Why is goto widely used in the Linux kernel, but many books do not advocate its use?

There is a lot of controversy about the goto statement in C language, and many books recommend "use it with caution or even avoid using it." However, in the practice of Linus, the father of Linux, he widely used the goto statement in Linux, which also inspired us to use this feature reasonably.

Because of the controversy, it is necessary for us to learn to use goto statements. Let’s look at some basic syntax and examples of goto statements:

1. Basic syntax of goto

The goto statement consists of two parts: the keyword goto and the label name. The naming rules for labels are the same as those for variables. Example:

goto label;

For this statement to work properly, the function must also contain another statement labeled label, which begins with the label name followed by a colon, such as:

label:printf(“goto here.\n”);

2. Examples of goto

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/*
编译环境：mingw32  gcc6.3.0
*/
#include 
#include 

/* goto测试 */
void TestGoto(void)
{
    int i;
 
    while (1)
    {
 for (i = 0; i if (i > 6)
     {
  goto label;
     }
     printf("%s : i = %d\n", __FUNCTION__, i);
 }
    }
 label:
     printf("test goto end!");
}
 
int main(void)
{
    TestGoto();
}

operation result:

From the running results, we can obviously know the usage of goto, which can jump out of multiple loops. When the goto statement is encountered during the execution of the program, it can jump to the label to continue execution.

One thing worth noting is that the goto statement and its jump label must be in the same function.

3. What is the difference between goto, break and continue?

It is also a jump statement. What is the difference between the goto statement and the break and continue statements?

Actually, break and continue are special forms of goto. The advantage of using break and continue is that their names already indicate their usage.

Let’s take a look at the usage of break and continue through code examples:

1. break test function

Use the above test program to build a function to test the break statement void TestBreak(void);, such as:

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/* break测试 */
void TestBreak(void)
{
    int i;
 
    while (1)
    {
 for (i = 0; i if (i > 6)
     {
         break; /* 第一个break：跳出for循环 */
     }
     printf("%s : i = %d\n", __FUNCTION__, i);
 }
 printf("Now i = %d\n", i);
 break;     /* 第一个break：跳出while循环 */
    }
    printf("test break end!");
}

operation result:

We can obviously know from the running results that break can exit the current loop.

In this example, the first break statement exits the current for loop, and the second break statement exits the current while loop. It can be seen that a break can exit a loop.

So, according to the characteristics of break and goto, if you want to jump out of many levels of loops, it will be more convenient to use goto.

2. continue test function

Similarly, build a function to test the continue statement void TestContinue(void);, such as:

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/* continue测试 */
void TestContinue(void)
{
    int i;
 
    for (i = 0; i if (i > 6)
 {
     printf("i = %d, continue next loop\n", i);
     continue; /* continue：结束本次循环（而不是终止这一层循环）继续进入下一次循环 */
 }
 printf("%s : i = %d\n", __FUNCTION__, i);
    }
    printf("test break end!");
}

operation result:

We can obviously know from the running results that continue can end this loop (not the entire loop) and enter the next loop (i represents the number of loops).

四、支持与反对goto的理由是什么？

1、不提倡使用goto

不提倡使用goto的占比应该比较多，不提倡的原因主要是：很容易把逻辑弄乱且难以理解。

2、使用goto的理由

这一部分人认为goto可以用在以下两种情况比较方便：

（1）跳出多层循环。

这个例子就类似于我们上面的goto测试程序。

（2）异常处理。

一个函数的执行过程可能会产生很多种情况异常情况。下面有几种处理方式，以代码为例：

方法一：做出判断后，如果条件出错，直接return。

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int mystrlen(char *str)
{
   int count = 0;
   if (str == NULL)
   {
      return-1;
   }

   if (*str == 0)
   {
      return0;
   }

   while(*str != 0 )
   {
      count++;
      str++;
   }
   return count;
}

方法二：先设置一个变量，对变量赋值，只有一个return。

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int mystrlen(char *str)
{
   int ret;
   if (str == NULL)
   {
      ret = -1;
   }
   elseif (*str == 0)
   {
      ret = 0;
   }
   else
   {
      ret = 0;
      while(*str != 0 )
      {
         ret++;
         str++;
      }
   }
   return ret;
}

方法三：使用goto语句。

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int mystrlen(char *str)
{
   int ret;
   if (str == NULL)
   {
      ret = -1;
      goto _RET;
   }

   if (*str == 0)
   {
      ret = 0;
      goto _RET;
   }
       
   while(*str !=0 )
   {
      ret++;
      str++;
   }

_RET:
   return ret;
}

其中，方法三就是很多人都提倡的方式。统一用goto err跳转是最方便且效率最高的，从反汇编语句条数可以看出指令用的最少，消耗的寄存器也最少，效率无疑是最高的。

并且，使用goto可以使程序变得更加可扩展。当程序需要在错误处理时释放资源时，统一到goto处理最方便。这也是为什么很多大型项目，开源项目，包括Linux，都会大量的出现goto来处理错误！

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