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Manual Pages  — SBREAD

NAME

sbget – read and write superblocks of a UFS file system

CONTENTS

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 sbreadand 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 <Mt jmallett@FreeBSD.org> Marshall Kirk McKusick <Mt mckusick@FreeBSD.org>
SBREAD (3) September 2, 2020
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