Copyright (c) 2002 Artur Grabowski <art@openbsd.org>
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.Dd $Mdocdate: December 31 2019 $ .Dt FALLOC 9 .Os .Sh NAME .Nm falloc , .Nm fdrelease , .Nm FREF , .Nm FRELE , .Nm fd_getfile , .Nm fd_getfile_mode , .Nm fd_checkclosed , .Nm getsock , .Nm getvnode .Nd an overview of file descriptor handling .Sh SYNOPSIS n sys/file.h n sys/filedesc.h .Ft int .Fn falloc "struct proc *p" "int flags" "struct file **resultfp" "int *resultfd" .Ft int .Fn fdrelease "struct proc *p" "int fd" .Ft void .Fn FREF "struct file *fp" .Ft int .Fn FRELE "struct file *fp" "struct proc *p" .Ft struct file * .Fn fd_getfile "struct filedesc *fdp" "int fd" .Ft struct file * .Fn fd_getfile_mode "struct filedesc *fdp" "int fd" "int mode" .Ft int .Fn fd_checkclosed "struct filedesc *fdp" "int fd" "struct file *fp" .Ft int .Fn getsock "struct proc *p" "int fd" "struct file **fpp" n sys/file.h n sys/filedesc.h n sys/vnode.h .Ft int .Fn getvnode "struct proc *p" "int fd" "struct file **fpp" .Sh DESCRIPTION These functions provide the interface for the UNIX file descriptors. File descriptors can be used to access vnodes (see .Xr vnode 9 ) , sockets (see .Xr socket 2 ) , pipes (see .Xr pipe 2 ) , kqueues (see .Xr kqueue 2 ) , and various special purpose communication endpoints.
p A new file and a file descriptor for it are allocated with the function .Fn falloc . The .Fa flags argument can be used to set the .Dv UF_EXCLOSE flag on the new descriptor. The larval file and fd are returned via .Fa resultfp and .Fa resultfd , which must not be .Dv NULL . .Fn 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 .Fn FRELE when it's done initializing it.
p
A file descriptor is freed with
.Fn fdrelease .
This releases the reference that it holds to the underlying file;
if that's the last reference then the file will be freed.
with
.Xr closef 9 .
The file descriptor table has to be locked on entry.
.Fn fdrelease
unlocks the table on return.
p The files are extracted from the file descriptor table using the function .Fn fd_getfile . .Fn 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. It also increases the descriptor's use count with .Fn FREF .
p .Fn fd_getfile_mode is like .Fn fd_getfile but also checks if the file has been opened with the given mode.
p .Fn fd_checkclosed checks if file descriptor .Fa fd has been closed and no longer points to file .Fa fp . The file must have been retrieved from the descriptor previously.
p The files are extracted from the process context using the function .Fn getsock and .Fn getvnode . These functions are special cases that besides doing .Fn fd_getfile also check if the descriptor is a socket or a vnode respectively, and return the proper errno on error. .Sh 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 .Fn FREF and .Fn FRELE . The function .Fn FREF increases the use count of a file descriptor. The function .Fn FRELE decreases the use count, and releases the file descriptor if the use count becomes zero. .Sh CODE REFERENCES The majority of those functions are implemented in
a sys/kern/kern_descrip.c . The function prototypes and the macros are located in
a sys/sys/filedesc.h . .Sh SEE ALSO .Xr vnode 9