how to fix a Runtime error in a char array

Question

I am currently working on a program and have run into a runtime error, ironically I cannot seem ti find the source of it at all, I have tried playing around with the char[] size, but since the problem's condition specifies the limit should be 10^5 there is not much I can do there.

Here is the code:

#include<stdio.h>
#include<string.h>


int main() {

char stations[100003], seq1[103], seq2[103];
int f = 0, b = 0, a = 0;
scanf("%s", stations);
scanf("%s", seq1);
scanf("%s", seq2);

if (strlen(stations) < strlen(seq1) + strlen(seq2))
printf("fantasy");
else {
if (strstr(strstr(stations, seq1), seq2) != 0) {
  f = 1;
}
if (strstr(strstr(strrev(stations), seq1), seq2) != 0) {
  b = 1;
}
if (f == 1 && b == 1) {
  a = 1;
}

if (a == 1)
  printf("both");
else if (f == 1)
  printf("forward");
else if (b == 1)
  printf("backward");
else
  printf("fantasy");
}
return 0;
}

and here is the compiler output:

 exit code: -1073741819 (STATUS_ACCESS_VIOLATION), checker exit code: 0, verdict: RUNTIME_ERROR

Any help is very much appreciated!

Answer 1

These if statements

if (strstr(strstr(stations, seq1), seq2) != 0) {
  f = 1;
}
if (strstr(strstr(strrev(stations), seq1), seq2) != 0) {
  b = 1;
}

are unsafe. The function strstr can return a null pointer. In this case the outer call of strstr invokes undefined behavior because a null pointer is passed as an argument.

You need to write something like

const char *p;

if ( ( p = strstr(stations, seq1) ) && strstr( p, seq2) ) {
  f = 1;
}
if ( ( p = strstr(strrev(stations), seq1) ) && strstr( p, seq2) )  {
  b = 1;
}

Or like

const char *p;

if ( ( p = strstr(stations, seq1) ) && strstr( p + strlen( seq1 ), seq2) ) {
  f = 1;
}
if ( ( p = strstr(strrev(stations), seq1) ) && strstr( p + strlen( seq1 ), seq2) )  {
  b = 1;
}

Also the array stations could be declared with the storage class specifier static

static char stations[100003];

And it will be more safer to write

scanf("%100002s", stations);
scanf("%102s", seq1);
scanf("%102s", seq2);