strstr not working with needles shorter than 3 char - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* findSequence(char s[], char ch, int n){
int i;
char*ptr;
char needle[n];
char*npdr=&needle;
for(i=0;i<n;i++){needle[i]=ch;}
ptr = strstr(s,ndpr);
printf("%s",ptr);
return ptr;
}
int main()
{
char stringa[]={"ciccciopasticcio"};
char carattere='c';
char*ptr;
int n=3;
ptr=findSequence(stringa, carattere,n);
return 0;
}
This quick code, should search for a matching between a string and a needle of non set lenght, it works just fine with any n>=3 the problem is with 1 and 2 as n values.
watching the debug i noticed that the pointer npdr adds a second and third value to the sequence on his own example: n=2 needle="cc" npdr=address of needle[0] "cc#"
Do you have any ideas of why this is happening?

Answer 1

The standard function strstr expects two pointers to strings. But you are calling the function passing an array as the second argument that does not contain a string - a sequence of characters terminated by the zero character '\0'
char needle[n];
char*npdr=&needle;
for(i=0;i<n;i++){needle[i]=ch;}
ptr = strstr(s,ndpr);
So the code invokes undefined behavior.
Also this initialization
char*npdr=&needle;
is incorrect. The initialized pointer has the type char * while the initializing expression has the type char ( * )[n]. At least you need to write
char*npdr=needle;
You could declare the array like
char needle[n + 1];
and
needle[n] = '\0';
Pay attention to that the function should not output any message. It is the caller of the function will decide whether to output a message. Moreover this call of printf
printf("%s",ptr);
will invoke undefined behavior if the pointer ptr is equal to NULL.
The function itself should be declared the following way
char* findSequence(const char s[], char ch, size_t n);
That is the first parameter should be declared with the qualifier const because passed strings are not changed within the function. And the second parameter should have unsigned integer type size_t instead of the signed integer type int.
Also using a variable length array within the function makes the function unsafe.
The function can be defined using standard C string function strchr instead of the function strstr.
Here is a demonstration program.
#include <stdio.h>
#include <string.h>
char * findSequence( const char s[], char ch, size_t n )
{
const char *p = NULL;
if (n != 0 && ch != '\0')
{
size_t count = 0;
do
{
count = 0;
p = strchr( s, ch );
if (p)
{
s = p;
do
{
++count;
} while (*++s == ch);
}
} while (p != NULL && count != n);
}
return ( char * )p;
}
int main( void )
{
char stringa[] = { "ciccciopasticcio" };
char carattere = 'c';
size_t n = 3;
char *ptr = findSequence( stringa, carattere, n );
if (ptr != NULL)
{
puts( ptr );
}
}
The program output is
ccciopasticcio