Hi,
I need a to write a system call. My question is: "WHAT EXACTLY IS A FILE DESCRIPTOR??!?". My system call is supposed to returns a number of open files for a process identified by a pid and file desctiptor for each file used by a process. The system call should look like this:

int get_open_files(pid_t pid, int *count, int** fds); returnt value is error code or an integer greater then 0 if evrything worked fine.

Geting the count is not a problem. The problem is getting (int **fds)!!! I guess it can be found by looking into task_struct of a proces and then into struct files_struct *files whitin task_struct. There is a field struct file ** fd in files_struct. This is my best. I am stuck here. Please help me. Once again: What exactly is a file descriptor? Ideal answer shoul point out one or more fields form a certain structure. For example "A file desctiptor, you idiot, is: some_stuct->some_field1, some_struct->some_field2...)
Thanx a lot
Dragan

When you declare
FILE *fp;

and initalise fp = fopen(file,mode);
fopent will return a pointer (memory location of that file starting location).
that is know as file descriptor and it is of type unsigned int.

even you declare
int fp;
fp = open(file,mode,flag); or
fp = creat(file,mode,flag); or read write and so on...

in your declaration,
int get_open_files(pid_t pid, int *count, int** fds);

change it to
int get_open_files(pid_t pid, int *count, unsigned int** fds);

hope its wright now...

hope i have cleared your doubt?

Hmmm, no. That is known as a file pointer (as the fp name suggests), and is certainly not of type unsigned int but of type pointer to a structure.
You got it better here, but you should use "fd", not "fp" to avoid confusion.
That's optimistic, I do not expect a process with more than 2147483648 open files
Hope I've cleared yours.
I guess the OP is asking about file descriptors in the kernel, not in userspace.

Wondered about your reply.
Thanks for your corrections....
appreciating you