strcspn( ) function: find the occurrence of one of a group of characters.
#include <stdio.h>
#include <string.h>
char s1[35]; int answer;
int main( )
{
strcpy(s1,"We are looking for great strings." );
answer=strcspn(s1,"abc");
printf("The first a,b,c appeared at position %d\n",answer+1); return (0);
}