标签:语句 goto bh C++ && pass buffers wait
注:以下代码仅用于学习交流,请勿用于商业用途
Authors:
yjljobrequest@163.com
- 01 "声明狼藉"的goto 语句
- 02 goto 语句 使用的集中场景
01 "声明狼藉"的goto 语句
goto语句的经常被贬斥为一种糟糕的编程方式,大学课程c语言课程中讲师这样称道,教科书很少见到goto语句的考题,甚至工程实践中也少见此类源码
有限的使用goto语句,可以提高代码的执行效率(但是可能会造成代码结构不易理解,比如回跳)
goto语句可以简化嵌套循环或者if else的复杂结构,从而使代码执行过程更加简洁
02 goto 语句 使用的几种场景
情景1 嵌套循环:简化代码结构,便于阅读
输出:val = 4704 i = 48 j = 98
#include <iostream>
int main()
{
using namespace std;
long val (0);
int i = 0;
int j = 0;
for(i = 1;i < 1000;i++)
{
for(j = 1;j < 100;j++)
{
val= i* j;
if(val > 4700)
goto bottom;
}
}
bottom:
cout << "val = " << val << " i = " << i << " j = " << j <<endl;
return 0;
}
情景2 频繁使用return语句:简化代码结构
借用一位同行的代码,原始链接如下:
https://bbs.csdn.net/topics/380068160
int getresult_use_goto()
{
int iResult = 0;
FILE* fp1 = NULL;
FILE* fp2 = NULL;
char* pMem1 = NULL;
char* pMem2 = NULL;
fp1 = fopen("C:\\file1", "rb");
if (NULL == fp1) goto End;
fp2 = fopen("C:\\file2", "rb");
if (NULL == fp2) goto End;
pMem1 = (char *)malloc(1000);
if (NULL == pMem1) goto End;
pMem2 = (char *)malloc(2000);
if (NULL == pMem2) goto End;
iResult = 1;
End:
if (fp1) fclose(fp1);
if (fp2) fclose(fp2);
if (pMem1) free(pMem1);
if (pMem2) free(pMem2);
return iResult;
}
情景3 优化程序执行效率
在linux内核代码中有非常多的这种使用范例(回跳),一个基本的使用思想是加强代码执行效率
static int sync_buffers(dev_t dev, int wait)
{
int i, retry, pass = 0, err = 0;
struct buffer_head * bh;
/* One pass for no-wait, three for wait:
0) write out all dirty, unlocked buffers;
1) write out all dirty buffers, waiting if locked;
2) wait for completion by waiting for all buffers to unlock.
*/
repeat:
retry = 0;
bh = free_list;
for (i = nr_buffers*2 ; i-- > 0 ; bh = bh->b_next_free) {
if (dev && bh->b_dev != dev)
continue;
#ifdef 0 /* Disable bad-block debugging code */
if (bh->b_req && !bh->b_lock &&
!bh->b_dirt && !bh->b_uptodate)
printk ("Warning (IO error) - orphaned block %08x on %04x\n",
bh->b_blocknr, bh->b_dev);
#endif
if (bh->b_lock)
{
/* Buffer is locked; skip it unless wait is
requested AND pass > 0. */
if (!wait || !pass) {
retry = 1;
continue;
}
wait_on_buffer (bh);
}
/* If an unlocked buffer is not uptodate, there has been
an IO error. Skip it. */
if (wait && bh->b_req && !bh->b_lock &&
!bh->b_dirt && !bh->b_uptodate)
{
err = 1;
continue;
}
/* Don't write clean buffers. Don't write ANY buffers
on the third pass. */
if (!bh->b_dirt || pass>=2)
continue;
bh->b_count++;
ll_rw_block(WRITE, 1, &bh);
bh->b_count--;
retry = 1;
}
/* If we are waiting for the sync to succeed, and if any dirty
blocks were written, then repeat; on the second pass, only
wait for buffers being written (do not pass to write any
more buffers on the second pass). */
if (wait && retry && ++pass<=2)
goto repeat;
return err;
}
标签:语句,goto,bh,C++,&&,pass,buffers,wait 来源: https://blog.csdn.net/uncle103/article/details/95610794
本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享; 2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关; 3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关; 4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除; 5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。