log_buf(ring buffer)(syslog)(printk)

printk 的输出都是保存在log_buf里的

How to read it the ring buffer? Here is a beautiful illustration from IBM Developerworks dmesg would be your first resort! How does dmesg accomplish its task? By a call to syslog()! How does syslog do its job? Through the system call interface which in turn call do_syslog(). do_syslog() does the finishing act like this

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// 模仿kernel/printk.c中do_syslog，在module 中读printk输出的最后4k

char **log_buf;
int *log_buf_len, *log_start, *log_end;

int MASK;
int end, len;
unsigned i, j, limit, count;
char c;
int ret;


log_buf = (char**) 0xffffffff803270e0;
log_buf_len = (int*)0xffffffff803270d8;
log_start = (int*) 0xffffffff804eabd0;
log_end = (int*) 0xffffffff804eabe0;

i = 0;
end = *log_end;
len = *log_buf_len;
MASK = len - 1;

count = L;
if (count > len) count = len;
limit = end;

for (i=0;i<count;i++) {
        j = limit-1-i;
        c = *(*log_buf + (j&MASK));
        if (c == '\0') c = '\n';
        buf[count-1-i] = c;
}

// 输出

file = filp_open(file_path, O_RDWR|O_CREAT|O_APPEND, 0777);
if(IS_ERR(file)) {
        printk("Open file %s failed..\n", file_path);
        return 0;
}
old_fs = get_fs();
set_fs(get_ds());
ret = file->f_op->write(file, buf, i, &file->f_pos);
file->f_op->fsync(file, file->f_dentry, 0);
set_fs(old_fs);
filp_close(file, NULL);

kk Blog —— 通用基础

date [-d @int|str] [+%s|"+%F %T"]
netstat -ltunp
sar -n DEV 1

printk 的输出都是保存在log_buf里的