学步园

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;
}

max_fdset值的分析和rlim_cur差不多，最初的值时从父进程继承过来的。

linux/arch/arm/kernel/init_task.c

struct task_struct init_task = INIT_TASK(init_task);

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

init_files的定义如下：

static struct files_struct init_files = INIT_FILES;

linux/init_task.h

#define INIT_FDTABLE \
{							\
	.max_fds	= NR_OPEN_DEFAULT, 		\
	.max_fdset	= __FD_SETSIZE, 		\
	.next_fd	= 0, 				\
	.fd		= &init_files.fd_array[0], 	\
	.close_on_exec	= &init_files.close_on_exec_init, \
	.open_fds	= &init_files.open_fds_init, 	\
	.rcu		= RCU_HEAD_INIT, 		\
	.free_files	= NULL,		 		\
	.next		= NULL,		 		\
}

#define NR_OPEN_DEFAULT BITS_PER_LONG
#define __FD_SETSIZE	1024

#define INIT_FILES \
{ 							\
	.count		= ATOMIC_INIT(1), 		\
	.file_lock	= SPIN_LOCK_UNLOCKED, 		\
	.fdt		= &init_files.fdtab, 		\
	.fdtab		= INIT_FDTABLE,			\
	.close_on_exec_init = { { 0, } }, 		\
	.open_fds_init	= { { 0, } }, 			\
	.fd_array	= { NULL, } 			\
}

BITS_PER_LONG是long型数据的字节数，即4*8=3，也就是说max_fds = 32。max_fdset为1024。max_fdset是进程打开文件描述符位图open_fds的大小。open_fds是fd_set的指针。

typedef __kernel_fd_set		fd_set;

#undef __NFDBITS
#define __NFDBITS	(8 * sizeof(unsigned long))

#undef __FD_SETSIZE
#define __FD_SETSIZE	1024

#undef __FDSET_LONGS
#define __FDSET_LONGS	(__FD_SETSIZE/__NFDBITS)

#undef __FDELT
#define	__FDELT(d)	((d) / __NFDBITS)

#undef __FDMASK
#define	__FDMASK(d)	(1UL << ((d) % __NFDBITS))

typedef struct {
	unsigned long fds_bits [__FDSET_LONGS];
} __kernel_fd_set;

fds_bits是一个long型数组，共有32个元素，共有1024bit。