Manually obtaining information about Linux File Descriptor Table

Question

How can I obtain the file descriptor flags 'fd flags' residing in the File Descriptor Table associated with any open file descriptor? I would like to know to write the code manually or at least what functions and structures to research further while not using the fcntl function or viewing the /proc entries.

The image below shows that the file descriptor table for a given process has a flags field and a file pointer field. How can I programatically access these fields?

Answer 1

You can access /proc/PID/fdinfo/FD file which contains flags in octal:

$ cat /proc/$$/fdinfo/0
pos:    0
flags:  0100002
mnt_id: 20

In this example, 0100002 = O_LARGFILE|O_RDWR

Answer 2

fcntl() is a system call, not "a library". It is the way you retrieve the flags associated with a file descriptor.