Provided by: freebsd-manpages_8.2-1_all bug

NAME

     sglist, sglist_alloc, sglist_append, sglist_append_mbuf,
     sglist_append_phys, sglist_append_uio, sglist_append_user, sglist_build,
     sglist_clone, sglist_consume_uio, sglist_count, sglist_free, sglist_hold,
     sglist_init, sglist_join, sglist_length, sglist_reset, sglist_slice,
     sglist_split -- manage a scatter/gather list of physical memory addresses

SYNOPSIS

     #include <sys/types.h>
     #include <sys/sglist.h>

     struct sglist *
     sglist_alloc(int nsegs, int mflags);

     int
     sglist_append(struct sglist *sg, void *buf, size_t len);

     int
     sglist_append_mbuf(struct sglist *sg, struct mbuf *m);

     int
     sglist_append_phys(struct sglist *sg, vm_paddr_t paddr, size_t len);

     int
     sglist_append_uio(struct sglist *sg, struct uio *uio);

     int
     sglist_append_user(struct sglist *sg, void *buf, size_t len,
         struct thread *td);

     struct sglist *
     sglist_build(void *buf, size_t len, int mflags);

     struct sglist *
     sglist_clone(struct sglist *sg, int mflags);

     int
     sglist_consume_uio(struct sglist *sg, struct uio *uio, size_t resid);

     int
     sglist_count(void *buf, size_t len);

     void
     sglist_free(struct sglist *sg);

     struct sglist *
     sglist_hold(struct sglist *sg);

     void
     sglist_init(struct sglist *sg, int maxsegs, struct sglist_seg *segs);

     int
     sglist_join(struct sglist *first, struct sglist *second);

     size_t
     sglist_length(struct sglist *sg);

     void
     sglist_reset(struct sglist *sg);

     int
     sglist_slice(struct sglist *original, struct sglist **slice,
         size_t offset, size_t length, int mflags);

     int
     sglist_split(struct sglist *original, struct sglist **head,
         size_t length, int mflags);

DESCRIPTION

     The sglist API manages physical address ranges.  Each list contains one
     or more elements.  Each element contains a starting physical address and
     a length.  Scatter/gather lists are read-only while they are shared.  If
     one wishes to alter an existing scatter/gather list and does not hold the
     sole reference to the list, then one should create a new list instead of
     modifying the existing list.

     Each scatter/gather list object contains a reference count.  New lists
     are created with a single reference.  New references are obtained by
     calling sglist_hold and are released by calling sglist_free.

   Allocating and Initializing Lists
     Each sglist object consists of a header structure and a variable-length
     array of scatter/gather list elements.  The sglist_alloc function
     allocates a new list that contains a header and nsegs scatter/gather list
     elements.  The mflags argument can be set to either M_NOWAIT or M_WAITOK.

     The sglist_count function returns the number of scatter/gather list
     elements needed to describe the physical address ranges mapped by a
     single kernel virtual address range.  The kernel virtual address range
     starts at buf and is len bytes long.

     The sglist_build function allocates a new scatter/gather list object that
     describes the physical address ranges mapped by a single kernel virtual
     address range.  The kernel virtual address range starts at buf and is len
     bytes long.  The mflags argument can be set to either M_NOWAIT or
     M_WAITOK.

     The sglist_clone function returns a copy of an exising scatter/gather
     list object sg.  The mflags argument can be set to either M_NOWAIT or
     M_WAITOK.  This can be used to obtain a private copy of a scatter/gather
     list before modifying it.

     The sglist_init function initializes a scatter/gather list header.  The
     header is pointed to by sg and is initialized to manage an array of
     maxsegs scatter/gather list elements pointed to by segs.  This can be
     used to initialize a scatter/gather list header whose storage is not
     provided by sglist_alloc.  In that case, the caller should not call
     sglist_free to release its own reference and is responsible for ensuring
     all other references to the list are dropped before it releases the
     storage for sg and segs.

   Constructing Scatter/Gather Lists
     The sglist API provides several routines for building a scatter/gather
     list to describe one or more objects.  Specifically, the sglist_append
     family of routines can be used to append the physical address ranges
     described by an object to the end of a scatter/gather list.  All of these
     routines return 0 on success or an error on failure.  If a request to
     append an address range to a scatter/gather list fails, the
     scatter/gather list will remain unchanged.

     The sglist_append function appends the physical address ranges described
     by a single kernel virtual address range to the scatter/gather list sg.
     The kernel virtual address range starts at buf and is len bytes long.

     The sglist_append_mbuf function appends the physical address ranges
     described by an entire mbuf chain m to the scatter/gather list sg.

     The sglist_append_phys function appends a single physical address range
     to the scatter/gather list sg.  The physical address range starts at
     paddr and is len bytes long.

     The sglist_append_uio function appends the physical address ranges
     described by a uio(9) object to the scatter/gather list sg.  Note that it
     is the caller's responsibility to ensure that the pages backing the I/O
     request are wired for the lifetime of sg.  Note also that this routine
     does not modify uio.

