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PBUF, GETPBUF, TRYPBUF, RELPBUF - reference manual online

Functions for managing physical buffers.

Chapter
July 9, 2001
PBUF(9)                           BSD Kernel Developer's Manual                           PBUF(9)

NAME pbuf, getpbuf, trypbuf, relpbuf — functions for managing physical buffers
SYNOPSIS #include <sys/param.h> #include <sys/systm.h> #include <sys/bio.h> #include <sys/buf.h> struct buf * getpbuf(int *pfreecnt); struct buf * trypbuf(int *pfreecnt); void 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 allo‐ cates 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 percent‐ age 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 buf‐ fers is stored in nswbuf which is defined in <sys/buf.h> and initialized in cpu_startup(). A recommended initialization value is 1/2 nswbuf. The getpbuf() function returns the first available buffer to the user. If there are no buf‐ fers available, getpbuf() will sleep waiting for one to become available. If pfreecnt is zero, getpbuf() will sleep until it increases. pfreecnt is decremented prior to returning. The trypbuf() function returns the first available buffer. If there are no buffers avail‐ able, NULL is returned. As well, if pfreecnt is zero, NULL is returned. pfreecnt is decre‐ mented prior to returning a valid buffer. If NULL is returned, pfreecnt is not modified. The 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). pfreecnt is incremented prior to returning.
RETURN VALUES getpbuf() and trypbuf() return a pointer to the buffer. In the case of trypbuf(), NULL can also be returned indicating that there are no buffers available.
AUTHORS This manual page was written by Chad David <@acns.ab.ca>.
BSD July 9, 2001 BSD
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getpbuf(9freebsd) referred by
refer to crfree(9freebsd) | pbuf(9freebsd)