s6-fdholder.7

.Dd September 29, 2021
.Dt S6-FDHOLDER 7
.Os
.Sh NAME
.Nm s6-fdholder
.Nd API for clients wanting to communicate with an
.Xr s6-fdholderd 8
daemon
.Sh DESCRIPTION
Check the
.Pa s6/s6-fdholder.h
header for the exact function prototypes.
.Ss A programming example
The
.Pa src/fdholder/s6-fdholder-*.c
files in the s6 package, for instance, illustrate how to use the
.Nm
library.
.Ss Synchronous functions with a specified maximum execution time
The explanation given in
.Xr s6-ftrigr 7
applies here too: the functions documented in this page are
synchronous, but can return early if the deadline is reached, in which
case the connection to the server should be closed immediately because
no protocol consistency is guaranteed.
.Pp
The
.Xr s6-fdholderd 8
server should be very quick to answer queries, so this mechanism is
provided as a simple security against programming errors - for
instance, connecting to the wrong daemon.
.Ss Starting and ending a session
.Bd -literal -offset indent
s6_fdholder_t a = S6_FDHOLDER_ZERO ;
int fd = 6 ;

tain_now_g() ;

s6_fdholder_init(&a, fd) ;
(...)
s6_fdholder_free(&a) ;
.Ed
.Pp
.Fn s6_fdholder_init
assumes that
.Va fd
is a socket already connected to an
.Xr s6-fdholderd 8
daemon.
The
.Va a
structure must be initialized to
.Dv S6_FDHOLDER_ZERO
before use.
.Pp
.Fn tain_now_g
initializes a global variable that keeps track of the current time,
for use with later functions.
.Pp
.Fn s6_fdholder_free
frees the resources occupied by
.Va a .
It does not, however, close
.Va fd .
You should manually close it to end the connection to the server.
Note that if your program has been started by
.Xr s6-ipcclient 8 ,
both fds 6 and 7 are open (and refer to the same socket), so you
should close both.
.Pp
Alternatively, if your connection to
.Xr s6-fdholderd 8
has not been created yet, you can use the following functions:
.Bl -bullet -width x
.It
.Ft int
.Fn s6_fdholder_start "s6_fdholder_t *a" "char const *path" "tain_t const *deadline" "tain_t *stamp"
.Pp
Starts a session with an
.Xr s6-fdholderd 8
instance listening on
.Va path .
.Va a
must be initialized to
.Dv S6_FDHOLDER_ZERO
before calling this function.
On success, returns nonzero and
.Va a
can be used as a handle for the next
.Fn s6_fdholder_*
function calls.
On failure, returns 0, and sets errno.
.It
.Ft void
.Fn s6_fdholder_end "s6_fdholder_t *a"
.Pp
Ends the current session and frees all allocated resources.
If needed,
.Va a
is immediately reusable for another
.Fn s6_fdholder_start
call.
.El
.Ss Storing an fd
.Bd -literal -offset indent
int r ;
int fd ;
tain_t limit = TAIN_INFINITE ;
char const *id = "my_identifier" ;
r = s6_fdholder_store_g(&a, fd, id, &limit, &deadline) ;
.Ed
.Pp
.Fn s6_fdholder_store
(and its variant
.Fn s6_fdholder_store_g
that uses the global timestamp variable) attempts to store a copy of
descriptor
.Va fd
into
.Xr s6-fdholderd 8 ,
using identifier
.Va id ,
with an expiration date of
.Va limit .
In this example,
.Va limit
is
.Dv TAIN_INFINITE ,
which means no expiration date.
The operation should return before
.Va deadline ,
else it will automatically return 0
.Dv ETIMEDOUT .
The result is 1 on success and 0 on failure, with an appropriate errno
code; refer to
.Xr s6-fdholder-error-codes 7 .
.Ss Deleting an fd
.Dl fd = s6_fdholder_delete_g(&a, id, &deadline) ;
.Pp
.Fn s6_fdholder_delete
attempts to delete the file descriptor identified by
.Va id .
It returns 1 on success and 0 on failure, with an appropriate errno
code; refer to
.Xr s6-fdholder-error-codes 7 .
.Ss Retrieving an fd
.Dl fd = s6_fdholder_retrieve_g(&a, id, &deadline) ;
.Pp
.Fn s6_fdholder_retrieve
attempts to retrieve the file descriptor identified by
.Va id .
It returns a valid fd number on success, and -1 on failure, with an
appropriate errno code; refer to
.Xr s6-fdholder-error-codes 7 .
.Pp
.Fn s6_fdholder_retrieve_delete
performs a retrieval and a deletion at the same time, if the client is
authorized to do so.
.Ss Listing the identifiers held by the server
.Bd -literal -offset indent
stralloc list = STRALLOC_ZERO ;
int n ;
n = s6_fdholder_list_g(&a, &list, &deadline) ;
.Ed
.Pp
.Fn s6_fdholder_list
gets the list of all identifiers currently held by the server.
It stores it into the stralloc[1]
.Va list ,
as a series of null-terminated strings, one after the other.
There are
.Va n
such strings.
The function returns
.Va n
on success, or -1 on failure, with an appropriate errno code; refer to
.Xr s6-fdholder-error-codes 7 .
.Ss Reading a dump
.Bd -literal -offset indent
genalloc dump = GENALLOC_ZERO ;
r = s6_fdholder_getdump_g(&a, &dump, &deadline) ;
.Ed
.Pp
.Fn s6_fdholder_getdump
attempts to retrieve the whole set of descriptors from the server.
It returns 1 on success, and 0 on failure, with an appropriate errno
code; refer to
.Xr s6-fdholder-error-codes 7 .
The set is stored into the genalloc[2]
.Va dump ,
which is to be interpreted as a stralloc containing an array of
.Vt s6_fdholder_fd_t .
.Pp
.Ql genalloc_s(s6_fdholder_fd_t, &dump)
is a pointer to this array, and
.Ql genalloc_len(s6_fdholder_fd_t, &dump)
is the number of elements in the array.
An
.Vt s6_fdholder_fd_t
contains at least a descriptor number, an identifier, and an
expiration date; see the
.Pa s6/s6-fdholder.h
header file.
.Ss Writing a dump
.Bd -literal -offset indent
unsigned int dumplen ;
s6_fdholder_fd_t const *dumparray ;
r = s6_fdholder_setdump_g(&a, &dumparray, dumplen, &deadline) ;
.Ed
.Pp
.Fn s6_fdholder_setdump
attempts to send a set of descriptors to the server.
The descriptors are contained in the array
.Va dumparray
of length
.Va dumplen .
The function returns 1 on success, and 0 on failure, with an
appropriate errno code; refer to
.Xr s6-fdholder-error-codes 7 .
.Sh SEE ALSO
.Xr s6-accessrules 7 ,
.Xr s6-ftrigr 7 ,
.Xr s6-ftrigw 7 ,
.Xr s6-libs6 7 ,
.Xr s6-s6lock 7
.Pp
[1]
.Lk https://skarnet.org/software/skalibs/libstddjb/stralloc.html
.Pp
[2]
.Lk https://skarnet.org/software/skalibs/libstddjb/genalloc.html
.Pp
This man page is ported from the authoritative documentation at:
.Lk https://skarnet.org/software/s6/libs6/s6-fdholder.html
.Sh AUTHORS
.An Laurent Bercot
.An Alexis Ao Mt flexibeast@gmail.com Ac (man page port)