在C中查找字符串长度时出现问题

Question

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. not 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?

Answer 1

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

Answer 2

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.

Answer 3

Use the following:

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

Answer 4

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...