tolower —
upper case to lower case letter conversion
Standard C Library (libc, -lc)
#include <ctype.h>
int
  
  tolower(int
    c);
The tolower() function converts an upper-case letter to
  the corresponding lower-case letter.
If the argument is an upper-case letter, the tolower()
  function returns the corresponding lower-case letter if there is one;
  otherwise the argument is returned unchanged.
ctype(3),
  isalnum(3),
  isalpha(3),
  isascii(3),
  iscntrl(3),
  isdigit(3),
  isgraph(3),
  islower(3),
  isprint(3),
  ispunct(3),
  isspace(3),
  isupper(3),
  isxdigit(3),
  stdio(3),
  toascii(3),
  toupper(3),
  ascii(7)
The tolower() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).
The argument to tolower() must be
  EOF or representable as an unsigned
  char; otherwise, the behavior is undefined. See the
  CAVEATS section of
  ctype(3) for more details.