function prototype 					returns 

int isalnum(int c) returns true when c is alphabetic or a digit
int isalpha(int c) returns true when c is alphabetic (upper or lower case) 
int iscntrl(int c) returns true when c is a control character 
int isdigit(int c) returns true when c is a digit character '0'-'9'
int islower(int c) returns true when c is a lowercase letter
int isprint(int c) returns true when c is printable character including space 
int ispunct(int c) returns true when c is a punctuation (printable, not space, not alnum)
int isspace(int c) returns true when c is any white-space character
int isupper(int c) returns true when c is an uppercase letter
int tolower(int c) returns lowercase equivalent of c if!isupper(c) then returns c unchanged
int toupper(int c) returns uppercase equivalent of c if!islower(c) then returns c unchanged 

Table 8.2: some functions in <ctype.h>