linux内核中的文件描述符(五)--fd的分配--locate_fd

Kernel version：2.6.14

CPU architecture：ARM920T

Author：ce123(http://blog.csdn.net/ce123)

继续上一篇博客的内容，分析另一个文件描述符fd的分配函数locate_fd。dup系统调用用于复制一个文件描述符对应的文件，返回值是个文件描述符。在前面的文章中，我们已经分析过了dup的源码(http://blog.csdn.net/ce123/article/details/8444482)，在这里我们深入分析locate_fd函数，其定义如下：

static int locate_fd(struct files_struct *files, 
			    struct file *file, unsigned int orig_start)//从orig_start位开始分配fd
{
	unsigned int newfd;
	unsigned int start;
	int error;
	struct fdtable *fdt;

	error = -EINVAL;
	if (orig_start >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//检查orig_start是大于进程最大可以打开文件的数量
		goto out;

repeat:
	fdt = files_fdtable(files);//文件描述符位图
	/*
	 * Someone might have closed fd's in the range
	 * orig_start..fdt->next_fd
	 */
	start = orig_start;
	if (start < fdt->next_fd)
		start = fdt->next_fd;//如果orig_start小于next_fd，那就从next_fd开始分配

	newfd = start;
	if (start < fdt->max_fdset) {//max_fdset是描述符问题的位数，下面会具体讲解
		newfd = find_next_zero_bit(fdt->open_fds->fds_bits,
			fdt->max_fdset, start);//分配fd
	}
	
	error = -EMFILE;
	if (newfd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)//进行判断，分配的fd不能大于进程最大可以打开的文件数量
		goto out;

	error = expand_files(files, newfd);//文件描述符表的扩展，这个我们留在下一篇文章中详细讲解
	if (error < 0)
		goto out;

	/*
	 * If we needed to expand the fs array we
	 * might have blocked - try again.
	 */
	if (error)
		goto repeat;

	/*
	 * We reacquired files_lock, so we are safe as long as
	 * we reacquire the fdtable pointer and use it while holding
	 * the lock, no one can free it during that time.
	 */
	fdt = files_fdtable(files);
	if (start <= fdt->next_fd)
		fdt->next_fd = newfd + 1;//更新next_fd值

	error = newfd;
	
out:
	return error;
}

linux/arch/arm/kernel/init_task.c

struct task_struct init_task = INIT_TASK(init_task);

#define INIT_TASK(tsk)	\
{									\
	...
	.files		= &init_files,					\
	...			
}