学习记录：snprintf 和scnprintf的差别

1，背景

      在学习Linux的过程中，偶然间看到了sys文件系统的说明文档：./Documentation/filesystems/sysfs.txt（系统版本为4.7.3），其中在说明文档中，有这么一段话：

- show() methods should return the number of bytes printed into the
  buffer. This is the return value of scnprintf(). 
- show() must not use snprintf() when formatting the value to be
  returned to user space. If you can guarantee that an overflow
  will never happen you can use sprintf() otherwise you must use
  scnprintf().

    但是，以前偶尔看到的Linux系统驱动源码中，却经常看到使用snprintf的情况，并且在某些博客甚至案例中点名了要使用这个函数，在Linux的官方文档中，却禁止使用这个函数的原因是什么？

2，snprintf和scnprintf 对比

首先看一下snprintf 

官方文档：http://www.cplusplus.com/reference/cstdio/snprintf/

The number of characters that would have been written if n had been sufficiently large, not counting the terminating null character.
If an encoding error occurs, a negative number is returned.
Notice that only when this returned value is non-negative and less than n, the string has been completely written.

运行一个demo查看一下：

#include <stdio.h>
#include <string.h>
int main (int argc, char *argv[])
{
    char buffer1 [10]= {""};
    int cx1;
    cx1 = snprintf (buffer1, 10, "%s", "justfor");

    printf("src lenth < n buffer1 is:%s, lenth is:%d return value:%d\n",buffer1, strlen(buffer1),cx1);

    char buffer2 [10]= {""};
    int cx2;
    cx2 = snprintf (buffer2, 10, "%s", "justfortes");

    printf("src lenth = n buffer2 is:%s, lenth is:%d return value:%d\n",buffer2, strlen(buffer2),cx2);
    
    char buffer3 [10]= {""};
    int cx3;
    cx3 = snprintf (buffer3, 10, "%s", "justfortest");

    printf("src lenth > n buffer3 is:%s, lenth is:%d return value:%d\n",buffer3, strlen(buffer3),cx3);
    return 0;
}

结果如下：

src lenth < n buffer1 is:justfor, lenth is:7 return value:7
src lenth = n buffer2 is:justforte, lenth is:9 return value:10//通过这个结果，可以看到其实buffer使用snprintf在n的位置是自动补\n的
src lenth > n buffer3 is:justforte, lenth is:9 return value:11//返回值过大，说明截断了

通过上面的返回值结果，我们能够看到，其实返回的数值是src的长度，而不是已经写入的长度，如果此时，再使用

buffer［xxlen］就有可能会出现越界的情况，比如上文中使用buffer[cx]就会越界；

关于 scnprintf

参考文章：https://www.kernel.org/doc/htmldocs/kernel-api/API-scnprintf.html

The return value is the number of characters written into buf not including the trailing '\0'. If size is == 0 the function returns 0.

因此，在linux kernel中作为返回用户态的一个value，建议使用scnpirnt

get