block.c

操作系统课程设计

/* block.c */

#include "block.h"

//返回当前在空闲块栈顶的空闲块号
int get_blk(FILE *fp)
{
    struct fs_super super;























































































































    fseek( fp, sizeof(struct fs_boot), SEEK_SET); /* skip boot block */
    fread( &super, sizeof(super), 1, fp);

    int i, blk_no;
    if(super.free_block_count > 0){ /* if more than one block */
	i = super.free_block_stack[0];//当前空闲块的索引值
	blk_no = super.free_block_stack[i];//当前空闲块的块号

	super.free_block_stack[0] -= 1; /* update the relative variable */
	super.free_block_count--;//空闲块数目少1

	//如果栈滿并且还有空闲块，则重新载入新的空闲块的栈
	if((super.free_block_stack[0]==0) && (super.free_block_count>0)){

	    fseek( fp, BLOCK_SIZE*(INODE_TABLE_BLOCKS + 1 + blk_no), SEEK_SET);
	    fread( &super.free_block_stack, sizeof(super.free_block_stack), 1, fp);
	}
	
	fseek( fp, sizeof(struct fs_boot), SEEK_SET);
	fwrite( &super, sizeof(super), 1, fp);

	return blk_no;
    }
    else			/* if free blocks is none*/
	return -1;
}

//释放磁盘块
int free_blk(FILE *fp, int blk_no)
{
    struct fs_super super;
    fseek( fp, sizeof(struct fs_boot), SEEK_SET);
    fread( &super, sizeof(super), 1, fp);

    int i;
    if(super.free_block_stack[0] == STACK_MAX){ /* if stack was full */

	//在释放的位置写入新的空闲块
	fseek( fp, BLOCK_SIZE*(INODE_TABLE_BLOCKS + 1 + blk_no), SEEK_SET);
	fwrite( &super.free_block_stack, sizeof(super.free_block_stack), 1, fp);

	super.free_block_count++; /* update the super */
	super.free_block_stack[0] = 1;
	super.free_block_stack[1] = blk_no;
    }
    else{
	super.free_block_count++;
	super.free_block_stack[0] += 1;
	i = super.free_block_stack[0];
	super.free_block_stack[i] = blk_no;//把刚释放的磁盘块号放在栈顶，这样就保证了总会先分配低块号的块
    }

    fseek( fp, sizeof(struct fs_boot), SEEK_SET); /* write back */
    fwrite( &super, sizeof(super), 1, fp);

    return 0;
}