isatty - test whether a file descriptor refers to a terminal
int isatty(int fd);
The isatty() function tests whether fd is an open file descriptor
referring to a terminal.
isatty() returns 1 if fd is an open file descriptor referring to a
terminal; otherwise 0 is returned, and errno is set to indicate the
EBADF fd is not a valid file descriptor.
EINVAL fd refers to a file other than a terminal. POSIX.1-2001
specifies the error ENOTTY for this case.
SVr4, 4.3BSD, POSIX.1-2001.
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