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