CGREAD(3)              FreeBSD Library Functions Manual              CGREAD(3)

NAME
     cgget, cgput, cgread, cgread1, cgwrite, cgwrite1 - read/write cylinder
     groups of UFS disks

LIBRARY
     UFS File System Access Library (libufs, -lufs)

SYNOPSIS
     #include <sys/param.h>
     #include <sys/mount.h>
     #include <ufs/ufs/ufsmount.h>
     #include <ufs/ufs/dinode.h>
     #include <ufs/ffs/fs.h>
     #include <libufs.h>

     int
     cgget(int devfd, struct fs *fs, int cg, struct cg *cgp);

     int
     cgput(int devfd, struct fs *fs, struct cg *cgp);

     int
     cgread(struct uufsd *disk);

     int
     cgread1(struct uufsd *disk, int cg);

     int
     cgwrite(struct uufsd *disk);

     int
     cgwrite1(struct uufsd *disk, int cg);

DESCRIPTION
     The cgget(), cgread(), and cgread1() functions provide cylinder group
     reads for libufs(3) consumers.  The cgput(), cgwrite(), and cgwrite1()
     functions provide cylinder group writes for libufs(3) consumers.

     The cgget() function reads the cylinder group specified by cg into the
     buffer pointed to by cgp from the filesystem described by the fs
     superblock using the devfd file descriptor that references the filesystem
     disk.  The cgget() function is the only cylinder group read function that
     is safe to use in threaded applications.

     The cgput() function writes the cylinder group specified by cgp to the
     filesystem described by the fs superblock using the devfd file descriptor
     that references the filesystem disk.  The cgput() function is the only
     cylinder group write function that is safe to use in threaded
     applications.  Note that the cgput() function needs to be called only if
     the cylinder group has been modified and the on-disk copy needs to be
     updated.

     The cgread1() function reads from the cylinder group specified by cg into
     the d_cg cylinder-group structure in a user-land UFS-disk structure.  It
     sets the d_lcg field to the cylinder group number cg.

     The cgread() function operates on sequential cylinder groups.  Calling
     the cgread() function is equivalent to calling cgread1() with a cylinder
     group specifier equivalent to the value of the current d_ccg field, and
     then incrementing the d_ccg field.

     The cgwrite() function stores on disk the cylinder group held in the d_cg
     cylinder-group structure in a user-land UFS-disk structure.

     The cgwrite1() function provides no additional functionality over the
     cgwrite() function as there is only one place that a given cylinder group
     can correctly be written.  If the caller gets the cg parameter wrong, the
     function fails with the error EDOOFUS.  This function remains only to
     provide backward compatibility.

RETURN VALUES
     The cgread() function returns 0 if there are no more cylinder groups to
     read, 1 if there are more cylinder groups, and -1 on error.  The
     cgread1() function returns 1 on success and -1 on error.  The other
     functions return 0 on success and -1 on error.

ERRORS
     The cgget(), cgread(), and cgread1() functions may fail and set errno for
     any of the errors specified for the library function bread(3).

     The cgput(), cgwrite(), and cgwrite1() functions may fail and set errno
     for any of the errors specified for the library function bwrite(3).
     Additionally the cgwrite1() will return the EDOOFUS error if the cylinder
     group specified does not match the cylinder group that it is requesting
     to write.

SEE ALSO
     bread(3), bwrite(3), libufs(3)

HISTORY
     These functions first appeared as part of libufs(3) in FreeBSD 5.1.

AUTHORS
     Juli Mallett <jmallett@FreeBSD.org>
     Marshall Kirk McKusick <mckusick@FreeBSD.org>

FreeBSD 13.1-RELEASE-p6        September 2, 2020       FreeBSD 13.1-RELEASE-p6