proc文件系统下seq_lseek（）用法

  1 #include

  2 #include

  3 #include

  4 #include

  5 

  6 int main(void)

  7 {

  8     int fd,len;

  9     fd = open("/proc/cpuinfo",O_RDONLY);

 10     if(fd<0)

 11     {

 12         printf("error");

 13         return(-1);

 14     }

 15     len = lseek(fd,0,SEEK_SET);

 16     printf("current pos1 is:%d\n",len);

 17     len = lseek(fd,0,SEEK_END); //这里出错！！！

 18     printf("current pos2 is:%d\n",len);//居然返回-1

 19 }

原来是procfs文件的系统的seq_lseek不支持SEEK_END这个选项，无语了！

#define SEEK_SET 0 /* seek relative to beginning of file */

#define SEEK_CUR 1 /* seek relative to current file position */

#define SEEK_END 2 /* seek relative to end of file */

#define SEEK_MAX SEEK_END

loff_t seq_lseek(struct file *file, loff_t offset, int origin)

{

struct seq_file *m = (struct seq_file *)file->private_data;

loff_t retval = -EINVAL;

mutex_lock(&m->lock);

m->version = file->f_version;

switch (origin) {

case 1: //--->SEEK_SET

offset += file->f_pos;

case 0: //--->SEEK_CUR

if (offset < 0)

break;

retval = offset;

if (offset != m->read_pos) {

while ((retval=traverse(m, offset)) == -EAGAIN)

;

if (retval) {

/* with extreme prejudice... */

file->f_pos = 0;

m->read_pos = 0;

m->version = 0;

m->index = 0;

m->count = 0;

} else {

m->read_pos = offset;

retval = file->f_pos = offset;

}

}

}

file->f_version = m->version;

mutex_unlock(&m->lock);

return retval;

}