getpbuf (9)

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Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved.

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(The comments found at the beginning of the groff file "man9/getpbuf.9freebsd".)

NAME

pbuf getpbuf trypbuf relpbuf - functions for managing physical buffers

SYNOPSIS

In sys/param.h In sys/systm.h In sys/bio.h In sys/buf.h Ft struct buf * Fn getpbuf int *pfreecnt Ft struct buf * Fn trypbuf int *pfreecnt Ft void Fn relpbuf struct buf *bp int *pfreecnt

DESCRIPTION

These functions are used to allocate and release physical buffers.

The physical buffers are allocated at system startup and are maintained in a separate pool from the main system buffers. They are intended for use by subsystems that cannot or should not be reliant on the main pool of buffers (for example the swap pager). The system allocates between 16 and 256 physical buffers depending on the amount of memory in the system.

Each subsystem that allocates buffers via these calls is expected to manage its own percentage free counter. If the value is initialized to -1 the number of buffers available to the subsystem is limited only by the number of physical buffers available. The number of buffers is stored in nswbuf which is defined in In sys/buf.h and initialized in Fn cpu_startup . A recommended initialization value is 1/2 nswbuf

The Fn getpbuf function returns the first available buffer to the user. If there are no buffers available, Fn getpbuf will sleep waiting for one to become available. If Fa pfreecnt is zero, Fn getpbuf will sleep until it increases. Fa pfreecnt is decremented prior to returning.

The Fn trypbuf function returns the first available buffer. If there are no buffers available, NULL is returned. As well, if Fa pfreecnt is zero, NULL is returned. Fa pfreecnt is decremented prior to returning a valid buffer. If NULL is returned, Fa pfreecnt is not modified.

The Fn relpbuf function releases the buffer back to the free list. If the buffers b_rcred or b_wcred structures are not NULL they are freed. See crfree(9).

Fa pfreecnt is incremented prior to returning.

RETURN VALUES

Fn getpbuf and Fn trypbuf return a pointer to the buffer. In the case of Fn trypbuf , NULL can also be returned indicating that there are no buffers available.

AUTHORS

This manual page was written by An Chad David Aq davidc@acns.ab.ca .