pagein.c

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#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <grass/segment.h>


static int segment_select(SEGMENT *, int);


int segment_pagein(SEGMENT * SEG, int n)
{
    int age;
    int cur;
    int i;
    int read_result;

    /* is n the current segment? */
    if (n == SEG->scb[SEG->cur].n)
        return SEG->cur;

    /* search the in memory segments */
    for (i = 0; i < SEG->nseg; i++)
        if (n == SEG->scb[i].n)
            return segment_select(SEG, i);

    /* find a slot to use to hold segment */
    age = 0;
    cur = 0;
    for (i = 0; i < SEG->nseg; i++)
        if (SEG->scb[i].n < 0) {        /* free slot */
            cur = i;
            break;
        }
        else if (age < SEG->scb[i].age) {       /* find oldest segment */
            cur = i;
            age = SEG->scb[i].age;
        }

    /* if slot is used, write it out, if dirty */
    if (SEG->scb[cur].n >= 0 && SEG->scb[cur].dirty)
        if (segment_pageout(SEG, cur) < 0)
            return -1;

    /* read in the segment */
    SEG->scb[cur].n = n;
    SEG->scb[cur].dirty = 0;
    segment_seek(SEG, SEG->scb[cur].n, 0);

    read_result = read(SEG->fd, SEG->scb[cur].buf, SEG->size);
    if (read_result != SEG->size) {
        G_debug(2, "segment_pagein: read_result=%d  SEG->size=%d",
                read_result, SEG->size);

        if (read_result < 0)
            G_warning("segment_pagein: %s", strerror(errno));
        else if (read_result == 0)
            G_warning("segment_pagein: read EOF");
        else
            G_warning
                ("segment_pagein: short count during read(), got %d, expected %d",
                 read_result, SEG->size);

        return -1;
    }

    return segment_select(SEG, cur);
}


static int segment_select(SEGMENT * SEG, int n)
{
    int i;

    SEG->scb[n].age = 0;
    for (i = 0; i < SEG->nseg; i++)
        SEG->scb[i].age++;

    return SEG->cur = n;
}