strcat, strncat, strcmp, strncmp, strcpy, strncpy, strlen, index − string operations

These functions operate on null-terminated strings.  They do not check for overflow of any receiving string.  appends a copy of string to the end of string copies at most characters.  Both return a pointer to the null-terminated result.  compares its arguments and returns an integer greater than, equal to, or less than 0, according as is lexicographically greater than, equal to, or less than makes the same comparison but looks at at most characters.  copies string to stopping after the null character has been moved.  copies exactly characters, truncating or null-padding the target may not be null-terminated if the length of is or more.  Both return returns the number of non-null characters in returns a pointer to the first  occurrence of character in string or zero if does not occur in  the string.