Linux系统调用read、write

0 前言

《Linux系统调用》整体介绍了系统调用，本文重点分析其中read、write的实现与使用方法。

1 定义

1.1 read

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
{struct file *file;ssize_t ret = -EBADF;int fput_needed;file = fget_light(fd, &fput_needed);if (file) {loff_t pos = file_pos_read(file);ret = vfs_read(file, buf, count, &pos);file_pos_write(file, pos);fput_light(file, fput_needed);}return ret;
}
// @file：fs/read_write.c

可见它调用的一个关键函数是vfs()_read():

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{ssize_t ret;if (!(file->f_mode & FMODE_READ))return -EBADF;if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))return -EINVAL;if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))return -EFAULT;ret = rw_verify_area(READ, file, pos, count);if (ret >= 0) {count = ret;if (file->f_op->read)ret = file->f_op->read(file, buf, count, pos); /* 调用file->f_op->read()！*/elseret = do_sync_read(file, buf, count, pos);if (ret > 0) {fsnotify_access(file);add_rchar(current, ret);}                                                                                                                                                                                                    inc_syscr(current);}   return ret;
}EXPORT_SYMBOL(vfs_read);
// @file：fs/read_write.c

分析上述vfs_read()可知，它最终调用了f_op->read()。在编写设备驱动程序时，很重要的一步就是实现f_op->read()，这里就是调用该f_op->read()的地方！