process that don't have file descriptor

Question

when I do

ls proc/[pid]/fd

sometimes I don't get output, It seems that there are no file descriptor in that file.

What does that mean when a process doesn't have file descriptor?

Answer 1

The process in question is more likely a deamon — daemon processed will intentionally close standard file descriptors in order to avoid hanging onto their resources. (They will also chdir to the root directory, invoke an extra fork() and perform a number of more obscure operations for the same reason.)