BUFCACHE(9) BSD Kernel Developer's Manual BUFCACHE(9)NAME
bufinit, bread, bwrite — Buffer Cache Functions
SYNOPSIS
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/buf.h>
#include <sys/buf2.h>
int
bread(struct vnode *vp, daddr_t blkno, int size, struct buf **bpp);
int
bwrite(struct buf *bp);
DESCRIPTION
The buffer cache functions are at the heart of all storage file systems;
they are used for reading from and writing to the underlying storage.
The bread() and bwrite() functions observe most activity in the kernel
from file systems, but other functions such as breadn() are also used.
At boot time, the bufinit() function is invoked to initialize various
accounting code. It also initializes nbuf number of buffers and inserts
them into the empty queue BQUEUE_EMPTY. The variable nbuf is a global
variable in the kernel that is tunable at boot time using the loader(8).
FUNCTIONS
bread(*vp, blkno, size, **bpp)
Retrieve a buffer with specified data. An internal function,
getblk() is called to check whether the data is available in
cache or if it should be read from the vp. If the data is
available in cache, the B_CACHE flag will be set otherwise size
bytes will be read starting at block number blkno from the block
special device vnode vp.
In case when the buffer is not in cache or not cacheable this
function will put the calling process or thread to sleep, using
bp as the wait channel and ‘biord’ as the wait message.
On successful return, the b_data field of bp will point to valid
data address and b_count will contain the number of bytes read.
bwrite(*bp)
Write a buffer back to the device pointed to by b_dev field.
Until the write operation is complete, the calling thread or
process will be put to sleep by the kernel using bp as the wait
channel and ‘biowr’ as the wait message.
Before calling this function, the following fields are the least
to be set:
b_data This field should be set to a valid data buffer to be
written by bwrite().
b_bcount
Size of buffer to be written, analogous to the size
argument of bread().
b_blkno Logical block number at which the buffer should be
written.
b_dev This can be set by using the vn_todev() function on the
device vnode.
b_vp This should be set to the vnode of the device to which
the buffer will be written.
This function will put the calling process or thread to sleep if
the data cannot be written when operating synchronously, using
bp as the wait channel and ‘biowr’ as the wait message. On suc‐
cessful return the b_resid field of bp will be set to the value
zero, thus indicating a successful write.
CODE REFERENCES
The file system code, located under sys/vfs directory are the main source
of reference.
SEE ALSObuf(9), VFS(9)AUTHORS
This manual page was written by Hiten Pandya ⟨hmp@freebsd.org⟩.
BSD July 29, 2005 BSD
