如何监控一个日志文件并验证，用tail的原理实现

最近在做网页爬虫的测试，需要通过apache的access-log来验证爬虫的行为，就想到了用类似linux的tail命令实现方式来实现。这里先把代码贴出来：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <malloc.h>
#include <sys/stat.h>

static size_t filesize(const char *filename) {
    struct stat sb;

    if (!stat(filename, &sb))
        return sb.st_size;
    return 0;
}

static void tailf(const char *filename, int lines) {
    char **buffer;
    int head = 0;
    int tail = 0;
    FILE *str;
    int i;

    if (!(str = fopen(filename, "r"))) {
        fprintf(stderr, "Cannot open \"%s\" for read\n", filename);
        perror("");
        exit(1);
    }

    buffer = (char **) malloc(lines * sizeof(*buffer));
    for (i = 0; i < lines; i++)
        buffer[i] = (char *) malloc(BUFSIZ + 1);

    while (fgets(buffer[tail], BUFSIZ, str)) {
        if (++tail >= lines) {
            tail = 0;
            head = 1;
        }
    }

    //the method here is very smart, if variable head is set, that is to say the file is longer than 10 lines,
    //and the first line of the last 10 lines is start from the index tail, and the last line is at index tail-1

    if (head) {
        for (i = tail; i < lines; i++)
            fputs(buffer[i], stdout);
        for (i = 0; i < tail; i++)
            fputs(buffer[i], stdout);
    } else {
        for (i = head; i < tail; i++)
            fputs(buffer[i], stdout);
    }
    fflush(stdout);

    for (i = 0; i < lines; i++)
        free(buffer[i]);
    free(buffer);
}

int main(int argc, char **argv) {
    char buffer[BUFSIZ];
    size_t osize, nsize;
    FILE *str;
    const char *filename;
    int count;

    if (argc != 2) {
        fprintf(stderr, "Usage: tailf logfile\n");
        exit(1);
    }

    filename = argv[1];

    tailf(filename, 10);

    for (osize = filesize(filename);;) {
        nsize = filesize(filename);
        if (nsize != osize) {
            if (!(str = fopen(filename, "r"))) {
                fprintf(stderr, "Cannot open \"%s\" for read\n", filename);
                perror(argv[0]);
                exit(1);
            }
            if (!fseek(str, osize, SEEK_SET))
                while ((count = fread(buffer, 1, sizeof(buffer), str)) > 0)
                    fwrite(buffer, 1, count, stdout);
            fflush(stdout);
            fclose(str);
            osize = nsize;
        }
        usleep(250000); /* 250mS */
    }
    return 0;

}