fd - file descriptor files
These files, conventionally called /dev/fd/0, /dev/fd/1, /dev/fd/2, and so on, refer to files accessible through file descriptors. If file descriptor n is open, these two system calls have the same effect:
fd = open("/dev/fd/n",mode); fd = dup(n);
On these files creat(2) is equivalent to open, and mode is ignored. As with dup, subsequent reads or writes on fd fail unless the original file descriptor allows the operations.
For convenience in referring to standard input, standard output, and standard error, an additional set of names is provided: /dev/stdin is a synonym for /dev/fd/0, /dev/stdout for /dev/fd/1, and /dev/stderr for /dev/fd/2.
open(2) returns −1 and EBADF if the associated file descriptor is not open.