freebsd/share/man/man9/ucred.9

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.Dd March 3, 2002
.Dt UCRED 9
.Os
.Sh NAME
.Nm ucred ,
.Nm crget ,
.Nm crhold ,
.Nm crfree ,
.Nm crshared ,
.Nm crcopy ,
.Nm crdup ,
.Nm cru2x ,
.NM cred_update_thread
.Nd "functions related to user credentials"
.Sh SYNOPSIS
.In sys/param.h
.In sys/ucred.h
.Ft "struct ucred *"
.Fn crget void
.Ft "struct ucred *"
.Fn crhold "struct ucred *cr"
.Ft void
.Fn crfree "struct ucred *cr"
.Ft int
.Fn crshared "struct ucred *cr"
.Ft void
.Fn crcopy "struct ucred *dest" "struct ucred *src"
.Ft "struct ucred *"
.Fn crdup "struct ucred *cr"
.Ft void
.Fn cru2x "struct ucred *cr" "struct xucred *xcr"
.Ft void
.Fn cred_update_thread "struct thread *td"
.Sh DESCRIPTION
The
.Nm
family of functions is used to manage user credential structures
.Pq Vt "struct ucred"
within the kernel.
.Pp
The
.Fn crget
function allocates memory
for a new structure, sets its reference count to 1, and
initializes its lock.
.Pp
The
.Fn crhold
function increases the reference count on the credential.
.Pp
The
.Fn crfree
function decreases the reference count on the credential.
If the count drops to 0, the storage for the structure is freed.
.Pp
The
.Fn crshared
function returns true if the credential is shared.
A credential is considered to be shared if its reference
count is greater than one.
.Pp
The
.Fn crcopy
function copies the contents of the source (template)
credential into the destination template.
The
.Vt uidinfo
structure within the destination is referenced
by calling
.Xr uihold 9 .
.Pp
The
.Fn crdup
function allocates memory for a new structure and copies the
contents of
.Fa cr
into it.
The actual copying is performed by
.Fn crcopy .
.Pp
The
.Fn cru2x
function converts a
.Vt ucred
structure to an
.Vt xucred
structure.
That is,
it copies data from
.Fa cr
to
.Fa xcr ;
it ignores fields in the former that are not present in the latter
(e.g.,
.Va cr_uidinfo ) ,
and appropriately sets fields in the latter that are not present in
the former
(e.g.,
.Va cr_version ) .
.Pp
The
.Fn cred_update_thread
function sets the credentials of
.Fa td
to that of its process, freeing its old credential if required.
.Sh RETURN VALUES
.Fn crget ,
.Fn crhold
and
.Fn crdup
all return a pointer to a
.Vt ucred
structure.
.Pp
.Fn crshared
returns 0 if the credential has a reference count greater than 1;
otherwise, 1 is returned.
.Sh USAGE NOTES
In general in a system call, 
you should always use the 
.Vt ucred
associated with the 
thread doing the work rather than that of the process attached to it,
As that associated with the thread is guaranteed to be safely accessed
under SMP, and will remain stable for the duration of the call, even
in the face of a multithreaded application changing the process credentials
from another thread.
.Sh SEE ALSO
.Xr uihold 9
.Sh AUTHORS
This man page was written by
.An Chad David Aq davidc@acns.ab.ca .