     The sglist_append_user function appends the physical address ranges
     described by a single user virtual address range to the scatter/gather
     list sg.  The user virtual address range is relative to the address space
     of the thread td.  It starts at buf and is len bytes long.  Note that it
     is the caller's responsibility to ensure that the pages backing the user
     buffer are wired for the lifetime of sg.

     The sglist_consume_uio function is a variation of sglist_append_uio.  As
     with sglist_append_uio, it appends the physical address ranges described
     by uio to the scatter/gather list sg.  Unlike sglist_append_uio, however,
     sglist_consume_uio modifies the I/O request to indicate that the appended
     address ranges have been processed similar to calling uiomove(9).  This
     routine will only append ranges that describe up to resid total bytes in
     length.  If the available segments in the scatter/gather list are
     exhausted before resid bytes are processed, then the uio structure will
     be updated to reflect the actual number of bytes processed, and
     sglist_consume_io will return zero to indicate success.  In effect, this
     function will perform partial reads or writes.  The caller can compare
     the uio_resid member of uio before and after calling sglist_consume_uio
     to determine the actual number of bytes processed.

   Manipulating Scatter/Gather Lists
     The sglist_join function appends physical address ranges from the
     scatter/gather list second onto first and then resets second to an empty
     list.  It returns zero on success or an error on failure.

     The sglist_split function splits an existing scatter/gather list into two
     lists.  The first length bytes described by the list original are moved
     to a new list *head.  If original describes a total address range that is
     smaller than length bytes, then all of the address ranges will be moved
     to the new list at *head and original will be an empty list.  The caller
     may supply an existing scatter/gather list in *head.  If so, the list
     must be empty.  Otherwise, the caller may set *head to NULL in which case
     a new scatter/gather list will be allocated.  In that case, mflags may be
     set to either M_NOWAIT or M_WAITOK.  Note that since the original list is
     modified by this call, it must be a private list with no other
     references.  The sglist_split function returns zero on success or an
     error on failure.

     The sglist_slice function generates a new scatter/gather list from a sub-
     range of an existing scatter/gather list original.  The sub-range to
     extract is specified by the offset and length parameters.  The new
     scatter/gather list is stored in *slice.  As with head for sglist_join,
     the caller may either provide an empty scatter/gather list, or it may set
     *slice to NULL in which case sglist_slice will allocate a new list
     subject to mflags.  Unlike sglist_split, sglist_slice does not modify
     original and does not require it to be a private list.  The sglist_split
     function returns zero on success or an error on failure.

   Miscellaneous Routines
     The sglist_reset function clears the scatter/gather list sg so that it no
     longer maps any address ranges.  This can allow reuse of a single
     scatter/gather list object for multiple requests.

     The sglist_length function returns the total length of the physical
     address ranges described by the scatter/gather list sg.

RETURN VALUES

     The sglist_alloc, sglist_build, and sglist_clone functions return a new
     scatter/gather list on success or NULL on failure.

     The sglist_append family of functions and the sglist_consume_uio,
     sglist_join, sglist_slice, and sglist_split functions return zero on
     success or an error on failure.

     The sglist_count function returns a count of scatter/gather list
     elements.

     The sglist_length function returns a count of address space described by
     a scatter/gather list in bytes.

ERRORS

     The sglist_append functions return the following errors on failure:

     [EINVAL]           The scatter/gather list has zero segments.

     [EFBIG]            There are not enough available segments in the
                        scatter/gather list to append the specified physical
                        address ranges.

     The sglist_consume_uio function returns the following error on failure:

     [EINVAL]           The scatter/gather list has zero segments.

     The sglist_join function returns the following error on failure:

     [EFBIG]            There are not enough available segments in the
                        scatter/gather list first to append the physical
                        address ranges from second.

     The sglist_slice function returns the following errors on failure:

     [EINVAL]           The original scatter/gather list does not describe
                        enough address space to cover the requested sub-range.

     [EINVAL]           The caller-supplied scatter/gather list in *slice is
                        not empty.

     [ENOMEM]           An attempt to allocate a new scatter/gather list with
                        M_NOWAIT set in mflags failed.

     [EFBIG]            There are not enough available segments in the caller-
                        supplied scatter/gather list in *slice to describe the
                        requested physical address ranges.

     The sglist_split function returns the following errors on failure:

     [EDOOFUS]          The original scatter/gather list has more than one
                        reference.

     [EINVAL]           The caller-supplied scatter/gather list in *head is
                        not empty.

     [ENOMEM]           An attempt to allocate a new scatter/gather list with
                        M_NOWAIT set in mflags failed.

     [EFBIG]            There are not enough available segments in the caller-
                        supplied scatter/gather list in *head to describe the
                        requested physical address ranges.

SEE ALSO

     malloc(9), mbuf(9), uio(9)

HISTORY

     This API was first introduced in FreeBSD 8.0.