在scull的proc read实现中，这些参数意味着什么？
在scull的proc read实现中，这些参数意味着什么？
提问于 2019-05-28 06:48:46
在LDD3 (Linux设备驱动程序第三版)的scullpipe中，static int scull_read_p_mem(char *buf, char **start, off_t offset, int count, int *eof, void *data)中的参数意味着什么？具体地说，我不理解start、page和offset之间的区别。
关于实际实现本身，我有一些问题(可以在下面找到)：
struct scull_pipe {
wait_queue_head_t inq, outq; /* read and write queues */
char *buffer, *end; /* begin of buf, end of buf */
int buffersize; /* used in pointer arithmetic */
char *rp, *wp; /* where to read, where to write */
int nreaders, nwriters; /* number of openings for r/w */
struct fasync_struct *async_queue; /* asynchronous readers */
struct semaphore sem; /* mutual exclusion semaphore */
struct cdev cdev; /* Char device structure */
};
int scull_p_nr_devs; /* the number of scull_pipe devices */
scull_pipe *scull_p_devices; /* scull_pipe devices to be malloc'ed */
/* ...... */
/* our proc read implementation */
static int scull_read_p_mem(char *buf, char **start, off_t offset, int count,
int *eof, void *data)
{
int i, len;
struct scull_pipe *p;
#define LIMIT (PAGE_SIZE-200) /* don't print any more after this size */
*start = buf;
len = sprintf(buf, "Default buffersize is %i\n", scull_p_buffer);
for(i = 0; i < scull_p_nr_devs && len <= LIMIT; i++) {
p = &scull_p_devices[i];
if (down_interruptible(&p->sem))
return -ERESTARTSYS;
len += sprintf(buf+len, "\nDevice %i: %p\n", i, p);
len += sprintf(buf+len, " Buffer: %p to %p (%i bytes)\n",
p->buffer, p->end, p->buffersize);
len += sprintf(buf+len, " rp %p wp %p\n", p->rp, p->wp);
len += sprintf(buf+len, " readers %i writers %i\n",
p->nreaders, p->nwriters);
up(&p->sem);
scullp_proc_offset(buf, start, &offset, &len);
}
*eof = (len <= LIMIT);
return len;
}
static void scullp_proc_offset(char *buf, char **start, off_t *offset, int *len)
{
/* QUESTION: what does this function do? */
if (*offset == 0)
return;
if (*offset >= *len) {
*offset -= *len; /* QUESTION: what is the purpose of this? */
*len = 0;
}
else {
*start = buf + *offset; /* QUESTION: why do you need to change "start"? */
*offset = 0;
}
}回答 1
Stack Overflow用户
发布于 2020-11-12 10:48:33
buf指向应该存储读取结果的内存页的基址。
offset建议在虚拟文件中从哪里开始读取。
作为返回值的start可以表示以下几点：
if start == null，我们有写入buf + offsetif start == some int <buf地址的结果，我们有写入buf内bufif start == some地址的结果，我们有启动
的写入结果
该函数应解释如下。
// context:
// 1. We have set start to be the beginning of buf
// *start = buf;
// 2. We have fill some string into the buf and increment len
static void scullp_proc_offset(char *buf, char **start, off_t *offset, int *len)
{
// If offset is zero, we are filling a new buf.
// (Or we have finished catching up as shown below)
// We can continue to do so until we reach buf size
if (*offset == 0)
return;
// Otherwise, we have some offset to catch up to.
if (*offset >= *len) {
// The offset might be still ahead given we have filled len.
// So, we reduce offset(meaning that we have done some catch up)
// and reset len(because we are still working on old data)
*offset -= *len;
*len = 0;
}
else {
// Or, the offset might be already behind us
// So, we set start to indicate that the write starts from this address
// (case 2 in fs/proc/generic.c#L76)
// and reset offset (meaning that we are done with catching up)
*start = buf + *offset;
*offset = 0;
// this part might lack the handling of len
// but it might be true if the structure is the same between two calls.
}
}总之，作者使用此函数来使写入始终从buf开始，但仅在赶上前一个偏移量之后计数len。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/56333180
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