Explain the usage of ttyname of function and isatty of function in C language

C language ttyname() function: returns the name of a terminal
The header file:

#include <unistd.h>

Definition function:

char * ttyname(int desc);

Function description: if the file descriptor represented by parameter desc is a terminal, the name of the terminal is returned by a string pointer, otherwise NULL is returned.

Return value: returns a string pointer to the name of the terminal on success, or NULL if an error occurs.

sample

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
main()
{
  int fd;
  char * file = "/dev/tty";
  fd = open (fiel, O_RDONLY);
  printf("%s", file);
  if(isatty(fd))
  {
   printf("is a tty. n");
   printf("ttyname = %s n", ttyname(fd));
  }
  else
   printf(" is not a ttyn");
  close(fd);
}

Perform:

/dev/tty is a tty ttyname = /dev/tty

C isatty() function: determines if the file descriptor isa terminal
The header file:

#include <unistd.h>

Definition function:

int isatty(int desc);

Returns 1 if the file descriptor represented by parameter desc is a terminal, or 0 if not.

Return value: returns 1 if the file is a terminal, or 0 if it is not.