#include <stdio.h>

char *strings[] = {
	"abcd",
	"qwerty",
	"xyz",
	"abcdef",
	"whatever",
	NULL,
};

int
common(char *a, char *b)
{
	int n = 0;

	while (*a++ == *b++)
		n++;

	return n;
}

int
main()
{
	char **s, **p;
	char *a = NULL, *b = NULL;
	int n = 0;

	for (s = strings; *s; s++) {
		for (p = s+1; *p; p++) {
			int c = common(*s, *p);
			if (c > n) {
				n = c;
				a = *s;
				b = *p;
			}
		}
	}

	if (a && b)
		printf("most common: %s %s\n", a, b);

	return 0;
}

most common: abcd abcdef

r=0
while(1){
  curch=listofstrings[0][r]
  if(!curch)return r; 
  for s in listofstrings:
     if(s[r]!=curch)"
         return r

 r=r+1;
}    

$man str\Tab\Tab
strace       strdup       strings      strncpy      strspn       strtonum
strcasecmp   strdupa      stringsize   strndup      strstr       strtoq
strcasestr   stream       strip        strndupa     strtod       strtoul
strcat       stream.im6   strlcat      strnlen      strtof       strtoull
strchr       strerror     strlcpy      strnstr      strtoimax    strtoumax
strchrnul    strerror_r   strlen       strpbrk      strtok       strtouq
strcmp       strfmon      strmode      strptime     strtok_r     strverscmp
strcoll      strfry       strncasecmp  strrchr      strtol       strxfrm
strcpy       strftime     strncat      strsep       strtold      
strcspn      string       strncmp      strsignal    strtoll