source: MondoRescue/branches/2.2.5/mindi-busybox/sysklogd/logread.c@ 1883

/* vi: set sw=4 ts=4: */
/*
 * circular buffer syslog implementation for busybox
 *
 * Copyright (C) 2000 by Gennady Feldman <gfeldman@gena01.com>
 *
 * Maintainer: Gennady Feldman <gfeldman@gena01.com> as of Mar 12, 2001
 *
 * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
 */

#include "libbb.h"
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>

#define DEBUG 0

enum { KEY_ID = 0x414e4547 }; /* "GENA" */

static struct shbuf_ds {
    int32_t size;           // size of data - 1
    int32_t tail;           // end of message list
    char data[1];           // messages
} *shbuf;

// Semaphore operation structures
static struct sembuf SMrup[1] = {{0, -1, IPC_NOWAIT | SEM_UNDO}}; // set SMrup
static struct sembuf SMrdn[2] = {{1, 0}, {0, +1, SEM_UNDO}}; // set SMrdn


static void error_exit(const char *str) ATTRIBUTE_NORETURN;
static void error_exit(const char *str)
{
    //release all acquired resources
    shmdt(shbuf);
    bb_perror_msg_and_die(str);
}

/*
 * sem_up - up()'s a semaphore.
 */
static void sem_up(int semid)
{
    if (semop(semid, SMrup, 1) == -1)
        error_exit("semop[SMrup]");
}

static void interrupted(int sig ATTRIBUTE_UNUSED)
{
    signal(SIGINT, SIG_IGN);
    shmdt(shbuf);
    exit(0);
}

int logread_main(int argc, char **argv);
int logread_main(int argc, char **argv)
{
    int cur;
    int log_semid; /* ipc semaphore id */
    int log_shmid; /* ipc shared memory id */
    smallint follow = getopt32(argv, "f");

    log_shmid = shmget(KEY_ID, 0, 0);
    if (log_shmid == -1)
        bb_perror_msg_and_die("can't find syslogd buffer");

    /* Attach shared memory to our char* */
    shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
    if (shbuf == NULL)
        bb_perror_msg_and_die("can't access syslogd buffer");

    log_semid = semget(KEY_ID, 0, 0);
    if (log_semid == -1)
        error_exit("can't get access to semaphores for syslogd buffer");

    signal(SIGINT, interrupted);

    /* Suppose atomic memory read */
    /* Max possible value for tail is shbuf->size - 1 */
    cur = shbuf->tail;

    /* Loop for logread -f, one pass if there was no -f */
    do {
        unsigned shbuf_size;
        unsigned shbuf_tail;
        const char *shbuf_data;
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
        int i;
        int len_first_part;
        int len_total = len_total; /* for gcc */
        char *copy = copy; /* for gcc */
#endif
        if (semop(log_semid, SMrdn, 2) == -1)
            error_exit("semop[SMrdn]");

        /* Copy the info, helps gcc to realize that it doesn't change */
        shbuf_size = shbuf->size;
        shbuf_tail = shbuf->tail;
        shbuf_data = shbuf->data; /* pointer! */

        if (DEBUG)
            printf("cur:%d tail:%i size:%i\n",
                    cur, shbuf_tail, shbuf_size);

        if (!follow) {
            /* advance to oldest complete message */
            /* find NUL */
            cur += strlen(shbuf_data + cur);
            if (cur >= shbuf_size) { /* last byte in buffer? */
                cur = strnlen(shbuf_data, shbuf_tail);
                if (cur == shbuf_tail)
                    goto unlock; /* no complete messages */
            }
            /* advance to first byte of the message */
            cur++;
            if (cur >= shbuf_size) /* last byte in buffer? */
                cur = 0;
        } else { /* logread -f */
            if (cur == shbuf_tail) {
                sem_up(log_semid);
                fflush(stdout);
                sleep(1); /* TODO: replace me with a sleep_on */
                continue;
            }
        }

        /* Read from cur to tail */
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
        len_first_part = len_total = shbuf_tail - cur;
        if (len_total < 0) {
            /* message wraps: */
            /* [SECOND PART.........FIRST PART] */
            /*  ^data      ^tail    ^cur      ^size */
            len_total += shbuf_size;
        }
        copy = xmalloc(len_total + 1);
        if (len_first_part < 0) {
            /* message wraps (see above) */
            len_first_part = shbuf_size - cur;
            memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
        }
        memcpy(copy, shbuf_data + cur, len_first_part);
        copy[len_total] = '\0';
        cur = shbuf_tail;
#else
        while (cur != shbuf_tail) {
            fputs(shbuf_data + cur, stdout);
            cur += strlen(shbuf_data + cur) + 1;
            if (cur >= shbuf_size)
                cur = 0;
        }
#endif
 unlock:
        /* release the lock on the log chain */
        sem_up(log_semid);

#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
        for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
            fputs(copy + i, stdout);
        }
        free(copy);
#endif
    } while (follow);

    shmdt(shbuf);

    fflush_stdout_and_exit(EXIT_SUCCESS);
}