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Annotation of researchv10no/libc/gen/seekdir.c, revision 1.1

1.1     ! root        1: /* Copyright (c) 1982 Regents of the University of California
        !             2:  * and modified by pjw in 1987 */
        !             3: 
        !             4: #include "ndir.h"
        !             5: 
        !             6: /*
        !             7:  * seek to an entry in a directory.
        !             8:  *
        !             9:  * offset `told' is interpreted in a non-obvious way:
        !            10:  * telldir computes it as  dd_offset*DIRBLKSIZ + dd_loc
        !            11:  * dd_loc is the byte offset into the current dd_buf contents;
        !            12:  * dd_offset is the number of times dirread(fd, buf, DIRBLKSIZ)
        !            13:  * must be called to reproduce the same buffer.
        !            14:  *
        !            15:  * Why not just use lseek?  Because told isn't a seek offset,
        !            16:  * and cannot conveniently be turned into one: the byte count
        !            17:  * in dd_loc is unrelated to the seek offset required to reach
        !            18:  * lseek(fd, 0L, 1)+dd_loc, because the seek offset refers to the raw
        !            19:  * directory, but dirread has reformatted the data.
        !            20:  * One might let dd_offset be the seek offset to the the beginning
        !            21:  * of dd_buf, compute told the same way, and let seekdir become
        !            22:  *     lseek(fd, told/DIRBLKSIZ, 0); dirp->dd_loc = told%DIRBLKSIZ;
        !            23:  * but a somewhat strident prior implementor worries that this unfairly
        !            24:  * constrains the size of directories.  This is probably a silly worry
        !            25:  * (will any real system have directories a megabyte long?) but it
        !            26:  * doesn't matter much except in programs that use seekdir a lot.
        !            27:  */
        !            28: void
        !            29: seekdir(dirp, told)
        !            30: register DIR *dirp;
        !            31: long told;
        !            32: {      long offset;
        !            33: 
        !            34:        offset = told / DIRBLKSIZ;
        !            35:        dirp->dd_loc = told % DIRBLKSIZ;
        !            36:        if (offset < dirp->dd_offset)
        !            37:                lseek(dirp->dd_fd, dirp->dd_offset = 0L, 0);
        !            38:        if(offset > dirp->dd_offset) {
        !            39:                do {
        !            40:                        dirp->dd_size = dirread(dirp->dd_fd, dirp->dd_buf,
        !            41:                                                DIRBLKSIZ);
        !            42:                        if(dirp->dd_size < 0)
        !            43:                                dirp->dd_size = pdirread(dirp->dd_fd,
        !            44:                                        dirp->dd_buf, DIRBLKSIZ);
        !            45:                        }
        !            46:                        while(++dirp->dd_offset < offset);
        !            47:                }
        !            48: }

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