FALLOC(9)

NAME
falloc, fdrelease, FREF, FRELE, fd_getfile, getsock, getvnode - an overview of file descriptor handling

SYNOPSIS
#include <sys/file.h>
#include <sys/filedesc.h>
int
falloc(struct proc *p, int flags, struct file **resultfp, int *resultfd);
int
fdrelease(struct proc *p, int fd);
void
FREF(struct file *fp);
int
FRELE(struct file *fp, struct proc *p);
struct file *
fd_getfile(struct filedesc *fdp, int fd);
int
getsock(struct proc *p, int fd, struct file **fpp);
#include <sys/file.h>
#include <sys/filedesc.h>
#include <sys/vnode.h>
int
getvnode(struct proc *p, int fd, struct file **fpp);

DESCRIPTION
These functions provide the interface for the UNIX file descriptors. File descriptors can be used to access vnodes (see vnode(9)), sockets (see socket(2)), pipes (see pipe(2)), kqueues (see kqueue(2)), and various special purpose communication endpoints.

A new file and a file descriptor for it are allocated with the function falloc(). The flags argument can be used to set the UF_EXCLOSE flag on the new descriptor. The larval file and fd are returned via resultfp and resultfd, which must not be NULL. falloc() initializes the new file to have a reference count of two: one for the reference from the file descriptor table and one for the caller to release with FRELE() when it's done initializing it.

A file descriptor is freed with fdrelease(). This releases the reference that it holds to the underlying file; if that's the last reference then the file will be freed.

The files are extracted from the file descriptor table using the functions fd_getfile() fd_getfile() performs all necessary checks to see if the file descriptor number is within the range of file descriptor table, and if the descriptor is valid.

The files are extracted from the process context using the function getsock() and getvnode(). These functions are special cases that besides doing fd_getfile() also check if the descriptor is a socket or a vnode respectively, return the proper errno on error and increase the use count with FREF().

CONCURRENT ACCESS
Since multiple processes can share the same file descriptor table, it's important that the file is not freed in one process while some other process is still accessing it. To solve that problem a special use count is kept with the functions FREF() and FRELE(). In most cases FREF() should be used on a file after it has been extracted from the file descriptor table and FRELE() should be called when the file won't be used anymore. There are cases when this isn't necessary, but since FREF() and FRELE() are cheap to use, there is no reason to risk introducing bugs by not using them.

CODE REFERENCES
The majority of those functions are implemented in sys/kern/kern_descrip.c. The function prototypes and the macros are located in sys/sys/file.h and sys/sys/filedesc.h.

SEE ALSO
vnode(9)