SBREAD(3)              FreeBSD Library Functions Manual              SBREAD(3)

NAME
     sbget, sbput, sbread, sbwrite - read and write superblocks of a UFS file
     system

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
     sbget(int devfd, struct fs **fsp, off_t sblockloc);

     int
     sbput(int devfd, struct fs *fs, int numaltwrite);

     int
     sbread(struct uufsd *disk);

     int
     sbwrite(struct uufsd *disk, int all);

DESCRIPTION
     The sbget() and sbread() functions provide superblock reads for libufs(3)
     consumers.  The sbput() and sbwrite() functions provide superblock writes
     for libufs(3) consumers.

     The sbget() function first allocates a buffer to hold the superblock.
     Using the devfd file descriptor that references the filesystem disk,
     sbget() reads the superblock located at the byte offset specified by
     sblockloc into the allocated buffer.  If successful, it returns a pointer
     to the buffer containing the superblock in fsp.  The sbget() function is
     safe to use in threaded applications.

     The sbput() function writes the superblock specified by fs to the
     location from which it was read on the disk referenced by the devfd file
     descriptor.  Additionally, the sbput() function will update the first
     numaltwrite alternate superblock locations.  To update all the alternate
     superblocks, specify a numaltwrite value of fs->fs_ncg.  The sbput()
     function is safe to use in threaded applications.  Note that the sbput()
     function needs to be called only if the superblock has been modified and
     the on-disk copy needs to be updated.

     The sbread() function reads the standard filesystem superblock into the
     d_sb, structure embedded in the given user-land UFS disk structure.

     The sbwrite() function writes the superblock from the d_sb, structure
     embedded in the given user-land UFS disk structure to the location from
     which it was read.  Additionally, the sbwrite() function will write to
     all the alternate superblock locations if the all value is non-zero.

RETURN VALUES
     The sbread() and sbwrite() functions return the value 0 if successful;
     otherwise the value -1 is returned and the global variable errno is set
     to indicate the error.  The sbget() and sbput() functions return the
     value 0 if successful; otherwise they return one of the errors described
     below.

ERRORS
     The errors returned by sbget() and sbread() include any of the errors
     specified for the library function bread(3).  Additionally, they may
     follow the libufs(3) error methodologies in situations where no usable
     superblock could be found.

     The errors returned by sbput() and sbwrite() include any of the errors
     specified for the library function bwrite(3).

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

HISTORY
     These functions first appeared as part of libufs(3) in FreeBSD 5.0.

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