©2004 by Marc J. Rochkind. All rights reserved. Portions marked "Open Source" may be copied under license.

 

                 

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c7/f1.c

Go to the documentation of this file.

/*
    File locking (bad example)
    AUP2, Sec. 7.11.1

    Copyright 2003 by Marc J. Rochkind. All rights reserved.
    May be copied only for purposes and under conditions described
    on the Web page www.basepath.com/aup/copyright.htm.

    The Example Files are provided "as is," without any warranty;
    without even the implied warranty of merchantability or fitness
    for a particular purpose. The author and his publisher are not
    responsible for any damages, direct or incidental, resulting
    from the use or non-use of these Example Files.

    The Example Files may contain defects, and some contain deliberate
    coding mistakes that were included for educational reasons.
    You are responsible for determining if and how the Example Files
    are to be used.

*/
#include "defs.h"
/*[rec]*/
struct rec {
    int r_data;
    off_t r_next;
};
/*[readrec]*/
bool readrec(int dbfd, struct rec *r, off_t off)
{
    ssize_t nread;

    if ((nread = pread(dbfd, r, sizeof(struct rec), off)) ==
      sizeof(struct rec))
        return true;
    if (nread != -1)
        errno = EIO;
    EC_FAIL
    return true;

EC_CLEANUP_BGN
    return false;
EC_CLEANUP_END
}

bool writerec(int dbfd, struct rec *r, off_t off)
{
    ssize_t nwrote;

    if ((nwrote = pwrite(dbfd, r, sizeof(struct rec), off)) ==
      sizeof(struct rec))
        return true;
    if (nwrote != -1)
        errno = EIO;
    EC_FAIL
    return true;

EC_CLEANUP_BGN
    return false;
EC_CLEANUP_END
}
/*[store]*/
bool store(int dbfd, int data)
{
    struct rec r, rnew;
    off_t end, prev;

    ec_neg1( end = lseek(dbfd, 0, SEEK_END) )
    prev = 0;
    ec_false( readrec(dbfd, &r, prev) )
    while (r.r_next != 0) {
        ec_false( readrec(dbfd, &r, r.r_next) )
        if (r.r_data > data)
            break;
        prev = r.r_next;
    }
    ec_false( readrec(dbfd, &r, prev) )
    rnew.r_next = r.r_next;
    r.r_next = end;
    ec_false( writerec(dbfd, &r, prev) )
    rnew.r_data = data;
    usleep(1); /* give up CPU */
    ec_false( writerec(dbfd, &rnew, end) )
    return true;

EC_CLEANUP_BGN
    return false;
EC_CLEANUP_END
}
/*[process1]*/
#define DBNAME "termdb"

static void process1(void)
{
    int dbfd, data;
    struct rec r;

    ec_neg1( dbfd = open(DBNAME, O_CREAT | O_TRUNC | O_RDWR, PERM_FILE) )
    memset(&r, 0, sizeof(r));
    ec_false( writerec(dbfd, &r, 0) )
    for (data = 100; data >= 0; data--)
        ec_false( store(dbfd, data) )
    ec_neg1( close(dbfd) )
    exit(EXIT_SUCCESS);

EC_CLEANUP_BGN
    exit(EXIT_FAILURE);
EC_CLEANUP_END
}
/*[process2]*/
static void process2(void)
{
    int try, dbfd;
    struct rec r1, r2;

    for (try = 0; try < 10; try++)
        if ((dbfd = open(DBNAME, O_RDWR)) == -1) {
            if (errno == ENOENT) {
                continue;
            }
            else
                EC_FAIL
        }
    ec_neg1( dbfd )
    for (try = 0; try < 100; try++) {
        ec_false( readrec(dbfd, &r1, 0) )
        while (r1.r_next != 0) {
            ec_false( readrec(dbfd, &r2, r1.r_next) )
            if (r1.r_data > r2.r_data) {
                printf("Found sorting error (try %d)\n", try);
                break;
            }
            r1 = r2;
        }
    }
    ec_neg1( close(dbfd) )
    return;

EC_CLEANUP_BGN
    exit(EXIT_FAILURE);
EC_CLEANUP_END
}
/*[main]*/
int main(void)
{
    pid_t pid;

    ec_neg1( pid = fork() )
    if (pid == 0)
        process1();
    else {
        process2();
        ec_neg1( waitpid(pid, NULL, 0) )
    }
    exit(EXIT_SUCCESS);

EC_CLEANUP_BGN
    exit(EXIT_FAILURE);
EC_CLEANUP_END
}
/*[]*/

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