isalpha —
alphabetic character test
Standard C Library (libc, -lc)
#include <ctype.h>
int
  
  isalpha(int
    c);
The isalpha() function returns zero if the character
  tests false and returns non-zero if the character tests true.
ctype(3),
  isalnum(3),
  isascii(3),
  isblank(3),
  iscntrl(3),
  isdigit(3),
  isgraph(3),
  islower(3),
  isprint(3),
  ispunct(3),
  isspace(3),
  isupper(3),
  isxdigit(3),
  stdio(3),
  toascii(3),
  tolower(3),
  toupper(3),
  ascii(7)
The isalpha() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).
The argument to isalpha() must be
  EOF or representable as an unsigned
  char; otherwise, the behavior is undefined. See the
  CAVEATS section of
  ctype(3) for more details.