Finding length of string using pointers

Question

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char *str=malloc(sizeof(char)*100);
    int length=0;
    printf("Enter string :\n");
    scanf("%c",str);
    while(*str)
    {
        length++;
        *str++;
    }
    printf("%d",length);
    return 0;
}

I'm trying to write a program to find length of string using pointers.But whatever the string, I'm getting the result as 1. Can somebody tell me what's wrong?

@federico-klez-culloca I think that's fine. `++` has higher precedence than `*` — adrtam, Feb 28 '19 at 18:14
Welcome to the wonderful world of [*undefined behavior*](https://en.wikipedia.org/wiki/Undefined_behavior). — Some programmer dude, Feb 28 '19 at 18:15
@AdrianTam Actually, that will increase the pointer and that is what leads to the UB. So, maybe not so fine. — Some programmer dude, Feb 28 '19 at 18:15
If you want to read a *line* of input, then use [`fgets`](https://en.cppreference.com/w/c/io/fgets). Besides solving your problem, it will also solve other *potential* problems. — Some programmer dude, Feb 28 '19 at 18:21
Finally, don't do the counting yourself. Use [`strlen`](https://en.cppreference.com/w/c/string/byte/strlen). — Some programmer dude, Feb 28 '19 at 18:22
Why does it lead to UB? aren't we incrementing the pointer to find the null terminator? — Combospirit, Feb 28 '19 at 18:23
@FedericoklezCulloca I would say that you don't *need* to dereference while incrementing in this case. It's an additional operation when the desired outcome is just to increment the pointer, but a decent compiler should optimize the dereference right out. — Govind Parmar, Feb 28 '19 at 18:38
With your current code there might not *be* a null-terminator. So the incrementing of the pointer will make you use uninitialized parts of the memory. And the contents of that memory is *indeterminate* (and should be seen as almost random or "garbage"). That could lead you to go out of bounds. — Some programmer dude, Feb 28 '19 at 18:39
@GovindParmar ok, but it obfuscates the meaning of the code, at best. — Federico klez Culloca, Feb 28 '19 at 18:41
@Someprogrammerdude so how does dereferencing it help in case null terminator is not present?Will it stop at the ubound of the array? — Combospirit, Feb 28 '19 at 18:45

score 2 · Accepted Answer · answered Feb 28 '19 at 18:33

You allocate 100 bytes ok

char *str=malloc(sizeof(char)*100);

int length=0;
printf("Enter string :\n");

You have a string but read one character

scanf("%c",str);

While that character is != 0 You increment the character with one e.g. 'A' becomes 'B' and so on the character overflows

while(*str)
{
    length++;
    *str++;

Instead, read a string using fgets()

const int maxlen = 100;
char *str=malloc(maxlen); 

if (fgets(str,maxlen,stdin) != NULL)
{
  // now to calculate the length
  int length = 0;
  char* p = str;  // use a temp ptr so you can free str 
  while (*p++) 
  { 
    ++length; 
  }
  printf("length=%d", length);
  free(str); // to avoid memory leak
}

Pim · Answer 2 · 2019-02-28T18:23:10.587

The %c modifier in scanf reads character sequences. As you did not provide a field width, it reads by default only one character per time. You might want to use the %s modifier.

Further, when no length modifier is added, the returned character sequence is not null terminated, which makes your loop to determine the length risky (you also might want to use the strlen function from the C standard library, but this function also expects a null terminated sequence).

score 0 · Answer 3 · answered Feb 28 '19 at 18:18

0

Problem is your scanf.

char *str=(char*)malloc(sizeof(char)*100);

printf("Enter string :\n");
scanf("%s",str);
int i = 0;
for (i = 0; i < 100 && str[i] != '\0'; i ++)
{
}
printf("%d",i);

answered Feb 28 '19 at 18:18

chris01

10,921
9
54
93

[Don't cast the result of `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). And add a length-modifier to the format so the buffer won't be overrun. And always check what `scanf` returns. – Some programmer dude Feb 28 '19 at 18:19
If I do not cast the malloc some compiles would give me an error. I tried with GCC 8.2. I did not try to optimize OPs code, just make it run. But you are generally right - C and memory needs some attention to be safe. – chris01 Feb 28 '19 at 18:33
1

Then you're using C++ and not C. – Some programmer dude Feb 28 '19 at 18:38

Finding length of string using pointers

3 Answers3