Problem finding the string length in C

Here is the code

#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
  clrscr();
  char a[20],rev[20];
  printf("enter the string");
  scanf("%s",a);
  int len=strlen(a);
  for(int i=0;i<len;i++)
  {
    rev[i]+=a[len-i-1];
  }
  printf("%d \t  \n string is \t %s",len,rev);
  getch();
}

It was correctly working when we gave it a string without spaces:

input: welcome

len:7

output: emoclew

When we give it a string with a space:

input : welcome to this world

len:7

output:some other ascii chars that I have not seen so far. and the "len" is again 7 only

When I change the following statement:

scanf("%s",a) to gets(a);

I get:

input :welcome to this world

len:21

output : something different. n开发者_开发知识库ot the reverse of string...

In this case "len" is correct but the output is wrong.

What is really happening? What is the problem with the above code?

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scanf will not read the entire line. Instead it'll read up to the first space... You need getline

Also, I notice you have things with length more than 19 but you allocated space with for 20 chars. Increase that or you get UB

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In addition to what others have said, rev is not guaranteed to be initialized to NUL characters, so your rev[i]+=a[len-i-1]; line can end up with garbage.

---

Use the following:

scanf("%[^\t\n]",string);

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To comments on it:

1. As Armen Tsirunyan wrote, you probably want getline
2. You only have a character array of size 20 - so passing a bigger string will occur in a buffer overflow (which makes your program very insecure - google for buffer overflow attack if you want to know more about it) - this is why you should make sure that you never read more than the size of your character array...