char str[]="S\065AB";
printf("\n%d", sizeof(str));

explain the output.
i am getting 5.

this is what it means fifth is null terminator 065 in oct is '5' check ascii table

#include <stdio.h>

int main()
{
   char str[]="S\065AB";
   char strSame[5] = { 'S','\065', 'A', 'B', '\0'};

   printf("str 1 %d \n", sizeof(str));
   printf("str 2 %d\n", sizeof(strSame));
   printf("same ? %d [%s] [%s]",strcmp(str, strSame),str, strSame);   

   return 0;
}
commented: nice :) +14

strlen find the exact length

use strlen

Maybe, but maybe not. Depends on what you want, strlen() and sizeof() may return different values such as when the buffer contains embedded '\0' characters.

strlen and sizeof should always return different values because strlen never includes the trailing '\0'. In a compleatly normal string such as "Hello World" with no embedded terminators strlen will return a value 1 less than sizeof returns.

It they don't return different values you have a buffer overrun in progress.