---
name: "sbread"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/sbfind"
---

# sbread(3)

read and write superblocks of a UFS file system

Part of [sbfind(3)](/sbfind/3.md).

## NAME

**sbget**, **sbsearch**, **sbput**, **sbread**, **sbfind**, **sbwrite** – read and write superblocks of a UFS file system

## LIBRARY

library “libufs”

## 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 flags*);

*int*\

**sbsearch**(*int devfd*, *struct fs \*\*fsp*, *int flags*);

*int*\

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

*int*\

**sbread**(*struct uufsd \*disk*);

*int*\

**sbfind**(*struct uufsd \*disk*, *int flags*);

*int*\

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

## DESCRIPTION

The **sbget**(), **sbsearch**(), **sbread**(), and **sbfind**() functions provide superblock reads for [libufs(3)](/libufs/3.md) consumers. The **sbput**() and **sbwrite**() functions provide superblock writes for [libufs(3)](/libufs/3.md) consumers.

The **sbget**() and **sbsearch**() functions first allocate 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. The value **UFS\_STDSB** may be specified for *sblockloc* to request that the standard location for the superblock be read. The **sbsearch**() function uses the *devfd* file descriptor that references the filesystem disk, to search first for the superblock at the standard location. If it is not found or is too damaged to use **sbsearch**() will attempt to find one of the filesystem's alternate superblocks. Flags are specified by *or*'ing the following values:

- **UFS\_NOCSUM**

  Causes only the superblock itself to be returned, but does not read in any auxiliary data structures like the cylinder group summary information.

- **UFS\_NOMSG**

  Indicates that superblock inconsistency error messages should not be printed.

If successful, **sbget**() and **sbsearch**() functions return a pointer to the buffer containing the superblock in *fsp*. The **sbget**() and **sbsearch**() functions are 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. The **sbfind**() function tries to find a usable superblock. It searchs first for the superblock at the standard location. If it is not found or is too damaged to use **sbfind**() will attempt to find one of the filesystem's alternate superblocks. If successful **sbread**() and **sbfind**() return a superblock in 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**(), **sbsearch**(), and **sbput**() functions return the value 0 if successful; otherwise they return one of the errors described below.

## ERRORS

The errors returned by **sbget**(), **sbsearch**(), **sbread**(), and **sbfind**(), include any of the errors specified for the library function [bread(3)](/bread/3.md). Additionally, they may follow the [libufs(3)](/libufs/3.md) 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)](/bwrite/3.md).

## SEE ALSO

[bread(3)](/bread/3.md), [bwrite(3)](/bwrite/3.md), [libufs(3)](/libufs/3.md)

## HISTORY

These functions first appeared as part of [libufs(3)](/libufs/3.md) in FreeBSD 5.0.

## AUTHORS

Juli Mallett ⟨*jmallett@FreeBSD.org*⟩ Marshall Kirk McKusick ⟨*mckusick@FreeBSD.org*⟩