cppreference.com > Standard C String and Character > isalnum isalnum Syntax:
      #include <ctype.h>
      int isalnum( int ch );
The function isalnum() returns non-zero if its argument is a numeric digit or a letter of the alphabet. Otherwise, zero is returned.
       char c;
       scanf( "%c", &c );
       if( isalnum(c) )
         printf( "You entered the alphanumeric character %c\n", c );
Related topics: isalpha iscntrl isdigit isgraph isprint ispunct isspace isxdigit