Pointers
IOBLOCK Anonymous Pipe data structure
A 'pipe' is a connection between (among) file handles (JFN's). Data written to the 'write end' of the pipe are made available for reading on the 'read end'. The $Pipe system call creates a pipe and returns two file handles, one for the read end and one for the write end. These handles are manipulated in the same way as normal file handles; they may be 'dup'ed and are inherited in the same way. Data are written into a pipe via a 'write' system call on the write end of the pipe. Likewise, data are read from the pipe via a 'read' call on the read end.
Data that are written to a pipe are captured in a circular buffer. The size of the buffer is specified when the pipe is created; if no size is specified, a default size is used.
The circular buffer is described by an 'ioblock'. The ioblock is the buffer's header; the circular buffer proper follows the ioblock in a heap memory object (mapped by a GDT selector) allocated when the pipe is created. The ioblock contains all of the per-pipe information, such as reader, writer, and reference counts, and also holds the pointers into the circular buffer proper.
The selector that points to the circular buffer is stored in the SFT, at sfi_hVPB.
When the in and out pointers are equal, the circular buffer is empty. When the in pointer trails the out pointer by 1, the buffer is full. Thus, a 512 byte buffer can hold only 511 bytes; one byte is lost so that full and empty conditions can be distinguished. So that the user can put 512 bytes in a pipe that they created with a size of 512, we allow for this byte lost when allocating the segment.
旼컴컴컴컴컴컴컫컴컴컴쩡컴컴컫컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커�Field Name �Offset�Length�Type�Description � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_inprogcnt �+0 �1 쿍 쿬ount of read/wrts in progress � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_refcnt �+1 �1 쿍 쿬ount of references � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_rdrcnt �+2 �1 쿍 쿬ount of readers � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_wtrcnt �+3 �1 쿍 쿬ount of writers � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_selector �+4 �2 쿥 쿫uffer selector � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_first �+6 �2 쿥 쿾tr to base of circular buffer � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_in �+8 �2 쿥 쿾tr to next free byte � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_out �+a �2 쿥 쿾tr to next byte of data � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_last �+c �2 쿥 쿾tr to end+1 of buffer � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_rdlksem �+e �2 쿥 퀁ead lock semaphore � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_wtlksem �+10 �2 쿥 퀇rite lock semaphore � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_rdsem �+12 �2 쿥 퀁ead sync semaphore � 쳐컴컴컴컴컴컴컵컴컴컴탠컴컴컵컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 쿶o_wtsem �+14 �2 쿥 퀇rite sync semaphore � 읕컴컴컴컴컴컴컨컴컴컴좔컴컴컨컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