char i=4;
    char str2[MAX_STR2_LEN]="0123456789abcdef";
    chat str1[MAX_STR1_LEN]="0123456789abcde";
    str2[MAX_STR2_LEN]=0;
    str1[MAX_STR1_LEN]=0;
    printf("1.str1:%s  str2:%s\n",str1,str2);
    strcpy(str1,str2);
   printf("2.str1:%s str2:%s\n",str1,str2);
   for(count=0;count<i;count++);
   str2[MAX_STR2_LEN]='a';
   str1[MAX_STR1_LEN]=0;
   strcpy(str1,str2);
   printf("3. str1:%s str2:%s\n", str1,str2);
   for(; count<i;count++);
   printf("4. count:%d\n",count);
   return;
}

The o/p i got in my compiler

Code:

1. str1:0123456789abcde str2:0123456789abcdef
2. str1:0123456789abcdef str2:
3. str1:    str2:
4. count:97

can anybody explain step by step procedure how i got this output... what is the concept beyond this program..........I am advance thankful to you

I don't see any question? What question did they give you?
Unless that's for an embedded system, don't use void main(), see the FAQ.
Try and indent it... and put a few line breaks in.horrid.

Code:

    char str2[MAX_STR1_LEN]="0123456789abcdef";
    chat str1[MAX_STR2_LEN]="0123456789abcde";
    str2[MAX_STR2_LEN]=0;
    str1[MAX_STR1_LEN]=0;

declares and initializes str2 and str1, then sets the last character to NUL, ie NUL to NUL (pointless).

prints them, blah blah blah - rest is straight forward (and somewhat error some, use of strcpy() etc).

Other than that, it's full of pointlessness.

Code:

for(count=0;count<i;count++);

Is just moronic, why not count = i; ? Duh.

As for the output, count should not be 97, Really it's just:

Code:

i = 4;
count = i;    /* 4 */

Therefore count is 4, as

Code:

for(; count<i;count++);

is the same as,

Code:

for(; 4 < 4;);

And last time I checked, 4 was not less than 4.

PS: try not to put your whole post in code tags :s

eh? What compiler.
And how the hell did you get count = 97?

Stripped down:

Code:

#include <stdio.h>

int main(void)
{
    int count, i = 4;
    
    for(count = 0; count < i; count++);
    printf("Count = &#37;d\n", count);
    
    for(; count < i; count++);
    printf("Count = %d\n", count);
    return 0;
}

Which is

Code:

Count = 4
Count = 4

As I said before.

Either:
* you're making it up
* you're compiler is old skool
* you're stepping past the end of an array and setting it's value. (Possibly from your bad use of strcpy(), str1 is not big enough to hold str2)

Code:

str2[MAX_STR2_LEN]='a';

strcpy(str1,str2);

Will remove the NUL terminator from str2, which is bad considering you then use strcpy() (Which will read from str2 until the NUL character) and copy it into str1, and this is probably one place where you set the value of count mistakingly, and you're probably unlucky enough to get a segfault.

If this was the actual question, it's moronic...

97 = 'a'
That's because the whole code is riddled with buffer overflows.

> str2[MAX_STR2_LEN]=0;
All these operations are out of bound memory accesses, so I guess at some point, the counter variable got trashed with some of the string manipulation.

Originally Posted by Salem

so I guess at some point, the counter variable got trashed with some of the string manipulation.

Code:

str2[MAX_STR2_LEN]='a';

strcpy(str1,str2);

Like there

He already has.

Exactly what skills are you looking to test?

The only skill you need here is the ability to recognise a buffer overflow when you see one (eg, a variable contains an unexpected value).

> could you now explain me how i will achieve this output?
What good will that do?
At the point where the code became undefined (in this case, it was with the 'void main'), exactly what the code does is just a matter of chance.

If you want to argue that
str2[MAX_STR2_LEN]='a';
is really
i = 'a';
because they're declared next to each other, then that's fine by me. There are several 'a' characters to begin with, maybe another one is responsible, or maybe it's none of them and it gets there some other way.

But that 'explanation' is valid ONLY for this program, with the current compiler, with the current compiler options.

But the only way to be sure would be to manually inspect the generated machine code.

Change anything at all, say the lengths of the arrays, remove some unused variables, change the options, change the compiler, change blah blah blah and you're likely to be scrabbling around for another explanation.

The problem is, whatever set of 'rules' you can come up with to explain such things, it will always be a subset. Sooner or later, a new bit of undefined code will force you to think again. Then you change the compiler, or patch your current one, or change the flags and you have to start over.