ParmBlkPtr

#include <Files.h>

typedef union ParamBlockRec{ Size Description

IOParam ioParam ; 50 Generally used in I/O for open files

FileParam fileParam ; 80 Used for unopened files

VolumeParam volumeParam ; 64 Used in volume-specific functions

CntrlParam cntrlParam; 50

SlotDevParam slotDevParam; 36

MultiDevParam multiDevParam; 38

} ParamBlockRec ; 80 (size of aggregate)

typedef ParamBlockRec *ParmBlkPtr; Note the spelling convention

Notes: All six structures on this union share the same names for the first eight

fields (the first 24 bytes). These common fields are defined in a macro

In lieu of Pascal's system of records and variants, C programmers can use

pre defined unions to access the various parts of the file system parameter

blocks. There are several options, but a common way to access the data is by

allocating a union (ie, storage for the largest of the union-member

structures) and creating pointers which refer to the relevant structure

data types:

ParamBlockRec pb; // allocate a union

ioParam *ipb=(IOParam *)&pb; // and structure ptrs

fileParam *fpb=(FileParam *)&pb; // all pointing same addr

volumeParam *vpb=(VolumeParam *)&pb;

pb.ioParam.ioVRefNum = 2; // as union member field

pb. fileParam.ioFlFndrInfo.fdType = 'TEXT';

pb. volumeParam.ioVolIndex = 0;

ipb->ioVRefNum = 2; // or as a structure field

fpb->ioFlFndrInfo.fdType = 'TEXT';

vpb->ioVolIndex = 0;

You can also do ad hoc type coercion:

unsigned char pb[80]; // big enough to hold a FileParam or IOParam

short theVRef;

theVRef = ((IOParam *)pb)-> ioVRefNum;

((FileParam *)pb)->ioFlLgLen = 1000L;

printf("File type is '%c%c%c%c'\n",pb[32],pb[33],pb[34],pb[35]);

Referenced by (65):