CODEWITHSUNDEEP
c
station
station

Reputation: 7145

C pointer problem!

Why does the following C code does not ouput 10

#include<stdio.h>
#include<stdlib.h>
void get_length(char s[]);
int main( ) 
{
char buff[128];
printf("\nEnter buffer data: ");
scanf("%s",buff);
get_length(buff);
}

void get_length(char *s){
    int count;  
for(count = 0 ; *s != '\0' ; s++)
    count++;
printf("count = %d\n",count);
}

The input is I am Rohit
The output is
count = 432
Can anyone please explain that.

Upvotes: 1

Views: 169

Answers (4)

Judge Maygarden
Judge Maygarden

Reputation: 27583

The scanf call is failing because you have used an invalid format. If you want a string of length 5, then the format should be "%5s" instead of "5s", or just "%s" if the five is a complete typo. So, the character buffer is uninitialized and you are getting the number of bytes before the first random '\n' character is encountered on the stack. Also, scanf ignores newline characters. So, your get_length function will never work as expected.

Upvotes: 1

Gareth McCaughan
Gareth McCaughan

Reputation: 19971

You appear to have a 5 instead of a % in your scanf format string. (Perhaps your keyboard's shift key needs cleaning.)

That means that scanf isn't actually reading anything into your buffer. Your code doesn't check the return value from scanf (it should!) and therefore never notices that it failed. So you're then trying to calculate the length of ... a buffer full of uninitialized junk. It could give you 432, or 0, or anything else at all.

[EDITED to add:] ... Oh. And even if you fix that, it still won't work because scanf will stop reading when it sees a whitespace character, including a newline, and therefore the \n your get_length function is looking for will not be in the buffer.

Incidentally, do you know about the strlen function in the C standard library?

Upvotes: 5

Aaron H.
Aaron H.

Reputation: 6587

I think that scanf is reading your string in and setting the last char to '\0', but you are searching for '\n', which I don't think is implicit in the scanf implementation.

That is, check for '\0' instead of '\n'.

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

I think you meant %s, not 5s.

BTW, value-initialising your array (char buff[128] = {}) will prevent your array from being unbounded (count = 432 is arbitrary for you).

You should then check for *s != '\0', not \n, in get_length — C-style strings are bounded by the NULL character (\0), not newlines, regardless that you took the data from user input. The newline is lost from that input.

Upvotes: 3

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